Bikini and 

Nearby Atolls

Part 2. Oceanography (biologic)

Biologic Economy of Coral Reefs 

Plankton of Northern Marshall Islands 

Recent Brachiopods

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-E, F, G
Bikini and 

Nearby Atolls

Part 2. Oceanography (biologic)

Biologic Economy of Coral Reefs

By MARSTON C. SARGENT and THOMAS S. AUSTIN

Plankton of Northern Marshall Islands

By MARTIN W. JOHNSON

Recent Brachiopods

By G. A. COOPER

GEOLOGICAL SURVEY PROFESSIONAL PA[PJER 260-E, F, G

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1954
UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary

GEOLOGICAL SURVEY 

W. E. Wrather, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office 

Washington 25, D. C., - Price 60 cents (paper cover)
CONTENTS

Page 

(E) Biologic economy of coral reefs, by MARSTON C. SARGENT and THOMAS S. AUSTIN._________---__-_-_-_________-__ 293

(F) Plankton of northern Marshall Islands, by MARTIN W. JOHNSON._______________--___-_____--____----____--- 301

(G) Recent brachiopods, by G. A. COOPER.________-_-_____________-__-____-___-_-_-_--______--_-__-_-_-___-__-- 315

in

Biologic Economy of 

Coral Reefs

By MARSTON C. SARGENT and THOMAS S. AUSTIN

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1954

CONTENTS

Page 

Abstract--_-_----_-_-----------_-__________________ 293

Introduction.______________________________________ 293

Geographical situation..._.-____-__-__________•_______ 293

Observations:

Maximum rate of oxygen production ______________ 294

Page 

Observations—Continued

Daily course of rate of oxygen production and

consumption ________________________________ 295

Oxygen exchange of individual reef organisms. _____ 297 

Literature cited__-_--_------_-------------_—— 300

ILLUSTRATION

FIGURE 100. Daily oxygen exchangePage 

296

TABLES

Page 

TABLE 1. Oxygen production by a reef north of Busch island-_______________________-__--___-____-_-----_-_____--__ 294

2. Oxygen production by a reef north of Busch island— __________________--_______-_--_-_-_--__-_--_-----_-- 295

3. Oxygen exchange on section across reef.________________________________________________________________ 296

4. Oxygen content of reef waters._________-_________________________-_____________--___-____-_------_---_ 297

5. Rate of production of oxygen--_------_-_--_-___--______-___________-_____-_-_-------------__------_-_- 297

ni

BIOLOGIC ECONOMY OF CORAL REEFS

MARSTON C. SARGENT l AND THOMAS S. AUSTIN 2

ABSTRACT

The eastern reefs of Rongelap Atoll, composed of animals and 

attached algae, produce more organic matter than they consume. 

The productivity per unit area is considerably higher than that of 

adjacent waters or any^other open marine areas. The zooxanthellae of the corals make a substantial contribution to the whole 

production, so that per unit of weight, actively growing reef 

corals may photosynthesize as fast as lithothamnia Review 

of work on other coral reefs indicates that these communities 

are generally self-maintaining.

INTRODUCTION

The coral atolls of the northern Marshall Islands are 

examples of a distinctive pelagic community of the 

Pacific Ocean. For thousands of miles in all directions 

they are surrounded by the communities common to 

all open oceans, of which the conspicuous members are 

plankton, fish, and birds. Only at some singular spots 

have a succession of crustal movements and biotic 

changes produced structures and biocycles strikingly 

different from their surroundings.

In one important respect the situation of these atolls 

is different from that of any coral reef where extensive 

studies in physiology and ecology have been conducted. 

Unlike fringing and barrier reefs, the reefs of these 

atolls are exposed to a steady one-directional wavedriven current of oceanic water as described by Von 

Arx (1948). As in a river, changes in the organic and 

inorganic content occur in this water as it crosses the 

reef, and if these changes are measured, conclusions can 

be drawn about the activities of the reef community. 

The most fundamental conclusion drawn from the observations recorded here is that atoll reefs are essentially 

self-sufficient communities, producing as much organic 

matter as they consume, or more. This is not necessarily true of barrier and fringing reefs where proximity 

of the land and stagnation of the waters overlying and 

adjoining the reefs may furnish conditions suitable for 

production of food of reef-building organisms quite independently of the presence of the reef. It may, how1 Scripps Institution of Oceanography, University of California.

2 Pacific Oceanic Fishery Investigations, Box 3830, Honolulu, T. H.

NOTE.—Contribution from the Scripps Institution of Oceanography New Series No. 703.

ever, be true of these reefs also, but not be easy to discern under the conditions prevailing.

In a previous brief report (Sargent and Austin, 1949) 

we have referred to reviews by Yonge (1940) and Shepard (1948) as covering the literature on coral biology 

and the geology of atolls. To these should be added 

several reports by Tracey, Emery, Ladd, and others on 

the geology of the northern Marshall Islands, which will 

be referred to below. Taylor's volume (1950) on the 

plants of Bikini Atoll also contains much unique and 

valuable information.

GEOGRAPHICAL SITUATION

In the present study several sets of measurements 

were made at two locations on the eastern reef of Rongelap Atoll and one set on the western reef. The morphology of these reefs is described by Tracey, Ladd, and 

Hoffmeister (1948) in a report which clearly demonstrates that it is difficult to draw a line between the 

geology and the biology of these structures or communities. Most measurements were made on the open 

reef between islands where the passage of water between sea and lagoon is unimpeded.

The eastern reef, in the classification of Tracey and 

others (1948), is type I-A. The smooth marginal 

Lithothamnion ridge rises only about a foot above the 

main reef flat and slopes gently seaward without development of high-standing buttresses. On the seaward 

face of the reef below low tide, spurs covered with living 

lithothamnia and corals and separated by grooves 

floored with debris reach to a depth of perhaps 15 

meters. Behind the Lithothamnion ridge, generally 5 

to 15 meters wide in these situations, is a roughly triangular zone of coral, narrow where islands approach 

closely to the Lithothamnion ridge but reaching back 

several hundred feet from the surf zone in the intervals 

between islands. The remainder of the reef to the 

lagoon edge is relatively barren of large fixed colonial 

creatures. Isolated coral colonies and small tufts of

293
294 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

algae project above the layer of sandy Foraminifera and 

inconspicuous matted algae that cover the flat. The 

total width of the reef at the locations studied is about 

300 meters. Close to the islands on each side of an 

interval the reef isMgher, exposed at low tide except for 

numerous tidepools, and largely devoid of life. This 

barren intertidal zone is in striking conrast with the 

densely populated intertidal zone of cooler seas.

The transport of water across the reef depends on the 

stage of the tide. The velocity change does not affect 

the, transport as much as the change in depth. At low 

tide the Lithothamnion ridge may in spots be momentarily awash between breakers, and over the whole flat 

the depth may not exceed 20 centimeters. At high tide 

the depth on the flat may be more than a meter. The 

velocity over the reef flat meanwhile changes perhaps 

threefold with a minimum velocity around 30 centimeters per second. Depths were determined with a 

meter stick or, when necessary, by measurements on an 

observer who was carefully calibrated for the purpose. 

Velocities were measured by timing the passage of a 

small patch of fluorescein along a measured line. At 

this time of year (June-July) the surface of the water 

flowing across the reef is smooth except close to the 

surf zone, although no doubt at seasons of high wind 

it is commonly marked by whitecaps. The smoothness 

of flow is important for the measurements of changes in 

oxygen content in that it diminishes any tendency to 

establish equilibrium with the atmosphere. The flow 

occurs hi pulses resulting from momentary changes of 

head as the breakers pile up on the face of the reef.

The western reef on which a few observations were 

made is type II-B-(l) according to the classification 

of Traceyand others (1948). The coral and algal reef 

with a steep front facing the sea and precipitous edges 

where blocks have been broken out has its highest level 

on the ocean side in the zone of weak surf. It gradually 

slopes down toward the lagoon, with larger and higher

pinnacles or islands of corals growing on it as the bottom 

gets deeper. On the day of our measurements there 

was a westerly wind, rare in these latitudes, which was 

sufficient, together with the tidal change, to maintain a 

considerable current in an easterly direction. We do 

not therefore feel qualified to describe typical current 

conditions on this reef and can only refer to Von Arx 

(1948), who observed that the direction of the current 

alternates with the tides.

OBSERVATIONS 

MAXIMUM RATE OF OXYGEN PRODUCTION

As the water flows across a reef, particularly at low 

tide, it undergoes changes in temperature and oxygen 

content. Table 1 shows the extent of these changes 

during a noon low tide north of Busch island on the 

eastern reef of Kongelap Atoll. The two lines were run 

from north to south parallel to the reef face from a 

point where the current near the seaward edge of the 

reef flowed southerly around the end of an island to a 

point on the other side of the interval between islands 

where it ran northerly around the end of the next 

island. It appears that the direction of flow was 

uniform except at the ends of the line. The velocity 

was highest in the middle of the interval, and less close 

to the islands.

At the four stations sampled near the surf zone, the 

temperature and oxygen content (as determined by 

Winkler titration) were uniform. Along the inner 

edge, both the temperature and the oxygen content were 

significantly greater. Particularly near the ends of the 

line where water was collected which could have run for 

some distance parallel to the reef before it rounded an 

island and crossed the reef, were high temperatures and 

oxygen contents found, In crossing the reef the water 

temperature increased, on the average, 0.4 C, the oxygen content 0.74 milliliters per liter. As the water was

TABLE 1.—Oxygen production by a reef north of Busch island 

[Observations at 1100-1300, July 20, 1946]

Seaward edge of reef

Station

1—— ——— —— —— —

2

8.—— .....„—————

4— ———— —— ————

5————————.—

6.——. _ -——..-.

7— —— — —— — —

8— —— — ———— ——

g

10—————— —

Mean.... .......

Distance 

between 

stations 

(m)

122 

61 

137 

146 

271 

61 

70 

73 

73

Depth 

(cm)

50 

40 

40 

35 

40 

25 

35 

35 

35 

35

35

Velocity 

(cm/sec)

19 

25 

8 

15 

34 

31 

29 

34 

34 

17

24

True 

direction 

of current 

(degrees)

160 

320 

270 

270 

270 

275 

260 

250 

250 

350

Water 

temperature 

CO

28.4

28.4

28.3

28.4

28.4

Concentration of 

oxygen 

(ml/1)

4.54

4.50

4.53

458

4.54

Concentration of 

dissolved 

phosphorus 

(mg-atoms/1)

0.54

.51

.66

.51

Lagoon edge of reef

Station

o

1-.——..———

3— .—— —— —— ~

4 _ .................

5— ... .... — ... ....

6—. ........... .....

7-— ................

8

Water 

temperature 

CO)

28.7 

28.6 

28.3 

28.8 

28.7 

28.9 

29.4 

29.0

28.8

Concentration of 

oxygen 

(ml/1)

5.28 

5.04 

5.44 

5.05 

5.06 

5.05 

5.55 

5.77

5.28

Concentration of 

dissolved 

phosphorus 

(mg-atoms/1)

0.51 

.45 

.54 

.47 

.49 

.49 

.48 

.31
BIOLOGIC ECONOMY OF CORAL REEFS 295

saturated with oxygen when it left the surf zone, the 

increase can be attributed only to photosynthesis by the 

reef organisms. The product of the mean depth in centimeters, the mean velocity in centimeters per second, 

and the mean change in oxygen content in milliliters per 

cubic centimeter,

- 35 X24X10-3 X (5.28-4.54),

gives the mean rate of oxygen production of the reef-top 

community as 0.62 milliliters per second per centimeter 

of reef normal to the current. This can be visualized 

as the oxygen production of all the organisms on a strip 

of reef top 1 centimeter wide running from surf to 

lagoon. A more elaborate calculation involving averaging individual products of depth, velocity, and oxygen 

change gives about the same numerical result (0.63 

ml/cm/sec), but it is not otherwise justified by the nature of the data.

As the reef top is about 270 meters wide, the rate of 

oxygen production can be restated as 23X10"6 milliliters per second per square centimeter of reef surface.

The line of stations in the surf zone was on top of the 

Lithothamnion ridge; consequently, only photosynthesis 

by the reef-top organisms lagoonward from the line was 

included in the measurement. Oxygen produced by the 

outer portion of the ridge and by the organisms of the 

seaward reef face was already dissolved before the water 

crossed the outer station line. Because of the large 

volume of water available and the effectiveness of the 

surf in maintaining oxygen-exchange equilibrium with 

the atmosphere, no measurable change in oxygen content due to activity of organisms could be expected seaward of the line. The measured increase in oxygen 

content was therefore found in what appears to the eye 

as an area rather barren of plants. Besides the Lithothamnion of the ridge, the only other plants readily apparent are scattered small erect Rhodophyceae (Liagora 

sp.) 5 to 10 centimeters high and 20 to 50 centimeters 

apart. Careful search shows that matted filamentous 

Myxophyceae and other minute algae are common embedded in the foraminiferal sand and in other locations 

somewhat protected from direct sunlight (Taylor, 1950, 

p. 11, 102-103). The most striking oxygen-producing 

organisms of the reef top, however, are undoubtedly the 

zooxanthellae of numerous corals, tridacnids, and other 

animals.

By contrast, the outer portions of the Lithothamnion 

ridge and the spurs on the face of the reef appear to be 

almost completely paved with dark red lithothamnia 

and we are tempted to assume a much higher rate of 

oxygen production per square centimeter of reef surface 

than the average for the reef top. The light intensity 

at a depth of 15 meters is probably 20 percent of that 

just below the surface and is presumably adequate for

maximum photosynthesis. The circulation of water is 

rapid (Munk and Sargent, Prof. Paper 260-C, p. 278); 

consequently the supply of gases and nutrients is as 

good as at any point on the reef top. The assumption 

that the reef face is at least as productive as an equivalent area of the most productive part of the reef top is 

not unreasonable. The area of this zone is such that it 

may add another 25 to 50 percent to the total oxygen 

production of the reef.

Table 2 shows additional data on conditions at this 

station in late morning on other days. The averages 

from table I are included for comparison. It appears 

at once that individual observations on the increase in 

oxygen lie within the range of individual observations 

made nearly simultaneously along a single stretch of 

reef. On June 30, when observations on depth and 

current velocity were made, the calculated rate of oxygen 

production was quite comparable with that on July 20.

Data shown in table 3 for another station also support 

the view that this value does not vary widely for reefs 

of similar characteristics and dimensions.

TABLE 2.— Oxygen production by a reef north of Busch island 

[Quantities marked by an asterisk (*) are averages]

Date

June 29 ..... 

June 29 _ _. 

June 30...-- 

July 20...---

Time

0940 

1130 

1120 

1100-1300

Dissolved oxygen (ml/1)

Seaward 

edge

4.51 

4.46 

4.55 

*4.54

Lagoon 

edge

5.95 

4.90 

5.47 

*5.28

Increase

1.44 

.44 

.92 *.74

Depth 

(cm)

20 *35

Velocity 

(cm/sec)

49 *24

Rate of 

oxygen 

production 

(ml/cm/see)

0.90 

.62

The single set of data for a station on the western 

reef shows that the oxygen content increased 0.10 

milliliter per liter at noon when the water was 107 

centimeters deep at a point where the velocity was 40 

centimeters per second. The calculated oxygen production was 0.49 milliliters per second per centimeter 

of reef normal to the current. This oxygen change 

occurred over a considerably narrower reef than that at 

Busch island. In any case, the rate of production of 

oxygen appears to be of the same order of magnitude 

even on somewhat dissimilar reef areas.

DAILY COURSE OP BATE OP OXYGEN PRODUCTION 

AND CONSUMPTION

These measurements show the rate of oxygen production by the reef community at noon. What is the daily 

cycle of changes in this rate? Measurements of this 

kind are not easy to make except at low tide. Whenever the water on the reef is deep, and the velocity 

great, obtaining samples is more difficult, and the differences in oxygen content between different samples 

diminish until they approach the experimental errors.
296 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Table 3 and figure 100 show the results of a set of observations made near Bokujarito island on the eastern

£ 0.500

0.000^

\

0600 1200 1800 2400 0600 

TIME OF DAY

FIGTJBE 100.—Course of daily oxygen exchange of reef.

reef on a line of four stations across the reef throughout 

a period of over 24 hours. It is clear that qualitatively 

the cycle of events was what would be predicted. The 

rate of oxygen production was highest around noon, and 

comparable with the rate at Busch island and on the 

western reef, while at night there was a considerable 

use of oxygen by the respiration of the reef organisms. 

Some parts of the curve in figure 100 are drawn on the 

basis of reasonable assumptions such as that the rate 

of respiration is constant all night and photosynthesis

begins about sunrise. The diurnal variation in rate of 

photosynthesis, as plotted, differs from the variation 

in insolation in that the rate of photosynthesis is relatively lower in the afternoon hours. There are previous 

reports (Kostytschew et al 1926, 1930) that under 

natural conditions, rate of photosynthesis of both land 

and water plants reaches a maximum before noon and 

then declines; Yonge, Yonge and Nichols (1932, p. 226) 

observed this in reef corals. Some observations of our 

own, described below, support this.

Graphical integration of the two arms of the curve 

shows that during the daylight hours 14,000 milliliters 

of oxygen were produced per centimeter of reef. During the night, 11,000 milliliters of oxygen were consumed. During the 24 hours there was then a slight 

excess of organic matter formed equivalent to 3,000 

milliliters of oxygen per centimeter of reef. Table 2 

shows that at Busch island the rate of oxygen production at noon was probably at least as high as at Bokujarito island. The one usable set of night observations 

at Busch island gives an oxygen-consumption rate of 

only 0.10 milliliter per centimeter per second.

Assuming that

CO2 +H2O=CH2O+O2 ,

where CH2O stands for the organic matter produced by 

photosynthesis, and less accurately, for organic matter 

in general, it follows that 3,000 milliliters of oxygen 

produced is equivalent to 1,600 milligrams of carbon 

fixed as organic matter. The reef width at this point is 

300 meters, and a 1-centimeter strip equals 3 square 

meters. The fixation of carbon as organic matter 

therefore amounted to 500 milligrams per day, or 190 

grams per year, per square meter. As there is no indication that the reef rock contains any such proportion 

of organic matter as this would indicate (Ladd, Tracey, 

and Lill, 1948), we may suppose that this amount of 

material is formed on and removed from the reef 

annually. This removal may be continuous, in the

TABLE 3.— Oxygen exchange on section across reef at Bokujarito island, June 17-18, 1946

Date and time

June 17 

1020--— —————— ————— —————————— —— ——

11SO..~.................— ..............................................

1440———————— —————— ———————————————

1750— - ————————— ——————————————————————— 

2200--:— —————— ————— ————————— ——————

June 18 

0100-— — ——————————————— —— ———— ——

0400— —————— ———————— —————————— —— - 

1030—— —— — ————— —— —— —— ——————————— ————

1256——————————————————————.————— ————

Station 1 (surf zone)

Oxygen 

content 

(ml/1)

4.21 

4.25 

4.28 

4.31 

4.09

4.19 

24.26 

' 4.49 

4.42

Depth 

(cm)

18

64

18

20 

36

Velocity 

(cm/sec)

35

84

135

73

78

Station 2 Station 3

Oxygen content 

(ml/1)

4.85 

4.50 

4.34 

4.23 

4.03

4.27

4.57 

4.57

5.19 

4.77 

4.31 

4.27 

3.94

5.02 

»4.25 

4.67 

4.58

Station 4 

(inner 

edge) 

oxygen 

content 

(ml/1)

4.80 

4.31 

4.29 

3.74

4.88 

4.59

Change in 

oxygen 

content 

(ml/1

0.98 

.55 

.03 

-.02 -.35

-.17 

-.01 

.39 

.17

Rate of 

oxygen 

production 

(ml/cm/sec)

0.62

.16

-.22

.57 

.48

i Estimate.

* Position approximate.
BIOLOGIC ECONOMY OP CORAL BEEPS 297

form of dissolved or fine particulate organic matter, or 

occasional and catastrophic during storms or high 

waves which break off whole colonies or large blocks 

encrusted with living organisms. Water flowing off the 

reef is limpid, remarkably free of plankton or any other 

sediment, and has a low biochemical oxygen demand (see 

Johnstone's data in Sargent and Austin, 1948). We 

therefore incline to the belief that removal of organic 

matter from reef or lagoon is occasional. Loss to the 

outer talus slope probably does not enter our calculations because these cover only the interval from the 

inner edge of the surf zone to the lagoon.

One other quantity of interest is the gross rate of 

photosynthesis—that is, the sum of the apparent rate 

of formation and the concurrent rate of consumption 

of oxygen. We have no wayiof deciding how much of 

the measured oxygen consumption is due to animals 

and how much to plants. We will guess that less than 

half of it is due to plants, but lump it all together in 

estimating the gross production rate.

During the daylight hours, 14,000 milliliters of oxygen 

were produced per centimeter of reef. Assuming that 

the day lasted 12 hours, a respiration rate of 0.22 

milliliters per centimeter per second concurrently consumed 9,500 milliliters of oxygen per centimeter. The 

gross rate of production per day was then 23,000 

milliliters of oxygen per centimeter. Using the conversion factors above, the equivalent in carbon amounts 

to 1,500 grams per square meter per year. We have 

shown previously (Sargent and Austin, 1949) that this 

is much higher than the production rate in the adjacent 

ocean and lagoon waters. According to Riley's compilation (see Sverdrup, Johnson, and Fleming, 1942, p. 938) 

of production rates, it is higher than that of any open 

waters where measurements have been made. General 

impressions lead us to believe that some estuarine 

areas and, in temperate zones, rocky shores between 

tides and to depths of a few dekameters may be as 

productive as these reefs, but we have no exact observations.

OXYGEN EXCHANGE OF INDIVIDUAL BEEF 

ORGANISMS

Oxygen was produced by all parts of the reef from the 

surf zone to the lagoon edge. Table 4 summarizes data 

taken towards midday on lines of stations crossing the 

reef. On each line, station 1 was on the Lithotkamnion 

ridge, station 2 was well back in the zone of rich coral 

growth, station 3 was in the apparently barren back 

part of the reef, and station 4 at the lagoon edge. Although there is great irregularity in the data, it is apparent that, on the average, oxygen was produced in 

each zone. Although the individual plants of the

lagoonward reef flat are not conspicuous, their combined 

productivity is considerable.

Three sets of observations were made on the activities 

of individual organisms. Table 5 shows the maximum 

rate of oxygen production, in milliliters per gram per 

hour, of Pocillopora, Acropora, and Forties, three reef 

corals; Porolithon, the characteristic lithothamnioid of 

the surf zone; and Liagom, an alga of the reef flat.

TABLE 4.—Oxygen content of reef waters, in milliliters per liter

Date Time

Oxygen content for station —

1 2 3 4

Bokujarito island

1020

1148 

1020

1255 

1220

4.21

4.25 

4.49

4.42 

4.28

4.85

4.50 

4.57

4.57 

4.34

5.19

4.77 

4.67

4.58 

4.44

4.80 

4.88

4.59 

4.41

Busch island

June 29. 

June 30.

0940

1120

4.51

4.55

5.18

5.32

5.06

5.04

5.95

5.47

TABLE 5. —Rate of production of oxygen by corals and algae

Specimen Air-dried 

weight (g)

3.1 

2.4 

11.3 

14.3 

13.2 

16.0

Rate of oxygen production (ml/hr) 

for indicated time of day

1015-1045

0.70 

.11 

.84 

1.68 

.56 

.36

0.00

1045-1300

0.30 

.02 

.16 

.34 

.20 

.37

-0.04

1300-1400

0.48 

.09 

.28 

.56 

.31 

.66

0.02

Maximum 

production 

rate 

(ml/g/hr)

0.23 

.046 

.075 

.118 

.042 

.035

The method of measurement was extremely simple. 

Samples were selected on the reef at low tide. Water 

was collected in an enameled pail, and the water stirred 

gently but thoroughly. The samples were put in pint 

mason jars; these jars were filled by gradual immersion 

in the pail, and the caps were screwed on under water 

so that bubbles were excluded. Also samples of water 

without added organisms were bottled, and samples 

were taken for immediate analysis. Each mason jar 

was then laid on its side in flowing water on the reef 

and propped in place with pieces of reef rock when 

necessary. During the exposure the temperature in 

the j&rs did not rise significantlyubove~the surroundings 

(28-29 °C). At the end of the exposure, each mason 

jar was shaken gently and a water sample (200 ml) 

siphoned off into a glass-stoppered bottle for oxygen 

measurement. The jars were then emptied and filled 

with freshly collected water for the next measurement. 

In order to save time and to keep the samples under as
298 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

natural conditions as possible during the whole set of 

measurements, all these operations were carried out on 

the open reef. Winkler reagents were added to the 

water samples within 20 minutes after they were drawn. 

The small activity found in the water indicates that this 

delay had no serious consequences.

The organisms collected as samples were selected as 

representatives of dominant members of the communities on different parts of the reef. The Porolithon was 

collected as far out in the surf as possible. The three 

species of corals—originally collected as four—occurred 

in the zone of abundant coral growth behind the Lithothamnion ridge. The Liagora is the most conspicuous 

alga of the lagoonward reef flat, where individual bushy 

plants a few centimeters high occur with a density of 

growth sometimes as high as 20 per square meter. 

According to Taylor (1950, p. 118), members of this 

genus are not as common at other times of year.

The coral samples and the Porolithon were in all cases 

single pieces taken from the tops of actively growing 

portions of the colony, apparently in good condition, 

very clean, and with no visible overgrowth of sessile 

organisms. The Liagora was essentially a whole plant 

about 5 centimeters high. All these samples consisted 

of a small amount of living tissue and a relatively massive deposit of lime. In Liagora, the ratio of tissue to 

skeleton was undoubtedly much higher than in the 

other organisms, and this is reflected in the relatively 

large oxygen production per gram.

Table 5 shows the results of a measurement of the 

relative maximum rates of oxygen production of the 

various organisms. The three measurements were made 

late in the morning, at noon, and early in the afternoon. 

Although there is no way of checking the results, we 

are sufficiently practiced in the technique to feel confident that'the apparent differences in rate represent 

real differences in activity rather than inadequacies in 

the mechanics of measurement. For instance, the lower 

activity at noon appears to be real and is probably an 

aspect of the phenomenon, observed by Yonge and 

others (1932), and by us (fig. 100), that activity reaches 

a maximum during the morning. To what extent the 

differences in activity reflect responses to differences— 

of which we were unconscious—in our manipulation of 

the individual specimens, we cannot say. Yonge (1937) 

has pointed out that some species of corals, notably 

members of the genus Acropora, yield quantities of 

mucus under handling and that the rapid consumption 

•of this material by bacteria may give spurious values 

for the respiratory activity of the coral. In table 5, 

however, Acropora shows a suprisingly high rate of 

oxygen production. Because in general our observations of this genus were over such short intervals, and 

^because the oxygen production rates so large, corrections for bacterial respiration should be only of secondary importance.

As shown in the last column of table 5, the maximum 

rate of oxygen evolution observed per gram of sample 

does not vary widely between the three species of corals 

or between the coral species and the alga Porolithon. 

Observations on Liagora, which is relatively slightly 

calcified, give a much higher value.

The data of Yonge and others (1932, pp. 224-228) 

recalculated to milliliters of oxygen per gram per hour 

(assuming a specific gravity of 2.5 for coral) include a 

range of values from 0.031 to 0.001—that is, all are 

somewhat lower than our values, some much lower. 

However, there is a clear inverse correlation between 

activity and size in their data, which was obtained with 

pieces of coral weighing from 15 to 80 grams, indicating 

probably that the larger pieces had lower activities 

because more dead coral was included. The heaviest 

of our pieces weighed 16 grams and all were heathy, 

presumably growing, tip pieces. Kawaguti (1937), 

using small pieces of coral, found activities as high as 

those we have observed. Therefore, our values for 

maximum rate of oxygen production by corals probably 

are dependable.

Only repetition of the measurements with careful 

selection of specimens of the alga would show how 

much importance should be attached to the fact that 

Porolithon had a rate of photosynthesis no higher than 

the corals. We also recognize, as did Yonge and others, 

that the activity per gram or per cubic centimeter is 

not a satisfactory unit for comparative studies.

Discussion.—We have previously shown (Sargent 

and Austin, 1949, p. 246) that the rate of consumption 

of organic matter by the reef community as measured 

by the oxygen consumption is much larger than the 

rate of supply of organic matter by the current driven 

across the reef. Neither the amount of material 

caught by a plankton net nor the amount available for 

oxidation by bacteria in the biochemical oxygen-demand measurement is sufficient to satisfy the requirements of the reef community. We have further found 

that between the surf zone and the lagoon the reef 

community as a whole produces in a 24-hour day 

probably somewhat more organic matter than it 

consumes. The picture of the reef as a self-supporting 

community, depending on the current only for dissolved nutrients (in a broad sense), and not for particulate or dissolved organic matter, is reasonably clean 

cut.

This may be true of the Great Barrier Reef, studied 

by Yonge and his associates, and of the coral reefs in 

the Bay of Batavia, studied by Verwey (1931). The 

absence of a steady current across the reef from the 

open sea at these locations makes the solution much
BIOLOGIC ECONOMY OF CORAL REEFS 299

more difficult. In each of these cases there is an 

adjacent land mass and a relatively stagnant lagoon to 

complicate the study of the situation. The growing 

reef edge even here, however, might exist only in a 

current of open ocean water maintained by the head 

piled up on the reef by surf. In this case it could turn 

out that these reefs also are self-supporting.

Our few experiments and the more numerous and 

more carefully performed ones of Yonge and others 

(1932) show that the contribution of the zooxanthellae 

of the corals to the total productivity of the reef is far 

from negligible. There is no doubt that in many 

cases the rate of production of organic matter by the 

zooxanthellae of a coral colony is quite sufficient to 

support the whole colony. Yonge and others (1932, p. 

234) found that 17 out of 22 colonies sealed in jars 

survived 14 days, and at the end of this time had not 

exhausted or had barely exhausted the orygen in the 

jars. As long as nearly as much oxygen remained at the 

end of the experiment as was available in the beginning, 

there must have been nearly as much organic matter 

present at the end as at the beginning; that is, unless 

the original piece of coral colony was very small, or its 

respiratory activitv very low, organic matter must have 

been produced by the zooxanthellae at a considerable 

rate or in any case almost as fast as it was consumed by 

the whole colony. In 6 of their 17 surviving colonies, 

there was more oxygen in the water and hence more 

organic matter in the jar at the end of the experiment 

than at the beginning. In another of their experiments (Yonge and others, p. 222), only 1 of 29 colonies 

produced more oxygen than it consumed in 24 hours. 

However, in another experiment (Yonge and others, 

p. 218) 8 of 22 specimens had a higher rate of oxygen 

production during 9 hours in the light than during 9 

hours in the dark, and there is a reasonable possibility 

that 12-hour exposures would have given the same 

results in most cases. In other words, during a natural 

24-hour day these colonies would have produced more 

oxygen than they consumed. Yonge and others, 

(1932) point out the possibility that epiphytic algae, or 

phytoplankton in the 14-day experiment, could have 

contributed to the apparent oxygen production. It 

should also be pointed out that bacterial activity, 

especially in the experiment lasting 14 days, could have 

consumed organic matter and oxygen at a rate sufficient 

to mask production by the zooxanthellae wholly or in 

part. Verwey (1931, pp. 175-176) found that pieces 

of coral colony produced as much organic matter as 

they consumed. The observations of Kawaguti (1937, 

p. 196) on the ratio of oxygen consumption to the rate 

of oxygen production also lend color to this view. 

The sum of this evidence is that the zooxanthellae of

corals produce organic matter at a rate quite comparable with its rate of consumption by the coral colony, 

under favorable circumstances probably exceeding it.

There is additional reason to believe that the reef 

corals not only could subsist on the products of their 

zooxanthellae but do. The product of the photosynthesis of the algae appears to remain within the 

coral colony. Evidence that organic matter produced 

on the reef remains there was presented on p. 297. In 

the 14-day experiments of Yonge and others (1932), no 

mention is made of large bacterial growth which would 

have indicated that the products of photosynthesis 

were being secreted by the coral colony. The results 

of these experiments strongly indicate a high rate of 

activity and a practically closed cycle of events within 

the colony.

What is true of the reef community as a whole 

appears now to be true of individual coral colonies. 

The external supply of organic nutrient -is inadequate 

to support a coral colony on the reef, but the daily 

production of organic matter by its contained zooxanthellae can be sufficient. Although Yonge (1940, p. 

365) has reviewed his own careful work and other data 

indicating that corals do not derive organic matter 

directly from their zooxanthellae, we believe that the 

weight of evidence is on the opposite side. Under conditions of growth on the eastern reef of Rongelap Atoll, 

the association between corals and zooxanthellae seems 

to be essential to the individual colonies precisely 

because the corals must derive organic matter from the 

algae or die.

This brings up another general question: zonation 

on the reef. As described by Tracey, Ladd, and Hoffmeister (1948), the reef top is composed of zones 

parallel to the edges. Behind the Lithothamnion ridge 

is a zone of rich coral development and behind this a 

reef flat populated principally by very small organisms. 

Our observations indicate that the temperature difference between the two sides of the reef even at low tide 

is relatively insignificant. In tide pools and backwaters around the islands large temperature differences 

occur, but not across the open reef between islands. 

The oxygen differences are likewise insignificant, and 

we believe that differences in carbon dioxide content 

and associated pH would be no greater. The possible 

gradient in organic matter, suspended or dissolved, is 

limited to a very small value by the absence of organic 

matter in the source water from the open sea. The 

difference in phosphate content (table I) is so small as 

not to be certainly measurable. The sediment load 

of the current is so small as to appear negligible.

Munk and Sargent (Prof. Paper 260-C, p. 278) have 

suggested that Lithothamnion grows luxuriantly only
300 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

in the zone of violent surf because of the scrubbing 

action of the momentary high-velocity currents which 

can remove various solids and also rapidly renew the 

water lamina close to the surface of the algae. Almost 

the only factor we can imagine varying sufficiently to 

account for the limitation of distribution of coral 

colonies is also velocity of moving water. The current 

velocity along a line across the reef does not vary much 

because the depth along the line is nearly constant 

inside the Lithothamnion ridge, except close to the 

lagoon edge where it begins to deepen. However, the 

average wave height and frequency diminish with 

increasing distance from the seaward edge, and below 

some critical value for these quantities conditions for 

growth of coral may become extremely unfavorable.

The fact that the reefs are more productive per unit 

area than the surrounding waters we have previously 

ascribed (Sargent and Austin, 1949) to a higher overall 

efficiency of attached compared with planktonic algae. 

We have no further evidence in support of this speculation, but we can find no other ultimate reason for the 

nourishing of coral atolls in an empty ocean.

LITERATURE CITED

Kawaguti, S., 1937, On the physiology of reef corals; I, On the 

oxygen exchanges of reef corals: Palao Trop. Biol. Sta. 

Studies, v. 1, p. 187-198.

Kostytschew, S., and Berg, V., 1930, Untersuchungen fiber den

Tagesverlauf der Photosynthese in Transkaukasien: Planta,

Band 11, p. 144-159. 

Kostytschew, S., and Soldatenkow, S., 1926, Der tagliche

Verlauf und die specifische Intensitat der Photosynthese

bei Wasserpflanzen: Planta, Band 2, p. 1-9. 

Ladd, H. S., Tracey, J. I., Jr., and Lill, G. G., 1948, Dnllingon

Bikini Atoll, Marshall Islands: Science, v. 107, p. 51-55. 

Sargent, M. C., and Austin, T. S., 1949, Organic productivity

of an atoll: Am. Geophys. Union Trans., v. 30, p. 245-249. 

Shepard, F. P., 1948, Submarine geology, Harper & Bros.,

New York. 

Sverdrup, H. U., Johnson, M. W., and Fleming, R. H., 1946,

The oceans, Prentice-Hall, Inc., New York. 

Taylor, W. R., 1950, Plants of Bikini, University of Michigan

Press, Ann Arbor. 

Tracey, J. I., Jr., Ladd, H. S., and Hoffmeister, J. E., 1948,

Reefs of Bikini, Marshall Islands: Geol. Soc. America Bull.,

v. 59, p. 861-878. 

Verwey, J., 1931, Coral reef studies; II, The depth of coral

reefs in relation to their oxygen consumption and the

penetration of light in the water: Treubia, v. 13, p. 169-198. 

Von Arx, W. S., 1948, The circulation systems of Bikini and

Rongelap lagoons: Am. Geophys. Union Trans., v. 29,

p. 861-870. 

Yonge, C. M., 1940, The biology of reef-building corals: Sci.

Repts. Great Barrier Reef Exp., v. 1, p. 353-389. 

Yonge, C. M., Yonge, M. J., and Nichols, A. G., 1932, Studies

on the physiology of corals; VI, The relationship between

respiration in corals and the production of oxygen by their

zooxanthellae: Sci. Repts. Great Barrier Reef Exped.,

v. 1, p. 213-251.
Plankton of Northern 

Marshall Islands

By MARTIN W. JOHNSON

Bikini and Nearby Atolls, Marshall Islands

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-F

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1954

CONTENTS

Page Page 

Abstract___-_-_____-_______________________________ 301 Bikini Lagoon—Continued

Introduction_______________________________________ 301 Inflow and discharge of water from the lagoon as

Methods of collecting.______________________________ 301 indicated by the drift of planktonic life__________ 306

Bikini Lagoon: Other lagoons of the northern Marshall Islands.________ 307

Concentration of plankton fauna________________ 302 Phytoplankton_____________________________________ 309

Capacity of the lagoon to foster an endemic fauna__ 303 Literature cited_____--_---_-__--___-_--_----_---_--_ 314

Maintenance and concentration of oceanic plankton 

in the lagoon_________________________________ 305

ILLUSTRATIONS

	Page

101. Positions of plankton hauls taken at Bikini Atoll____-_____-_____-__---__-_----__-__-_____-_-----_--__ 302

102. Distribution of Undinula vulgaris, by stations____________--______-___--_-_______-__-__-___--_-------__ 303

103. Diurnal migration of Undinula vulgaris-------.--.-.--.----------------------------------------------- 304

104. Diurnal migration of ostracodes__ ___________________________________________________________________ 304

105. Diurnal migration of zooplankton__________________________________________________________________ 304

106. Distribution of Tretomphalus planus, by stations______--____-_-_______________----_-_------------_----_ 305

107. Distribution of Globerigina, by areas_________________________________-_______----_-_____-____---_-_-_ 306

108. Distribution of copepods, by areas_-_____________________--______-______-_-__---_-_-_-_-_-_----__--_- 307

109. Distribution of Appendicularia, by areas_. ____________________________________________________________ 308

110. Distribution of the pteropod, Creseis sp., by areas__ ___________________________________________________ 309

111. Distribution of arrow worms, Sagitta spp. by areas_______--________________________-___-----_-----_,._ _ _ 310

112. Distribution of crab larvae, by areas__________________l__-_____-_--___-__---__-__----------__-------- 311

113. Distribution of shrimp larvae, by areas__-__-_-____-_____----_----_-__---_----------___-----_--------- 312

114. Distribution of fish eggs, by areas______________________________•___________________________-__-__-__ 313

115. Distribution of fish larvae, by areas__________________________________________________________________ 314

TABLE

TABLE 1. Relative number per cubic meter of certain plankton organisms in four lagoons in the northern Marshall Islands__ 308

m

PLANKTON OF NORTHERN MARSHALL ISLANDS

MARTIN W. JOHNSON

ABSTRACT

Most of the holoplankton species found in Bikini Lagoon occur 

also in the surrounding open sea. The plankton fauna of the 

lagoon, however, is characterized by (1) a number of endemic 

species not found outside the lagoon except where washed out by 

outflowing currents, and (2) a concentration within the lagoon of 

many forms that occur normally also in the outside surface 

waters. Both features are dependent upon the degree of dispersal of the plankton and are therefore directly correlated with 

the semiclosed circulation which the lagoon imposes upon the 

water that flows in over the eastern and northeastern reefs.

A study of the vertical distribution and diurnal vertical migration of the endemic copepod Undinula vulgaris, and of the general 

plankton, reveals that much of the lagoon plankton lives mainly 

in the deeper, slowly (0.1 knot) eastward-moving counter current 

which as a water mass represents 70 to 90 percent of the living 

space in the lagoon. Even during the night when much of the 

plankton migrates toward the surface into the faster (0.3 knot) 

westward-moving water, about two-thirds of the plankton still 

remains within the deeper counter current.

Much of the oceanic plankton swept into the lagoon thrives 

there and becomes concentrated so that the average concentration per cubic meter of water of the 11 most common animal 

groups is about four times higher than in the outside northeastern 

area, and twice higher than the outside western-southern area 

of principal outflow. The increased concentration found in the 

outside water to the west and south indicates an outwash from 

the lagoon in that area and is shown especially by such forms as 

Undinula vulgaris and Tretomphalus planus (foraminifer), which 

occur abundantly in the lagoon but were not found in the area 

immediately to the east and north of the atoll.

The restricted water flow within the lagoon is biologically 

important in conserving also the transitory planktonic larval 

stages of lagoon and reef fishes and bottom-living animals.

A comparison of the plankton in Bikini, Eniwetok, Rongerik, 

and Rongelap Lagoons showed the last to be the richest for the 

period of the survey. Phytoplankton was exceedingly sparse 

in all but Rongelap, where a small "bloom" was in progress.

INTRODUCTION

By the term "plankton" is meant all the organisms, 

usually microscopic or semimicroscopic, that float about 

in the sea with little or no resistance to water currents. 

Thus, in general, they are dispersed and transported 

in a manner dependent upon the prevailing current 

system. The animal plankton consists not only of 

such forms as are permanently planktonic all of their

lives (holoplanktonic), but also of the transitory young 

or larval stages of bottom-living forms—such as crabs, 

certain shrimp, clams, and others—and fishes.

In an earlier report (Johnson, 1949) the plankton was 

studied with a view to its use in corroborating the 

physical oceanographic findings relative to the water 

exchange between Bikini Lagoon and the surrounding 

sea.

From a biologist's standpoint, however, the greatest 

significance of the study involves an evaluation of the 

ecological effect of the water currents in the maintenance of a consistently endemic or concentrated 

population within the lagoon and on the wave-washed 

reefs that form its banks.

Bikini Lagoon rests as a marine lake on the summit 

of a coral-capped volcanic mountain which rises 

abruptly from the floor of the open ocean at a depth 

of more than 2,000 fathoms. The lagoon has a maximum depth of about 35 fathoms (Emery, Tracey, and 

Laddj 1949). At low tide it is completely isolated from 

the sea except for a number of passages forming gaps 

6 to 35 fathoms deep in the reef along the southern and 

southwestern periphery. (See fig. 101.) Depending 

upon tidal action and wind conditions, the waters of 

this "lake" are partially renewed daily by a flow of 

water over the reefs from the sparsely populated surrounding high seas.

METHODS OF COLLECTING

The quantitative plankton samples were taken in 

two ways in each of four lagoons:

1. Horizontal tows were made with Clark-Bumpus samplers, 

which give a measure of the volume of water filtered through the 

net. The nets used with these samplers were of No. 2 bolting 

silk having apertures of about 0.366 millimeter.

Inside the lagoons, two samplers were usually towed 20 to 30 

minutes simultaneously at 1- and 10-fathom depths. Deeper 

horizontal towing within the lagoon was hazardous because of the 

many coral pinnacles rising from the bottom. For the 24-hour 

series in Bikini Lagoon, 3 samplers were towed in series at depths 

of 1, 10, and 25 fathoms over a course previously surveyed to 

assure an absence of shallower coral obstructions. Outside the

1 Scripps Institution of Oceanography, University of California.

NOTE.—Contribution from the Scripps Institution of Oceanography New Series No. 702

301
302 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

A

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NAUTICAL MILES

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1012345678

FIGUEE 101.—Positions of plankton hauls taken at Bikini Atoll March 10 to May 18,1946; the arrows indicate course of horizontal hauls; the circles indicate vertical hauls;

the figures are depths in fathoms.

lagoon the samplers were towed simultaneously at 1-, 10-, and 

40-fathom depths. The station positions and courses over which 

the Bikini samples were taken are shown in figure 101.

2. Vertical tows were made with a 10-inch net of No. 20 

bolting silk having apertures of 0.076 millimeter. These nets 

were pulled up vertically from a depth of 50 meters or, where 

depths were shallower, from the bottom to the surface. The 

water filtered in the regular hauls was about 2.5 cubic meters. 

The purpose of obtaining this series was to sample the more 

minute organisms of the phytoplankton together with the 

tintinnids, copepod larvae, and clam larvae that pass through 

the coarser nets.

The following numbers of stations were occupied for 

quantitative hauls in each of the four lagoons: Bikini, 

81; Eniwetok, 18; Rongelap, 35; Rongerik, 11. The 

No. 20 net was used at about 50 percent of the stations 

in each lagoon.

The regular quantitative hauls were checked qualitatively by many irregular hauls with various types of 

nets. A total of 437 tows are involved in the whole 

survey.

BIKINI LAGOON 

CONCENTRATION OF PLANKTON FAUNA

Insofar as Bikini Atoll is concerned, this report deals 

mainly with observations made between March 10 and 

May 18, 1946, before the atomic-bomb tests. A few 

samples (not shown in fig. 101) taken outside the lagoon 

as late as August 8, 1946, and contrasted with the spring 

samples, showed no appreciable change in concentration in the plankton.

Perhaps 90 percent or more of the holoplanktonic 

forms found in Bikini Lagoon occur also in the outside 

surrounding water. The lagoon plankton fauna is, 

however, characterized by the following two striking 

features: (1) A number of the important species are 

endemic to the lagoon and are seldom or never found 

outside except at localities where they have been 

washed out of the lagoon over the reef or through passages; (2) there is a concentration of many forms that 

also occur normally in the outside surface water. In
PLANKTON OF NORTHERN MARSHALL ISLANDS 303

general, the plankton concentration in the Marshall 

Islands area is relatively sparse, especially the phytoplankton.

CAPACITY OF THE LAGOON TO FOSTER AN ENDEMIC

FAUNA

The capacity of the lagoon to foster an endemic fauna 

and to concentrate the fauna common to the lagoon 

and the outside water results largely from the same 

mechanism. This mechanism is inherent in the current 

pattern which results mainly from the retarding effect 

that the lagoon has on the oceanic water which it intercepts from the North Equatorial Current. This type 

of dependence on water currents is doubtless characteristic of other lagoons similarly constructed.

In some areas, such as the Gulf of Maine, for example, 

the endemic population is correlated with a large circular 

flow of water which produces a semiclosed system 

(Bigelow, 1924; Redfield, 1941). In Bikini Lagoon the 

current mechanism is much smaller and operates mainly 

in a vertical rather than horizontal plane.

Surface water in the lagoon is driven by the prevailing trade winds in a west-southwest direction, whereas 

the water at intermediate and greater depths flows in 

the opposite direction as a counter current. This gives 

rise to sinking in the western portion of the lagoon and 

to up welling against the eastern reefs (Von Arx, 1948).

In the search for endemic species in Bikini Lagoon, 

the most useful thus far considered are Undinula, 

vulgaris (Dana), a copepod crustacean, and Tretomphalus planus Cushman, a pelagic foraminifer. This 

foraminifer was identified by the late J. A. Cushman, 

who described the species originally from Samoa and 

stated that it occurs also in Fiji. Both species occur 

also in the lagoons of Eniwetok, Rongelap, and 

Rongerik.

Undinula vulgaris.—This is a widely distributed 

species which, though reported from the open ocean, 

appears to be preponderantly a lagoon form in this 

area. Large (more than 2 millimeters) and easily 

recognized, it occurs in varying numbers throughout 

most of the lagoon. During the period March 10 to

£NYU

I 0 I 2345678

FIOUEE 102.—Number of Undinula vulgaris per cubic meter of water, by stations (black dots).
304 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

May 18 it was present at about 90 percent of the lagoon 

stations. The largest numbers (as many as 255 per 

cubic meter of water) have been taken near the center 

of the lagoon. (See fig. 102.) In the daytime the 

animals seek the deeper water strata, 10 fathoms or 

more deep. Very few specimens were caught in the 

net towed at or above the 1-fathom depth except at 

night. It is obvious that to maintain an endemic 

population of an animal with a relatively long life 

history like that of Undinula vulgaris, and in such 

numerical concentrations, there must be a pattern of 

currents which leads to a relatively closed system with 

very slow flushing of at least a large part of the lagoon. 

This would require the presence of a sizable counter 

current or some other effective retarding system. The 

hydrographic study has indicated that a deep slowly 

moving countercurrent exists. Thus the biological and 

hydrographic observations support each other on this 

point.

During the time the animals are in the upper surface 

layer near the outwash of the lagoon, they are in danger 

of being swept out through passages or over the reef. 

Therefore, reproduction in the deeper layer would have 

to be at a rate sufficient to replenish the loss of numbers 

washed away by the surface layer, or where there is 

slow reproduction the bulk of the population would 

have to spend the greater part of the time in the deep, 

slowly moving countercurrent. That the latter occurs 

in Bikini Lagoon is indicated by a study of the diurnal 

vertical migratory journeys of Undinula, vulgaris and 

a number of other planktonic forms. (See figs. 103, 

104, and 105.) It will be seen from the illustrations 

that not only Undinula but the bulk of the other

SUNRISE 

4 3

UNDINULA VULGARIS

FIGURE 103.—Diurnal migration of Undinula sulgaris; 35-minute horizontal hauls 

taken at surface and at 10 and 25 fathoms at 4-hour intervals over course shown by 

double dashed arrow in figure 101.

planktonic forms avoid the more lighted surface waters 

during the day and that a goodly portion (two-thirds 

or more) of the population remains even at night in 

the slowly moving (average 0.1 knot) countercurrent. 

This current involves 70 to 90 percent of the water 

mass of the lagoon—that is, 70 to 90 percent of the 

living space. Nevertheless, at night much of the population must be transported towards the western and

SUNSET . SUNRISE 

MAY2 24 MAY3 4 1 8

SCALE 1""1 *! INOIVIOUAL/M3

OSTRACODES 

FIGURE 104.—Diurnal migration of ostracodes.

SUNSET , SUNRISE

MAY 2 ' MAY 3 I 24

SCALE i-i=0.l C.C./M 3

SETTLING "VOLUME ZOOPLANKTON

FIGURE 105.—Diurnal migration of total zooplankton as represented by settling

volume.

southern margins of the lagoon by the relatively fastmoving (average 0.3 knot) surface current. Depending upon wind velocity, the depth of this current varies 

from 2.5 to 10 fathoms, with an average of 6.5 fathoms 

(the maximum depth of the lagoon is about 35 fathoms). 

At the approach of dawn a downward movement of the 

animals occurs, and a slow journey eastward begins. 

In this manner about one-third of the distance lost 

during the night is recovered during the day. It is 

probable that this is the order of events with many 

larval forms of bottom-living adults in the lagoon and 

also with the oceanic forms swept into the lagoon over 

the eastern reefs. Biologically this is very important 

in conserving not only the holoplankton but also the 

transitory larvae that eventually settle to the bobtom 

of the lagoon or on the reefs to restock the bottomliving animals.

Tretomphalus planus,—This species was found in all 

parts of the lagoon, sometimes in numbers up to 175 

individuals or more per cubic meter of water. The 

average was, however, only about 25 per cubic meter. 

This species was present throughout the whole period 

of investigation (Mar. 10 to Aug. 8) and occurred at all 

depths within the lagoon. The greatest concentrations 

were in the upper 10 fathoms except in a few instances 

where numbers were greater in the deeper strata.

The plankton samples collected outside the lagoon 

yielded this species only at stations along the western 

and southern periphery of the atoll. The greatest 

outside concentrations were found near the passages, 

thus suggesting the main route of outwash for the 

species as indicated in figure 106. Apparently small
PLANKTON OF NOETHEEN MAESHALL ISLANDS 305

BIKINI I

NAUTICAL MILES

•0,3

1012345678

FIGURE 106.—Tretomphalus planus, number per cubic meter of surface water by stations (black dots).

numbers are washed out also over the long (shallow reef 

extending from Bokororyuru Pass to Namu island. 

This is in keeping with the animal's habit of floating 

near the surface. The deeper-living lagoon animals, 

such as Undinula vulgaris, are discharged through the 

passages (depths varying from 6 to 35 fathoms) rather 

than over the reef.

From the above observations it appears that Tretomphalus is endemic to the lagoon, perhaps requiring 

shallow water during the early stages of its life history 

before the float chamber has been developed. This 

would explain its absence or scarcity in deep water 

outside the lagoon except where washed out on the 

down-current side by the prevailing flow through the 

lagoon. It should be pointed out here that this relationship suggests the use of Tretomphalus as an index 

of past oceanographic conditions of the atoll. Provided 

its tests are well preserved as fossils in the sediments, it 

should serve to show the general direction of currents 

that have swept through the lagoon in past history, for 

though the animal is kept suspended by the float

chamber while alive, there must be a gradual precipitation of defunct tests to the bottom along the course of 

the outflowing water.

MAINTENANCE AND CONCENTRATION OF OCEANIC 

PLANKTON IN THE LAGOON

The salinity and temperature of the water within 

the lagoon differ but little from that outside. The 

highest lagoon salinity (34.85 parts per thousand) is 

only about 0.1 part per thousand higher than in the 

open ocean,- and the temperature change from about 

27.02 C to 27.10 C is negligible. The vertical gradients 

of these factors are also insignificant. In these respects 

the lagoon water is essentially oceanic and would therefore not be seriously inimical to most of the surface 

oceanic plankton coming in over the reef.

The concentration of oceanic plankton that has 

drifted into the lagoon results from the semiclosed 

circulatory pattern in the lagoon. Presumably this 

circulation would operate in a mechanical manner 

similar to its effect on the maintenance of the endemic
306 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

fauna. In the following section it will be shown that 

certain forms that are common to the inside and outside may be as much as 27 times more numerous per 

unit of water inside the lagoon. This increased concentration doubtless results in part from better food 

conditions inside as a result of local production, but 

the reduction of dispersal must be very important.

The presence in the lagoon of many egg-bearing 

specimens and a large proportion of larval and juvenile 

stages of species common to the outside is evidence of 

successful reproduction inside. In the absence of wide 

and uninterrupted dispersal, such as occurs outside, a 

concentration from both reproduction inside and 

recruitment from outside should occur.

In summarizing the average concentration by areas 

of the 11 most common animal groups, it appears that 

the ratios of concentration per cubic meter for (a) the 

outside eastern-northern area, (6) the inside lagoon 

area, and (c) the outside western-southern area of 

principal outflow, are roughly a:b:c: = l:4:2.

INFLOW AND DISCHARGE OF WATER FROM THE 

LAGOON AS INDICATED BY THE DRIFT OF PLANKTONIC LIFE

When we compare the number of specific organisms 

occurring per cubic meter of water at stations occupied 

inside the lagoon with the number found in the surrounding water just outside the reefs and passages, it is 

seen that the concentration is almost invariably smallest 

in the waters outside the eastern and northern reefs. 

It is nearly always largest by far at the stations within 

the lagoon, and intermediate at stations just outside 

the western and southern passages and reefs. The 

largest outside catches occur near Rukoji Pass and the 

western portion of Enyu Channel (figs. 102, and 106 

to 115). Figures 102 and 106 are especially useful 

since they deal with two endemic species and show 

clearly the route by which the species are washed out 

of the lagoon. According to hydrographic studies, the 

outflow is through these passages and over these reefs, 

and this is clearly corroborated by the biological eviNAUTICAL MILES

1012345678

-————————————I___________________'

FIOUEE 107.—Olobigerina, average number per cubic meter of water by areas; black dots indicate stations averaged in each area.
PLANKTON OF NORTHERN MARSHALL ISLANDS 307

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FIGURE 108.—Copepods, average number per cubic meter of water by areas; black dots indicate stations averaged in each area.

dence. The biological evidence does, however, indicate a somewhat broader outwash from Enyu Channel 

in the early part of the survey than the hydrographic 

charts indicate.

From the hydrographic survey we learn that in the 

open sea off Bikini Atoll there is a small northward 

gradient in temperature and salinity so that the water 

entering the lagoon from the north is about 0.1 degree 

(C) colder and 0.3 part per thousand more saline than 

that which enters the southern passes.

The plankton has not yet been studied in sufficient 

detail to know the extent of its usefulness in indicating 

other than the general routes of entry into the lagoon. 

The distribution of one copepod species, Copilia 

mirabUis, does, however, suggest that its center of 

distribution may be in the southern water with which 

it enters the lagoon through Enyu Channel and over 

the reef between Bikini and Enyu islands. Forty-four

percent of the stations in the southern half of the lagoon 

yielded this species; only 11 percent in the northern 

half. Outside the lagoon from near Bikini island 

around the southern periphery of the atoll to near 

Arriikan Pass on the west side, 75 percent of the stations yielded the species. None was taken at outside 

stations north of these points. Nowhere was the 

species found in abundance.

OTHER LAGOONS OF THE NORTHERN MARSHALL 

ISLANDS

Plankton surveys were made in the lagoons of 

Eniwetok, Rongelap, and Eongerik Atolls for comparison with Bikini Lagoon. Table 1 gives the result in 

type and average concentration of the more important 

organisms for the periods indicated.

The first part of table 1 is based on collections with 

the Clark-Bumpus samplers with No. 2 mesh net, the
308 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

NAUTICAL MILES

1012345678

FIGURE 109.—Appendicularia, average number per cubic meter of water by areas; black dots indicate stations averaged in each area.

TABLE 1.—Relative number per cubic meter of certain planktonic 

organisms in four lagoons of atolls in the northern Marshall 

Islands

Where density could not be determined, mere presence of an organism is indicated 

by a plus sign (+)]

Bikini 

(March 26— May 18)

Inside 

lagoon

Outside 

lagoon

Eniwetok 

(June 2-14) 

Inside 

lagoon

Rongelap 

(June 18-22) 

Inside 

lagoon

Rongerik 

(June 23-24) 

Inside 

lagoon

Collected with Clark-Bumpus samplers—No. 2 mesh net

Tretomphalus planus. . ..

0#Aowaspp-_- _____

Echinopluteus. ........

Fish larvae _______

5.01

25.15 

5.05

15.89

3.31

18.51

19.59

3.31

4.41

ffi

11.30

7.47

25.25

1.45

.61

5.56

1.3

i 51.2

.32

1.08

(+)

+)

.95

1.83

7.6

.66

.16

0.41

2.30 

8.46

.68

.38

4.11

1.88

5.92

5.06

.75

2.30

.36

5.67

1.76

.09

1.15

10.88

6.64 

3.93

16.44

6.17

(+)

22.66

5.05

13.84

139.1

3.50

.34

5.76

26.18

2.27

3.64

0.62

3.07 

4.60

3.04

2.31

1.94

3.95

2.44

4.78

(+)

H)

.02

4.03

47.62

.73

1.43

Collected with No. 20 mesh net in hauls from 50 meters to surface

Dinoflagellates -.. _ ....

95

551

334

435

1,137

76

298

6

85

273

271

555

905

595

326

1,065

1,091

534,345

1,572

Sf>0

483

774

734

631

second part on 50-0 meter hauls with the No. 20 mesh 

net. The amount of water filtered by the No. 20 net is 

subject to some error because of the drift of the ship, 

but this was not usually too serious since the hauls were 

so shallow. The numbers given for the No. 2 net 

hauls are doubtless minimal for some groups with small 

members because of the selective size of the mesh. This 

is especially true for the small copepods Acartia, Corycaeus, Oithona and for some less common cyclopoids.

On the basis of these surveys some important differences come to light relative to the standing crop within 

the lagoons. It is seen that Rongelap and Bikini 

Lagoons appear to have a somewhat richer planktonic 

fauna than do the other lagoons, especially Eniwetok. 

This was noticeable especially for the copepods as a 

group. For these the numbers per cubic meter of water 

with the No. 2 net were: Bikini, 62.37; Rongelap, 

57.17; Rongerik, 18.08; and Eniwetok 15.74.

Pending further taxonomic analysis of the collections 

it will be useful to list the following holoplanktonic 

species as being among the more common for the 

lagoons:
PLANKTON OF NORTHERN MARSHALL ISLANDS 309

Copepoda:

Acartia hamata Mori.

Calanus pauper (average 4.62 per cubic meter in Bikini

Lagoon).

Calanopia minor (very common in Rongelap Lagoon). 

Calocalanus pavo. 

Candacia spp. 

Centropages elongalus.

sp.

Clausocalanus pungens. 

Copilia mirabilis. 

Corycaeus spp. 

Eucalanus attenuates. 

Labidocera sp. 

Lubbockia squillimana. 

Oithona plumifera. 

Oncea venusta. 

Sapphirina stellata. 

Scolecithrix danae.

sp. 

Tortanus murrayi.

gracilis. 

Undinula darwini (found outside only).

vidgaris.

The list of Copepoda is far from complete for the 

whole area and for the rarer species.

Chaetognaths:

Sagitta enflata. 

robusta. 

serratodentata.

The siphonophores occurring in the samples have 

been discussed by Sears (1950).

One conspicuous difference in the zooplankton of the 

lagoons was the relatively large number of different 

larval forms, including fish, in Rongelap Lagoon. 

Specific identification could not be made of the larvae; 

most of the shrimp larvae were snapping shrimp.

PHYTOPLANKTON

Another notable difference pertains to the phytoplankton caught with the No. 20 net. The standing 

crop of phytoplankton was exceedingly small in most of 

the lagoons, and this was verified by the blue color and 

great transparency of the water. In Rongelap Lagoon,

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310 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

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FIGURE 111.—Arrow worms, Sopitta spp., average number per cubic meter of water by areas; black dots indicate stations averaged in each area.
PLANKTON OF NORTHERN MARSHALL ISLANDS 311

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FIQUBK 112.—Crab larvae, average number per cubic meter of water by areas; black dots indicate stations averaged in each area.
312 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

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FIGTJEE 113,—Shrimp larvae, average number per cubic meter of water by areas; black dots indicate stations averaged in each area.
PLANKTON OF NORTHERN MARSHALL ISLANDS 313

1012345678

FIGTJEE 114.—Fish eggs, average number per cubic meter of water by areas; black dots indicate stations averaged in each area.

however, there was somewhat of a phytoplankton 

"bloom" in process during June 17-22. This gave a 

perceptible discoloration to the water in some parts of 

the lagoon. A second visit to the lagoon July 22 to 23 

showed the phytoplankton to have diminished considerably, but it was still more abundant than observed in 

any of the other lagoons during the course of the survey. 

The report on bottom sediments does not indicate 

that siliceous material was encountered. While a considerable quantity of diatom frustules may be precipitated to the bottom during the blooms, the diatoms 

found in the plankton were for the most part weakly 

silicified and therefore might add but little material to 

the sediments. The dominant diatom was Rhizosolenia, sp., which constituted nearly 100 percent of the 

diatoms over most of the lagoon. The small northeastern bulge of the lagoon showed a similar bloom of 

diatoms, but these were strangely nearly all a species of 

Chaetoceros. Diatoms such as Coscinodiscus with 

heavier type of frustules were exceedingly rare in the 

plankton of all lagoons.

Dinoflagellates formed a substantial part of the 

phytoplankton and were the dominant photosynthetic 

forms observed in net collections in Bikini Lagoon.

Judging from the small standing crop of phytoplankton in all of the lagoons, with the possible exception of Rongelap, one is tempted to conclude that the 

sampling method used failed to reveal a portion of the 

photosynthetic forms or that production and utilization 

proceed at nearly the same rate. That the water must 

produce a fair crop of plants seems evident in view of 

the rather considerable concentration of zooplankton, 

including a substantial number of forms such as tintinnids and appendicularia that require very finely divided 

particulate material either as minute living cells or as 

detrital particles that readily pass through the meshes 

of the net.

Among the corals the problem of obtaining food is 

thought to be met by the presence of zooxanthellae that 

synthesize utilizable organic substance within the coral 

tissue. (See Prof. Paper 260-E.) However, this is 

controversial. Yonge (1951) is of the opinion that the

CONTENTS

Page 

Abstract- _-_-__________-_--_--_-___-______--____-__ 315

Occurrence and associations.--.-.-_-___-__-_-_-____-_ 315

Systematic descriptions.------------.---------------- 315

Genus Crania, __.___-____--__-___-_____-____-_- 315

Genus Thecidellina- __-__-_-___-___---___-_______ 316

Systematic descriptions—Continued 

Genus Thecidellella-.-- _--_-_-_. 

Genus Argyroiheca-------------

Selected references._-__-_--_--_----

Page

318

318

316

ILLUSTRATIONS

[Plates 80 and 81 are inserted after list of references]

PLATE 80. Crania sp. and Thecidellina congregata. 

81. Thecidellina maxilla and Argyrotheca sp.

in

RECENT BRACHIOPODS

By G. A. COOPER *

ABSTRACT

Three genera and four species of brachiopods were found at 

Bikini Atoll. Some occur in the lagoon, others on the outer 

slopes on reef slabs at depths of 300 feet. The genera Argyrotheca 

and Crania have one species each of minute forms, possibly immature individuals. The genus Thecidellina is represented by 

2 species, one new and one already described.

The extensive collecting at Bikini Atoll prior to the 

bomb-testing activities there produced four species of 

brachiopods. The specimens are all small and not 

easy to see. They represent three genera, none of which 

has so far been found commonly in Pacific waters. At 

Bikini Atoll, Crania appears as tiny yellow to orange 

shells that probably represent young rather than mature 

individuals. Argyrotheca occurs as small individuals 

one or two millimeters in width with characters that 

clearly indicate young shells. The two remaining 

species, which belong to the genus Thecidellina (Thomson, 1927), are fairly common. Although the first two 

genera may represent new species, respectively, these 

are not described as new because of the youthful character of the shells. Of the species of Thecidellina one 

is new, the other is a species previously described from 

Funafuti Atoll.

OCCURRENCE AND ASSOCIATIONS

The association of these species and their habitats 

are interesting. Three of the species occur together: 

Crania sp., Argyrotheca sp., and Thecidellina congregata, 

n. sp. This association appears on a large corralline 

slab about 8 by 10 niches in horizontal dimensions and 

about 1 inch thick. Although the life position of the 

slab could not be determined at the time the piece was 

collected, because it was dredged from 300 feet, the 

brachiopods are clustered on one side of the slab only. 

The opposite side to the one bearing the brachiopods 

and numerous other attached forms is rough and irregular and contains only an occassional young pelecypod 

(Spondylus) but is encrusted by nullipores. The side 

having abundance of life is smooth compared to the 

reverse side. This evidence combined with the nullipores suggests that the smooth surface is the lower side

of the slab and that the brachiopods were clustered on 

the side where the water was quiet. The most abundant specimens on this slab are Thecidellina, congregata. 

This species also appears at the northwest end of 

Bikini Atoll in 240 fathoms (1,440 feet) of water. The 

same species was taken at 180 to 200 feet in the lagoon 

(locality S-46-44).

Thecidellina maxilla occurs on the outer slopes of 

Bikini Atoll and in the lagoon as well. It does not, 

however, have as great a depth range as T. congregata. 

At the northwest end of Bikini Atoll T. maxilla ranges 

from 154 feet to 450 feet. In the lagoon one specimen 

was taken between 180 and 200 feet at the same 

locality as mentioned above for T. congregata. This 

particular specimen is loose, consequently its attachment surface and associates are not known.

Specimens of Thecidellina maxilla from K. O. Emory 

(Coll. 421) consists mostly of dead shells attached to 

the coral Lobophyllia. The individual shells are large 

and are somewhat encrusted by limy material. The 

entire mass may not have been found in place.

SYSTEMATIC DESCRIPTIONS

Genus CRANIA Retzius 1781

Crania sp. 

Plate 80, Figures 1-5

Shell small, pale salmon-colored; pedicle valve thin, 

attached by its entire surface and conforming to the 

contours of its place of attachment; brachial valve a 

low, eccentric cone, the apex directed toward the 

posterior, which is the shorter side; apex located onethird the total shell length anterior to the posterior 

margin; posterior slope moderately steep, less steep 

than the long and gentle anterior slope. Anterior 

profile broadly convex with the valve narrowly pointed 

at the middle. Measurements of the largest specimen: 

length 2.8 mm, width 3.0 mm, height 0.8 mm.

Figured specimens: USNM 549272, 549273, 549274.

Locality and depth: On underside of coralline expansion, on the south side of Bikini Atoll at 300 feet.

• U. 8. National Museum. This report is published by permission of the Secretary of the Smithsonian Institution.

315
316 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Genus THECIDELLINA Thomson 1915

Thecidellina congregata Cooper, n. sp.

Plate 80, figures 6-15

Small, but of about medium size for the genus; white 

to blue gray; attached by nearly the entire surface of 

the pedicle valve; shape variable as usual with attached 

forms, most specimens wider than long and polygonal 

in outline; deepest just anterior to the beak of the 

brachial valve. Hinge narrow. Brachial valve marked 

by obscure concentric growth lines and wrinkles only.

Pedicle valve with only the apical region visible; 

interarea small, sharply triangular; palintrope plane, 

descending at an angle from the commissure ranging 

from 90 to about 30 degrees, but "steep in most specimens. Interior of pedicle valve with large teeth 

buttressed by strong dental ridges; hemispondylium 

attached to the floor of the valve in the delthyrial 

chamber and consisting of two short nearly vertical 

plates bearing sharply pointed processes; adductor 

scars located in deep, elongate pits on each side of the 

valve just anterior to the delthyrial cavity.

Brachial valve provided with a short, broad triangular plane apical interarea. Gently convex in lateral and 

anterior profiles. Interior with thick narrow and 

strongly elevated quadrangular cardinal process; sockets 

moderately deep; bridge thin; septum strongly elevated; 

valve margin granulated; a deep, narrow trough inside 

the granulated margin running around the anterior 

side of the bridge and terminating on the flanks of the 

median septum at about the middle of the valve; narrow 

trough occupied by lophophore; posterior occupied by a 

chamber located just anterior to bridge and at posterior 

end of median septum; entrance to chamber located on 

anterior side of bridge at its middle and at posterior end 

of median septum, the "reversed spondylium" of 

Hayasaka; posterior side of bridge and just anterior to 

cardinal process marked by deep pit divided by a 

small septum. Area between lophophore groove and 

median septum occupied by a curving row of protruberances rising in height from mere nodes to erect processes 

which may have served as straining devices.

USNM No.

549275a.-- — ... ... .

C4QO7IV

K/1Q977

Measurements, in millimeters

Length

3.3

4.9 

4.7

Brachial 

length

3.1 

3.9 

4.0

Maximum 

shell width

3.4

4.5 

4.2

Hinge 

width

1.4 

1.9 

2.2

Thickness

1.2 

1.1

Deltidial plate: The abundance of material of this 

species makes it possible to add some information on 

the generic characters of Thecidellina. The cover of 

the delthyrium is a flat plate resembling that of the

Triplesiidae and suggestive of the terebratuloid symphytium. It is not known how this plate originates, 

but it is clear that it must have been deposited by 

the mantle because its growth lines are transverse to 

the sides of the delthyrium as in the terebratuloid 

symphytium. Of all the specimens studied, not one 

shows the deltidial plate to be punctate. Most of the 

specimens of T. congregata reveal the punctae as small 

white dots or in some instances as black dots. Neither 

of these types shows on the deltidial plate. It is 

therefore concluded that the deltidial plate is impunctate, although much of the remainder of the shell is 

strongly and coarsely punctate.

Hemispondylium: This structure, recently named by 

Elliott (1948), appears as two small pointed processes 

that extend obliquely anteriorly and are located in the 

delthyrial chamber. These are sharply pointed at their 

free end, but expand toward the floor of the valve and 

are attached in the apex. The floor between these 

plates is somewhat thickened and the plates extend 

posteriorly to the apex at their base so that they bound 

a small shallow chamber at the rear. This is quite 

unlike the hemispondylium of T. maxilla, which forms 

a free tube under the deltidial plates. The hemispondylium is the seat of attachment of didcutor muscles 

and inner adductor muscles whose other extremities are 

attached to the cardinal process or quadrilateral plate 

and the pit behind the bridge.

Teeth: The teeth are stout and are located at the 

base of the deltidial opening. They are buttressed by 

a ridge that runs along the delthyrial edge. This ridge 

is the growth track of the tooth.

Brachial valve: This part of Thecidellina is quite 

complicated, although the lophophore is fairly simple 

in form. The cardinal process or "quadrilateral plate" 

is fairly complex. Its external expression when the 

brachial valve is viewed from the exterior is that of a 

broad sloping, nearly smooth plate fitting under the 

deltidial plate. Seen from the inner side, the plate is 

squarish posteriorly but is buttressed by two lateral 

thickenings or ridges which bound an inner recess and 

lateral recesses. The latter serve as the deep and 

large sockets which receive the teeth. The lateral buttresses flare to each side at the base of the adductor 

attachments and bound the cavity posterior to the 

bridge.

The cavity just anterior to the base of the cardinal 

process is divided into halves by a ridge and a small 

septum arising from the top of the arch or vault covering 

the body cavity. Adductor muscle ends are attached 

to the upper part of the vault on either side of the 

septum.

The lophophore is in the schizolophus stage and is 

attached in the groove just inside the granulated mar-
RECENT BRACHIOPODS 317

gin, forming a big loop from the bridge and running 

not only down each side to curve narrowly in a posterior 

direction but also along each side of the median septum. 

The filaments of the lophophore are long and are separated in pairs or singly by thick or thin horizontal 

partitions that become progressively smaller toward the 

posterior.

The opening into the body cavity is located just 

posterior to the posterior end of the median septum and 

just anterior to the bridge, which separates it from the 

somewhat larger opening on the opposite side of the 

bridge.

The musculature is not easy to determine in these 

small shells except for the large adductor muscles that 

are attached at the outside base of the cardinal process. 

The scars of these muscles can be seen in the pedicle 

valve on each side of the hemispondylium on the underside of the palintrope. Two sets of muscles appear to be 

attached to the hemispondylium. One of these sets 

attaches to the posterior edge of the cardinal process and 

the other is attached inside the pit just anterior to the 

cardinal process and just behind the bridge. This is 

like the musculature described for Lacazella. (Lacaze 

duThiers, 1861.)

Types: Holotye, USNM 549277; figured paratypes, 

USNM 549275a-d, 549276, 549278.

Locality and depth: On underside of coralline expansion, on the south side of Bikini Atoll at 300 feet.

Discussion: Thecidellina congregata is especially distinguished by its moderate size, irregular spreading outline; expansively adherent mode of attachment, and 

short, small interarea. The interior of the brachial 

valve has a large and conspicuous cardinal process extending far posterior to the bridge while the pedicle 

valve has the hemispondylium resting on the floor of the 

valve at the apex. The external form and interior distinguish this species readily from T. maxilla. The 

form and outline of the pedicle valve distinguish it 

from either T. blochmanni or T. barretti which it resembles in size.

Thecidellina maxilla Hedley 

Plate 81, figures 1-10

Thecidea maxilla Hedley. Mollusca of Funafuti, pt. 2, Pelecypoda and Brachiopoda, Mem. Australian Mus., v. 3, pt. 8, 

p. 508-510, fig. 57, 1899.

Shell large for the genus, longer than wide and with 

an elongate oval outline, the beak forming the narrow 

end. Shell attached by the beak of the pedicle valve 

and usually in an upright position. Hinge slightly less 

than the greatest shell width, which is about at the 

middle. Both valves marked by concentric growth 

wrinkles.

Pedicle valve with interarea long, equaling more than

one-third of valve length; interaera smooth and elongate triangular. Lateral profile gently convex; anterior profile narrowly rounded; umbo more or less irregular and occupied by the scar of attachment. Median region swollen; sides narrowly rounded and steep. 

Teeth large and having strong, thick dental ridges; 

hemispondylium (Elliott, 1948) a long rounded tubular 

plate with two stout processes at the distal end; hemispondylium apparently free above delthyrial cavity and 

not attached to floor of valve.

Brachial valve with short and deeply concave cardinal process; socket ridges not well developed; granulated 

margin narrow, elevated toward the posterior; lophophore groove narrow and deep; bridge elevated and 

broad, almost hiding the cardinal process when viewed 

from the pedicle valve; median septum high and thin, 

with the lophophore groove located on its sides about 

midway in the direction of the brachial valve. Space 

between lophophore grooves occupied by elaborate 

nodes and irregular processes. Posterior chamber 

large, with smooth walls; opening into chamber at 

posterior end of septum small, openings just anterior to 

cardinal process large.

USNM No.

549286a. — .—— ——

549286b ———— — —

549285..... — - — -

Measurements, in millimeters

Length

7.2 

8.8 

8.2

Brachial 

length

4.7 

5.5 

5.4

Midshell 

width

4.8 

5.1 

5.6

Hinge 

width

3.6 

3.9 

3.4

Thickness

3.3

4.2 

4.0

Length of 

interarea

2.5 

3.3

2.8

Types: -figured specimens, USNM 549284a, 549285, 

549287, 584676.

Locality and depth: USNM 549284a and 549285, 420 

to 450 feet at Bikini Atoll, lat. 10°39'10" N., long. 

165°12'59" E.; USNM 549287, at 154 feet, Bikini 

Atoll, lat. 11°38'15" N., long. 165°18'05" E.; USNM 

584676, at 180-200 feet in the lagoon, Bikini Atoll. 

At Funafuti Atoll in the Ellice Islands, the species was 

found in considerable numbers on dead coral at depths 

of 240 to 480 feet on the lee slope of the atoll (Hedley, 

1899, p. 509).

Discussion: The specimens here referred to T. maxilla 

are the largest specimens of this genus yet found. They 

are identified as T. maxilla because of their elongate 

outline, erect habit of growth, long median septum and 

small cardinal process. These features combined with 

the unusually long interarea make the species easy to 

recognize. Thecidellina maxilla is entirely different 

from T. congregata, which never attains the size of T. 

maxilla, is usually attached by most of the surface of 

the pedicle valve, and has a more variable shape. 

Thecidellina maxilla is quite unlike T. barretti, an Atlantic species occurring in the waters about the West
318 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS

Indies. Thecidellina blochmanni from Christman Island is another small species quite unlike T. maxilla.

Genus THECIDELLELLA Hayasaka

Hayasaka (1938) described the genus Thecidellella as 

being very similar to Thecidellina but differing from that 

genus in having a "reversed spondylium." This latter 

structure is located at the posterior end of the median 

septum. In it is the opening just anterior to the bridge 

and just behind the septum. The "reversed spoadylium" is present in both of the Bikini species of Thecidellina and also in T. barretti, contrary to Hayasaka's. 

statement. Hayasaka reproduces Davidson's figure of 

T. barretti which seems to be inaccurate. All species of 

Thecidellina in the U. S. National Museum are internally 

like Hayasaka's species. It is therefore concluded that 

Thecidellella is a synonym of Thecidellina.

ARGYROTHECA

Argyrotheca sp.

Plate 81, figures 11-22

Shell minute, slighly wider than long with broadly 

rounded sides but indented anterior margin; hinge narrower than the midshell width; cardinal extremities 

rounded; subequally biconvex but with the pedicle 

valve having a somewhat greater depth than the brachial valve; most specimens smooth but some with about 

three indistinct radial costae on each side of the medianline. Coarsely punctate.

Pedicle valve gently convex in lateral profile and with 

maximum convexity anterior to umbo; anterior profile 

broadly but moderately convex, somewhat narrowly 

convex in the median region; umbonal region, with steep 

slopes; posteromedian region moderately swollen; anterior region somewhat flattened and forming a long, 

gentle anterior slope, sides with moderately steep slopes. 

Interior with ponderous teeth; apex with a moderate 

callosity: median ridge low, long, extending nearly to 

the front margin.

Brachial valve gently convex in lateral profile and 

with the maximum convexity just anterior to the umbonal region; anterior profile broadly and gently convex; beak small; umbo moderately swollen; median sulcus originating about one-third the valve length anterior 

to the beak; sulcus moderately deep, widening rapidly 

anteriorly to form a moderately broad indentation at 

the anterior margin; flanks bounding sulcus somewhat 

narrowly rounded to form board plicae with steep 

posterolateral slopes but fairly gentle lateral and anteroUSNM No.

549279a

549279b.._. ..........

Measurements, in millimeters

Length

1.5 

1.7

Brachial 

length

1.3 

1.4

Midshell 

width

1.6 

1.9

Hinge 

width

0.8 

1.2

Thickness

0.6 

2

lateral slopes. Interior with delicate cardinalia; socket 

plates erect and with narrowly rounded posterolateral 

boss that articulates with the teeth of the opposite 

valve; sockets deep, enclosed; loop short, widely divergent, attached at the base of the socket plates and extending laterally to attach to the floor at the posterolateral edge of the visceral region. Inner margin 

marked by seven prominent nodes on each side of the 

median line. Median septum arising about one-third 

the valve length anterior to the beak, and rising rapdily 

from the valve floor to extend nearly to the front 

margin; median septum highest near the valve middle 

and serrate on its anterior margin.

Figured specimens: USNM 549279a, b, 549288, 

549289.

Locality: 300 feet on the south side of Bikini Atoll.

Discussion: The small size, translucent shell, and 

coarse punctae of this species suggest the young of a 

larger form. However no fully grown specimens were 

seen. Blochmann described A. australis from South 

Australia and A. mayi from Tasmania, but both of 

these species are much larger forms with a definite 

costation. The Bikini specimens cannot be satisfactorily identified with either of Blochmann's species.

SELECTED REFERENCES

Blochmann, F., 1910, New brachiopods from South Australia, in 

Verco, J. C., The brachiopods of South Australia: Royal 

Soc. South Australia Trans., v. 34, p. 88-89. pis. 27-28.

———— 1914, Some Australian brachiopods: Royal Soc. Tasmania 

Papers and Proc. 1913, p. 112-115, pis. 10-12.

Elliott, G. F., 1948, Palingenesis in Thecidea (Brachiopoda): 

Annals and Mag. Nat. History, v. 12, no. 1, p. 1-30, pis. 1, 2.

Hayasaka, I., 1938, A new neotreme genus of Brachiopoda from 

Japan: The Venus, v. 8, p. 9-13, figs. 1, 2.

Hedley, Charles, 1899, Mollusca of Funafuti, pt. 2, Pelecypoda 

and Brachiopoda, Australian Mus. Mem., v. 3, pt. 8, p. 508- 

510.

Lacaze du Thiers (Lacaze-Duthiers], H., 1861, Histoire naturelle 

des brachiopodes vivants de la Me"diterrane"e. Premier 

me'moire sur la The"cidie: Annales sci. Nat. Paris, zool., 4th 

ser., v. 15, p. 260-330.

Thomson, J. A., 1927, Brachiopod morphology and genera (Recent and Tertiary): New Zealand Board of Sci and Art., 

Manual 7, p. i-vi, 1-338, pis. 1, 2.
PLATES 80-81
PLATE 80

FIGURES 1-5. Crania sp. (p. 315).

1-3. Apical, side, and interior views of the brachial valve, X6, figured specimen USNM 549272.

4. Interior of the pedicle valve showing punctate and granular margins, X5, figured specimen USNM 549273.

5. Individual in position of attachment, X5, figured specimen USNM 549274. 

6-15. Thecidellina congregata Cooper, n. sp. (p. 316),

6. Exterior of a medium-sized specimen showing the small triangular interarea, X4, paratype USNM 549275c.

7. Small are? of coralline surface showing four individuals, one showing the granular inner surface of the pedicle 

valve, X6, paratype USNM 549276.

8. Anterior view of a specimen with brachial valve agape and showing hemispondylium and nodose border of

the pedicle valve, X5, paratype USNM 549275a.

9, 10. Interior of two brachial valves showing the lophophore in action, X 10, paratypes USNM 549275b, 549275d.

11-14. Side, view slightly tilted toward the observer, full interior view and posterior view of a brachial valve with

all flesh removed and showing median septum, bridge, brachial groove and ridges, orifice leading into

body cavity, cardinal process and small septum at its base, X10, holotype USNM 549277.

15. Small piece of coralline expansion showing specimens of Thecidellina in natural size, paratype USNM 549278.
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 80

CRANIA SP. AND THECIDELLINA CONGREGATA
GEOLOGICAL SURVEY PROFESSIONAL PAPER 260 PLATE 81

THECIDELLINA MAXILLA AND ARGYROTHECA SP.
PLATE 81 

FIGURES 1-10. Thecidellina maxilla Hedley (p. 317).

1. Cluster of dead shells on the coral Lobophyllia, XI, showing large size of the species, hypotype USNM

549287.

2-4. Brachial, side, and anterior views of a large adult, X4, hypotype USNM 584676. 

5, 6. Pedicle valve in interior view and inclined to the side to show teeth and the ends of the hemispondylium,

X4, hypotype USNM 549285.

7. Interior of another pedicle valve, X4, showing interarea and large teeth, hypotype USNM 549284a. 

8-10. Posterior, side, and interior views of the brachial valve, X5, showing median septum and bridge, the latter

completely obscuring the cardinal process, hypotype USNM 549285. 

11-22. Argyrotheca sp. (p. 318).

11-13. Brachial, side, and pedicle views of a complete specimen, X8, figured specimen USNM 549279a.

14. Coralline surface showing three attached specimens, X6, figured specimen 549288.

15. Another specimen attached to coralline surface in close proximity to a specimen of Thecidellina congregata,

X6, hypotype USNM 549289.

16, 17. Interior of the brachial valve, X10 and X20, showing the median septum, square cardinal process, row of 

subperipheral nodes, and loop; figured specimen USNM 549279b.

18. Same interior before removal of dried flesh, X20, showing lophophore.

19. Same interior, X15, turned to the side to show the serrated median septum.

20. Interior of the pedicle valve, X10, showing teeth, concave apical plate, median septum, subperipheral row

of pits, and deep pits anterior to median septum.

21, 22. Same interior, X20, tilted to side and in normal view showing the same features in more detail. In figure 

22 the flesh is still in place, and the short, stout pedicle is visible.

U. S. GOVERNMENT PRINTING OFFICE: 1954