Predation on Sub-Holocene Scaphopod 

Mollusks from Southern Louisiana 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1282 
Predation on Sub-Holocene Scaphopod 

Mollusks from Southern Louisiana 

By ELLIS L. YOCHELSON, DAVID DOCKERY, and HEIDI WOLF 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1282 

Boring of scaphopod shells by naticid gastropods, 

which can be documented back to the Late 

Cretaceous, is most common on coarsely ribbed 

spec1es 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1983 
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Library of Congress catalog-card No. 83-600542 

For sale by the Distribution Branch, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 
CONTENTS 

Page 

Abstract - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Acknowledgments - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 

Identification of the mollusks from "mud lumps" and predation on Dentalium - - - - - - - - - 2 

Longitudinal and circumferential distribution of borings in D. (Dentalium) laqueatum - - - 3 

Morphology of the borehole - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 5 

Other possible predation on Dentalium - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 7 

Ontogenetic change in D. (Dentalium) laqueatum - - - - - - - - - - - - - - - - - - - - - - - - - - - - 8 

Distribution of borings in fossil and present-day scaphopods of eastern North America - - - 8 

Identity of the predator - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 9 

Selection of predation site - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 10 

Other considerations - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 12 

References cited - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 12 

ILLUSTRATIONS 

[Plates follow index] 

Plate 1. General views of Dentalium (Dentalium). 

2. Scanning electron microscope photographs of a boring in a six-sided variant of Dentalium (Dentalium) laqueatum Verrill. 

3. Broken and repaired shells and apertural views of Dentalium (Dentalium) laqueatum Verrill. 

4. Ontogenetic change in the apex of Dentalium ( Dentalium) laqueatum Verrill. 

Page 

Figure 1. Circumferential distribution of borings in rib variants in two samples of Dentalium (Dentalium) laqueatum Verrill - -- 4 

2. Relationship of length and width of 40 specimens of Dentalium (Dentalium) laqueatum Verrill to length and width of borehole in the shell - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 6 

III 

PREDATION ON SUB-HOLOCENE SCAPHOPOD MOLLUSKS 

FROM SOUTHERN LOUISIANA 

By ELLIS L. YOCHELSON, DAVID DOCKERY, 1 and HEIDI WOLF2 

ABSTRACT 

A scaphopod mollusk, Dentalium (Dentalium) laqueatum Verrill, 

is common in the sub-Holocene fauna from the "mud lumps" at the 

mouth of the Mississippi River. More than 50 percent of the individuals have been bored; nearly 20 percent of these organisms had nonfatal shell breakage during life. The infaunal life habit of the 

scaphopods and the characteristics of the boreholes suggest that 

naticid gastropods were the predators. The holes are not bored at 

random, but were evidently positioned where they would intercept 

internal organs of the scaphopod. Holes are slightly more common on 

the right side, as an observer would look into the aperture, and more 

common in a dorsal position. Some relationship also exists between 

the .size of the hole and the diameter of the shell. Apparently, the 

naticid predator crawled along the dorsal side of the tube from the 

aperture toward the apex until it found an appropriate site to drill. 

Predation on scaphopods by naticid gastropods can be documented 

back to the Late Cretaceous. The rate of predation observed in fossil 

and Holocene specimens commonly is low, in the 1-5 percent range; 

however, coarsely ribbed scaphopod species show a high rate of predation. Theoretical considerations suggest that coarse ribs would facilitate burrowing into coarse sediments, such as shell pavements, 

though no observations on the relationships of scaphopod shell morphology and sediment type exists. N aticid gastropods seldom penetrate fine-grained sticky sediments, so the degree of predation seen 

in collections of fossil scaphopods may give some indication of the 

original character of the sediment. 

INTRODUCTION 

The ephemeral islets, or "mud lumps," at the mouth 

of South Pass of the Mississippi River have long commanded interest. They are now accepted as diapir 

structures, in which strata of sub-Holocene age (about 

15,000 years old) from beds probably more than 100m 

below the sea floor of the Gulf of Mexico (Morgan, Coleman, and Gagliano, 1968) are raised to the surface. 

Clay and silt forming these "mud lumps" contain a diverse molluscan fauna, including scaphopods. 

Scaphopods feed by using their captaculae to gather 

microorganisms such as foraminifers from the surrounding sediment (Dinamani, 1964b). The abundant 

foraminifers in the sediments of the "mud lumps" pro-

'Mississippi Bureau of Geology, Jackson, Miss. 39216. 

•u.s. National Museum of Natural History, Washington, D.C. 20560. 

vide a logical explanation for the abundance of 

scaphopods at the locality. Palmer (1975) observed piles 

of broken foraminifer tests adjacent to Jurassic 

scaphopods, suggesting that these microorganisms have 

been an important part of the scaphopod diet. An unusually large proportion of scaphopods at South Pass 

have been bored, and it is this feature that is investigated. 

The surprisingly high degree of predation at this locality independently attracted two of us to this study. 

Y ochelson made a~ surface collection from the "mud 

lumps"; this collection was measured and studied in 

Washington, D.C., by Wolf. Dockery examined 

scaphopods in the collections of the Department of 

Earth Sciences, Tulane University, New Orleans, La., 

which had accumulated as a result of repeated trips to 

the "mud lumps." These Tulane specimens were obtained by screening and washing of sediment as well as 

by surface collecting. Because the Tulane collection was 

larger than the Washington collection, a more sophisticated statistical approach was used there to handle data 

on predation. Some differences in the results of the two 

investigations may be related to the much larger sample size at Tulane and to the greater number of smaller 

individuals obtained by sieving of the sediments. Observations of scaphopod morphology, apart from data from 

boreholes, are based on the Washington collection. 

The specimens studied by Dockery are housed at 

Tulane University. The figured and unfigured specimens studied by Wolf and Y ochelson are housed in the 

Division of Mollusks, National Museum of Natural History, Smithsonian Institution, Washington, D.C. 

ACKNOWLEDGMENTS 

A trip to the Mississippi "mud lumps" was arranged 

by Emily Vokes, Department of Earth Sciences, Tulane 

University, with the good offices of Shell Oil Corporation. While at the locality on April 23, 1977, 15 members of the party donated the scaphopods that they col1 
2 PREDATION OF SUB-HOLOCENE SCAPHOPOD MOLLUSKS FROM SOUTHERN LOUISIANA 

lected to Y ochelson. Wolf 's participation in this project 

was during a Colgate University program in January 

1978, under the supervision of Robert M. Linsley. Computer time for Dockery was obtained through the Department of Earth Sciences, Tulane University. 

Norman F. Sohl, U.S. Geological Survey, 

Washington, D.C., shared his knowledge of Mesozoic 

predation with us. M. R. Carriker, University of Delaware, Lewes, Del.; John Kraeuter, Virginia Institute of 

Marine Sciences, Gloucester Point, V a.; Louie Marincovich, U.S. Geological Survey, Menlo Park, Calif.; and 

C. P. Palmer, British Museum (Natural History), London, gave helpful advice and guided us to pertinent literature. Joseph Rosewater, Division of Mollusks, National Museum of Natural History, Smithsonian Institution, Washington, D.C., permitted the examination of 

types and the large collection of Holocene scaphopods 

under his charge. 

IDENTIFICATION OF THE MOLLUSKS FROM 

THE "MUD LUMPS" AND PREDATION ON 

DENTAUUM 

Gastropods and pelecypods in the fauna were identified by consulting Abbott (1974) and Warmke and Abbott (1961). The overall degree of predation of the mollusks was low, as shown in the Tulane collections. Predation by boring was more widespread among bivalve 

genera than among gastropods. The bivalves Gouldia 

cerina (C. B. Adams), Mulinia. lateralis (Say), small 

valves of Anadara boughrnani Herlein, Pleuromeris 

armilla (Dall), and Corbula sp. show a moderately high 

percentage of bored valves. The gastropod Turritella is 

noteworthy for the amount of boring. For example, 57 

percent of 281 specimens of Turritella exoleta (Linnaeus) and 39 percent of specimens ofT. acropora Dall 

were bored; a few additional species of gastropods 

showed evidence of predation at a much lower level. Although none of these borings have been studied in detail, they are similar to the borings on Dentalium. 

In contrast to the number of bored gastropods and 

pelecypods, only 18 of 183 specimens of Cadulus 

(Platyschismoides?) cf. C. (Platyschismoides?) elongatus Henderson (39 in Washington, under USNM 

780590, and 144 in Tulane collections) were bored. All 

these borings were positioned along the middle third of 

the tube; details of the boreholes are similar to those in 

Dentalium. Evidently, Cadulus was not a preferred 

prey. 

Scaphopods were identified with reference to Henderson (1920). His figured specimens as well as specimens identified by him in the biologic collection of the 

National Museum of Natural History, Washington, 

D.C., were examined by us. We are confident of the assignment of specimens to D. (Dentalium) obscurum 

Dall (under USNM 780952) (pl. 1, fig. 3), but we are 

less certain of the identity of the most abundant species 

(USNM 780951) (pl. 1, figs. 1, 2, 4). This latter species 

shows the fine reticulate ornament of D. (Dentalium) 

laqueatum Verrill, but that species characteristically 

has nine ribs. D. (Dentalium) texasianum Phillipi is 

less well known but is reported as six ribbed and lacking fine reticulation. We have decided to use Verrill's 

name; although eventually it may be appropriate to 

name a new species for this material, such action should 

be delayed until the variation in both D. (Dentalium) 

laqueatum and D. (Dentalium) texasianum is better 

known. In addition to these two common species of 

Dentalium, two specimens, lacking boreholes, of a 

third undetermined species of scaphopod were present 

(USMN 780953). 

In the Washington collection, 235 specimens of D. 

(Dentalium) laqueatum Verrill are available, of which 

121 show evidence of predation. These are divided into 

groups on the basis of the number of longitudinal ribs. 

A few specimens show one to three supplementary ribs 

bifurcating an interarea; they are also tabulated below, 

denoted by a "+" sign: 

Specimens Specimens Percentage 

No. ofribs not bared bared Total bored 

5+1 0 2 2 100 

6 15 13 28 46 

6+1 2 1 3 33 

6+2 1 0 1 0 

7 85 89 174 51 

7+3 1 0 1 0 

8 10 15 25 60 

8+3 0 1 1 100 

The overall degree of predation was 52 percent. Not all 

the unbored shells are complete, and a few may have 

contained a hole in the missing part of the shell. However, we judge that, at most, only 1 or 2 percent additional specimens might have been bored. Two specimens each bear two borings; four individuals bear only 

incomplete borings, but they are counted as having 

been bored. 

Borings in D. (Dentalium) laqueatum are more common in the Tulane collection, 291 of 478 specimens having been bored. 

Three variants of rib number were distinguished: 

Specimens Specimens Percentage 

No. ofribs not bared bared Total bared 

6 33 42 75 56 

7 145 223 368 61 

8 8 26 33 79 

The overall degree of predation is 61 percent. The 

three variants of D. (Dentalium) laqueatum show that 

the percentage of individuals bored increases with the 

increase in number of ribs. This trend also app~ars in 

the smaller Washington collection. 
LONGITUDINAL AND CIRCUMFERENTIAL DISTRIBUTION OF BORINGS IN D. (DENTALIUM) LAQUEATUM 3 

The Washington, D.C., collection also contains 29 

specimens of D. (Dentalium) obscurum Dall, of which 

13 (45 percent) were bored. This species shows several 

variants in rib number, but individuals are too few to 

indicate any relationship between number of ribs and 

degree of predation. In the Tulane collection, 16 of 48 

specimens (34 percent) of this species were bored, but 

the predation rate varies widely among the rib-number 

variants. The numbers summarized below probably are 

too small to be significant in demonstrating increase of 

predation with increase in number of ribs: 

Specimens Specimens Percentage 

No. ofribs not bared bored Total bored 

9 13 9 21 45 

10 16 6 22 27 

11 2 0 2 0 

12 1 1 2 50 

LONGITUDINAL AND CIRCUMFERENTIAL 

DISTRIBUTION OF BORINGS IN 

D. (DENTALIUM) LAQUEATUM 

The position of the borings along the length of the 

shells of D. (Dentalium) laqueatum was determined by 

two different methods. Shells in the Washington, D.C., 

collection were measured by Wolf by moving a Swiss 

Map Measurer (Dietzgen Inc. )

3 along the convex 

(longest) surface from apex to aperture. The relative 

position of the borehole was projected onto this ventral 

surface, regardless of where it was on the circumference. 

Specimens were judged to be complete when the 

apertural margin was thin and translucent to light. A 

few of the specimens may have lacked part of the apex, 

but, at most, only 1-2 mm was missing. All specimens 

were measured at least three times to insure accuracy. 

Forty specimens of the seven-sided variant of D. (Dentalium) laqueatum were measured; they ranged in 

length from 1.9 to 5.0 em (mean 3.4 em ±0.92). The 

position of the borings was calculated as percentage of 

the distance from apex to hole against the total length. 

The lowest value was 30 percent and the highest 70; 

these extremes are atypical but are not a consequence 

of poor measurement or broken material. The average 

distance to the borings was approximately 45 percent of 

the length; 38 of the 40 holes were between 38 and 59 

percent of the total shell length. 

To check for possible relation between size of the organism and longitudinal position of the holes, measurements of the six shortest shells were pooled. Average 

length was 2.3 em, and percentage of distance to boring 

was 44 percent. For the six largest shells, the average 

length was 4. 9 em and the percentage distance 45 per-

•Any use of trade names in this publication is for descriptive purposes only and does not 

constitute endorsement by the U.S. Geological Survey. 

cent. To check for possible differences around the circumference, the distance to holes on or near the plane 

of bilateral symmetry in the dorsal and the ventral 

quadrants were averaged separately; both were essentially the same. 

One five-sided variant, 7 six-sided variants, and 10 

eight-sided variants were also measured. These ranged 

from 1.3 to 5.2 em in length, with an average of 2.9 em. 

The lowest percentage distance to boring was 33 and 

the highest 73; the average percentage was 49. The average percentage for six-sided specimens was 44, 

whereas that for eight-sided individuals was 52. The 

number of specimens probably is too small for these 

percentages to be significant. 

Figure lA - D illustrates the bore radial position of 

123 boreholes on 121 specimens of D. ( Dentalium) 

laqueatum in the Washington collection, divided into 

four variants of rib number. With the exception of a 

single hole centered on the dorsal rib of a six-sided 

specimen, all borings were between ribs. A large hole 

may impinge upon an adjacent rib, but, with the one 

exception, a rib was not a site for boring. On those 

specimens in which an extra rib was intercalated, the 

boring was situated on a wider interspace. As noted, 

four of these specimens each show an incomplete boring. In addition, one specimen had one complete hole 

and an incomplete one in an adjacent interarea. One 

specimen contained two holes, both in the same interarea. 

In the Tulane collection, emphasis was given to the 

seven-ribbed variant of D. (Dentalium) laqueatum, for 

which 323 mostly complete specimens were available 

(fig. lE-G). Procedure for measurements made on this 

variant differs from those used by Wolf. Definitions of 

measurements are: 

(1) Arch height (A), greatest displacement of the concave dorsal margin of the tube from the apex to 

anterior cord. 

(2) Bore diameter (BD), greatest diameter of the 

borehole at the exterior surface. 

(3) Bore longitudinal position (BLP), percentage of the 

shell length to which the borehole was displaced 

from the anterior aperture. This percentage was 

calculated from the measurement of total length 

along the chord from the apex to the aperture and 

the chord length from the aperture to the boring. 

(4) Bore radial position (BRP), radial location in which 

interspace the borehole occurred. The interspaces 

are numbered from 1 to 7 in a clockwise manner 

as viewed looking into the aperture, the apex upward in the life position. 

(5) Diameter (D), diameter of the anterior aperture. 

(6) Length (L), length of the cord from the apex to the 

dorsal edge of aperture. 
4 PREDATION OF SUB-HOLOCENE SCAPHOPOD MOLLUSKS FROM SOUTHERN LOUISIANA 

6 

17 10 

14 91 17 

FIGURE I.-Circumferential distribution of borings in rib variants in two samples of D. (Dentalium) laqueatum: A-D, Washington collection, 

and E-G, Tulane collection. Incomplete borings are circled; total number of complete and incomplete borings is given in the center of 

each outline sketch. In designating position, side 1 is to the upper right in every rib variant. The sketch is a view of the aperture as 

seen by an observer looking into it, the apex upward in life position. 

Measurements of 204 bored specimens were tested in 

a correlation matrix for any relationship between position and size of the boring and morphology and size of 

the shell. The means and standard deviations of measurements for specimens bored and not bored were also 

calculated. All values are in millimeters except for the 

longitudinal position of the borehole, which is a percentage, and the bore radial position, which is a value from 

one to seven. The correlation matrix is as follows: 

D L A BLP BRP BD 

1.000 0.944 0.700 0.010 0.081 0.223 D 

1.000 .731 .013 .040 .210 L 

1.000 .020 .022 .206 A 

1.000 -.006 -.132 BLP 

1.000 .114 BRP 

1.000 BD 

Means and standard deviations (SD) measurements 

of the 204 bored specimens are: 

D 

4.037 

1.122 

L 

34.542 

10.557 

A 

3.096 

1.063 

BLP 

53.603 

8.085 

BRP 

3.932 

1.984 

BD 

1.402 Mean 

1.377 SD 

Means and standard deviations for measurements of 

the 119 specimens not bored are: 

D 

3.500 

1.121 

L 

29.130 

10.493 

A 

2.567 Mean 

1.063 SD 

Relationships among the shell dimensions are obvious 

without the use of a correlation matrix, but their relationship, if any, to the selected position of boring by 

the predator is less evident. There is no correlation of 

the longitudinal or radial borehole position with the size 

of the Dentalium shell. A correlation slightly higher 

than the 5-percent level of significance (the 5-percent 

level at two degrees of freedom is 0.138) occurs between the bore diameter and the shell dimensions, not 

unexpectedly, as the larger predators might select the 

larger prey. The correlation of the borehole diameter 

with the radial position of the borehole is slightly less 

than the 5-percent level. The mean and standard deviations of the longitudinal position of the borehole indicate a strong preference by the predator to bore along 

the midsection of the shell, as noted in the Washington 

collection. 

The larger Dentalium shells were also those more 

commonly bored, as indicated by the differences between the means of specimens bored and not bored. 
LONGITUDINAL AND CIRCUMFERENTIAL DISTRIBUTION OF BORINGS IN D. (DENTALIUM) LAQUEATUM 5 

The various size groups of seven-sided individuals are 

tabulated below: 

Specimens Specimens PercenW,ge 

Lengthinmm 1Wt bored bored Total bored 

60+ 1 2 3 67 

5~9.9 0 4 4 100 

50-54.9 5 11 16 69 

~9.9 4 18 22 82 

40-49.9 10 31 41 76 

3&-39.9 16 33 49 67 

30--34.9 20 31 51 61 

25-29.9 17 35 52 67 

20--24.9 20 25 45 56 

15-19.9 19 14 33 42 

10--14.9 7 0 7 0 

As observed in the smaller Washington collection, 

the circumferential distribution of boreholes around D. 

( Dentalium) laqueatum shells is not random; more 

boreholes are found at some interspaces between ribs 

than at others, as shown in figure lE-G. On the sixribbed shells, the boreholes are concentrated in interspaces 2, 5, and 6. On the seven-ribbed form, borings 

are most common in interspaces 2, 3, 6, and 7. On the 

eight-ribbed form, they are concentrated in interspaces 

2, 4, and 7. No count was made of the five-ribbed variant in the Tulane collections. In all these variants, the 

opposing interspaces just to the right of the dorsal rib 

and on, or just to the left of, the ventral margin (as 

viewed toward the aperture) show the lowest frequency 

of boreholes. 

A few specimens show more than one incomplete boring. One seven-ribbed specimen has three shallow incomplete borings in interspaces 1, 2, and 4 and a complete boring in interspace 6, all of which are similar in 

size. As interspaces 1 and 4 are least commonly bored, 

a predator may have abandoned these sites in favor of 

one in which it could better grasp the shell with its 

foot. Another seven-sided specimen had two complete 

borings. One specimen has an incomplete boring that is 

partly on a rib. As in the Washington collection, all 

other borings were centered in the interspaces; the increased shell thickness along the rib may have discouraged the predator from siting there, but more likely the 

upward projection interfered with positioning of the 

predator's foot and thereby discouraged drilling. 

th~;~ugh a rib. 

Relatively few incomplete boreholes are found compared with the number of complete ones. If each boring 

is considered a separate attack by a predatory · gastropod, the percentage of incomplete boreholes in the 

Tulane collection is 15 percent for the six-ribbed individuals, 5 percent for the seven-ribbed forms, and 4 

percent for the least common eight-ribbed variant. The 

seven-ribbed individuals are by far the most common. 

The percentage of incomplete boreholes is low for all 

variants; when considered along with the number of attacks, the six-ribbed variant is the least vulnerable. 

There seems to be no· obvious reason why drilling is apparently less effective on this variant. 

Of the 13 bored specimens of D. (Dentalium) 

obscurum examined in Washington, 12 show one boring. None of the holes is centered on a rib, but, because 

of the more numerous ribs, holes impinge more on adjacent ribs; predation was not seen in the areas where an 

intercalated rib was present. Almost all the holes are 

on the dorsal half of the circumference, but they are 

more commonly lateral than directly dorsal. About as 

many are on the right side of the shell as on the left. 

One specimen with two complete borings has one on the 

upper lateral right and one on the upper lateral left 

side. 

Five drilled specimens of D. (Dentalium) obscurum 

were complete enough to treat by the map-measurer 

method. The shells range in length from 2.1 to 3. 0 em, 

averaging 2.3 em. The percent distance to the borehole 

ranges from 39 to 52, averaging 44. 

Except for the count of predation given earlier, no 

study of the Tulane specimens of D. ( Dentalium) 

obscurum was undertaken. 

MORPHOLOGY OF THE BOREHOLE 

Because the scaphopod shell breaks smoothly around 

its circumference, some specimens have "half holes" 

and permit easy examination of the cross section of the 

boring (pl. 2, fig. 1; pl. 3, fig. 1). Viewing the hole from 

the exterior gives an impression that the walls slope inward more or less smoothly, but the cross sections 

show that only in the exceptional hole is part of the wall 

uniformly inclined. Characteristically, the sides of the 

hole are concave, so that the hole is bowl shaped. 

The borehole forms an area of weakness, and a third 

of the Washington specimens examined are broken; 

these specimens show the circumference of the shell 

and allow ready observation of both shell thickness and 

borehole morphology. Because of the predominantly 

crosslamellar structure of the shell (pl. 2, figs. 2, 4) 

(Hass, 1972), the fracture tends to be radial (Joseph 

Carter, oral commun., 1977), and the fractured surface 

is commonly smooth and sharp (pl. 2, fig. 6). 

Although we did not measure shell thickness of any 

individuals, our impression is that thickness shows little 

individual variation along the length of the tube, except 

at the apertural margin. The farther from the aperture 

that the hole was bored, the greater the thickness of 

shell that had to be penetrated. In a small hole, the 

shell thickness penetrated was 0.4 mm; in a large hole, 

it was 0.9 mm. No data are available on rates of boring 

for naticid gastropods, but muricids can bore Q.3 mm 

per day through an oyster shell (Carriker, Williams, 

and Van Zandt, 1978, p. 126); no doubt, shell structure 

and mineralogy affect rates of penetration. 
6 PREDATION OF SUB-HOLOCENE SCAPHOPOD MOLLUSKS FROM SOUTHERN LOUISIANA 

The gross morphology of the borehole in 40 specimens of D. (Dentalium) laqueatum in the Washington 

collection was examined. Length and width measurements of the holes were taken on the exterior and interior using a 20 x 20 grid under 18 x magnification. 

One significant point is that the long axis of the hole 

was invariably alined along the long axis of the shell. 

On the external shell surface, the length of holes 

ranged from 0.2 to 4.1 mm, with a mean of 2.4 mm 

( ± 0.69); the width ranged from 1.0 to 3.3 mm, with a 

mean of 2.0 mm ( ± 0.55) On the internal shell surface, 

length of holes ranged from 0.9 to 2.8 mm, with a mean 

of 1.6 mm ( ± 0.44); width ranged from 0. 7 to 2.3 mm, 

with a mean of 1.1 mm ( ± 0.3 ). 

We also plotted the hole size relative to width of the 

scaphopod at the point of boring for specimens in the 

Washington collection (fig. 2). The plot shows a wide 

scatter, but in general the wider the position where the 

scaphopod was bored, the larger the hole. Carriker and 

Van Zandt (1972b, p. 221-225) found a general relationship of borehole size to size of predator in the muricid 

Urosalpinx. Berg (1975) suggested this relationship in 

naticids, but provided no supporting data. 

The holes were grouped into four quadrants-one 

centered ventrally, one dorsally, and two laterally- to 

test for a preference in the size of the hole and its circumferential position. Holes were also divided into 

groups according to the maximum borehole length on 

the outer shell surface: small (2.4--4.2 mm), medium 

(4.3-6.2 mm), and large (6.3--8.1 mm). Of the 15 holes 

in the ventral quadrant, 54 percent were small, and, of 

the 14 holes in the dorsal quadrant, 36 percent were 

small. The average length of the borings on the dorsal 

and ventral sides was the same, 3.5 em. 

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X X 

X 

-

I I I I 

5.5 6.0 6.5 7.0 7.5 

FIGURE 2.-Relationship of length and width of 40 specimens of D. (Dentalium) laqueatum Verrill to length and width of borehole in the 

shell. 
MORPHOLOGY OF THE BOREHOLE 

It is instructive to describe the shape of the hole as 

well as its size. Because the external margin of a 

borehole is bilaterally symmetrical and the inner margin very nearly so, the ratio of width to length indicates 

deviation from circularity. For the exterior in the 40 

holes, this ratio ranged from 100 percent-a circular 

hole-to 72 percent. The mean was 86 percent, with a 

standard deviation of 7 percent. For the interior margin, the maximum was 96 percent-nearly a circular 

hole, but an unusually wide one-and the minimum was 

53 percent. The mean of the interior was 69 percent, 

with a standard deviation of 10 percent. The inner margin is more oval than the exterior; as noted earlier, the 

long axis of the hole is parallel to the ribs. So far as we 

can determine, the lateral margins of the holes are uniform in outline, except for the few boreholes that impinge on a rib on one side and not on the other. 

The external margin of the hole commonly is sharp 

and without any beveling of the edge, for the fine ornament on the interarea may be followed to the edge of 

the depression at almost every one of the holes (pl. 1, 

fig. 4). The inner margin of the hole is also quite sharp. 

No irregular shelf of unbored shell material is left as a 

partial floor to the hole, and, in almost all examples, 

the inner edges are bilaterally symmetrical. Both the 

inner and outer margins of the borehole appear oval, 

but the external opening is closer to circularity. 

Many of the drillings were examined at moderate 

magnification, and collectively they conveyed an impression of general similarity of form. One "half hole" was 

examined in more detail by scanning electron microscope (SEM) (fig. 2). The close relationship of the fine 

ornamentation to the crosslamellar structure of the 

shell is evident (pl. 2, figs. 1--3). This same point emphasizes how sharp is the contact between the outer 

circumference of the hole and the outer shell surface. 

No area of etching surrounds the hole (pl. 2, fig. 3). Because even the microscopic sculpture of the shell is 

sharp, it seems unlikely that erosion by geologic processes has affected the margin of the borehole. 

The SEM photographs show the concave inward 

bowl-shaped walls of the hole (pl. 2, figs 1, 4). The thin 

inner shell appears fibrous and is strikingly different 

from the thick outer crosslamellar layer (pl. 2, figs. 2, 

5). Near the juncture of the two layers, the curvature 

of the drill-hole wall changes dramatically, and this 

change may be related to the different shell structure 

(pl. 2, figs. 2, 4). Finally, there is no evidence whatsoever of any radular rasp marks on the walls of the 

hole (pl. 2, figs. 2, 4, 5). Other specimens examined at 

lower magnification also showed the drill-hole walls to 

be smooth and sharp. 

In examining the borings on 15 specimens of D. 

(Dentalium) obscurum, we did not observe any percep7 

tible difference in size or shape from those on the more 

abundant species. 

OTHER POSSIBLE PREDATION ON 

DENT ALlUM 

We have observed an apertural peculiarity of D. 

(Dentalium) laqueatum in that some specimens show 

an irregularity at the apertural edge, which is then repaired by subsequent shell growth (pl. 3, figs. 4, 5). 

This irregularity was noted in 43, or 18 percent, of the 

specimens in Washington. Many were moderately complete specimens, but there might have been an irregularity in those lacking the most mature part of the shell; 

thus, perhaps as many as 20 percent of the specimens 

may have been broken and repaired. A depression in 

the longitudinal profile is present at the side of the irregularity, but within the subsequent millimeter or two 

the shell has increased to its former size, and expansion 

continues uniformly through later growth (pl. 3, figs. 4, 

5). 

These breaks characteristically have a jagged outline, 

and almost none are circular or subcircular around the 

circumference of the margin. Breakage is random in 

terms of the dorsal, ventral, or lateral parts of the 

aperture, for only eight shells show breakage confined 

to part of the. aperture. Almost certainly the breaking 

only affected the shell and did not involve the mantle 

edge, because during later growth of the shell, the ribs 

and fine ornament continue in the same pattern and the 

same position as on the earlier part. 

A few shells show multiple breaks. Of the 43 

examined, four have two growth interruptions (pl. 3, 

fig. 5), one has three, and two have four; after the 

fourth break, the shell of one specimen grew with slight 

deviation from the plane of bilateral symmetry, but the 

ribs were still straight and uniformly spaced (pl. 3, fig. 

6). An example of wasted effort is recorded in a shell 

repaired twice and then bored. 

Similar breakage and repair has been seen by John 

Kraeuter (oral commun., 1978) in other species of 

Dentalium, though seldom such a high percentage. We 

speculate that these scaphopods may have been attacked by a fish or a crustacean but that those individuals who survived either were not protruded or were 

able to retract into the shell rapidly enough to avoid injury to the soft tissues. 

Some of the . specimens might have been exhumed 

during life by storms and broken before they reburrowed; however' in·· spite of thinness near the aperture, 

the shells are strong, and this possibility seems unlikely. · 

Of 29 specimens of D. (Dentalium) obscurum, 5 have 

been broken and then repaired. The breaks are similar 
8 PREDATION OF SUB-HOLOCENE SCAPHOPOD MOLLUSKS FROM SOUTHERN LOUISIANA 

to those in D. (Dentalium) laqueatum. We did not observe any breakage in the Cadulus specimens. 

ONTOGENETIC CHANGE IN 

D. (DENTAUUM) LAQUEATUM 

The available specimens of D. (Dentalium) 

laqueatum show no apparent variation in logarithmic 

curvature, the prime feature of growth. The five, six, 

and eight-ribbed variants are uncommon relative to the 

seven-ribbed form. Even rarer are those specimens in 

which one or two of the sides is bifurcated by an accessory rib (pl. 1, fig. 2). Commonly these accessory ribs 

do not begin at the apex but are intercalated about a 

centimeter below it. The relative height of the ribs decreases during ontogeny (pl. 3, figs. 2, 3); apparently 

this is a regular feature of growth and not an abrupt 

gerontic change, but more observations of large individuals are needed to confirm the interpretation. The 

fine ornamentation appears constant on all individuals 

examined (pl. 1, fig. 4; pl. 2, fig. 1). Growth lines are 

obscure in the apical area but are slightly coarser along 

most of the shell length. Specimens do not show crowding of the growth lines near the aperture, which might 

indicate gerontic growth. However, the one shell area 

of significant ontogenetic change is at the apical area 

(pl. 4). 

The sequence of change appears as follows: During 

early growth stages, the· apex bears a tiny circular 

opening that increases in diameter as shell length increases. At the next stage, the margin is indented by 

a "V" formed ventrally, making a keyhole shape, the 

sides of the "V" being inclined at an angle as great as 

45° to the line of bilateral symmetry (pl. 4, fig. 1); this 

"V" is commonly between major ribs but need not be 

directly ventral and may be inclined to the left or right. 

Concurrent with increase in length, the base of the "V" 

then gives rise to an incipient slit parallel to the ribs 

(pl. 4, fig. 2). Increase in the diameter of the apical hole 

continues so that the upper part of the "V" is steadily 

reduced in width and depth until it ultimately disappears. The final stage consists of the apical hole, 

breached by an elongated slit occupying all or most of 

an interarea between two ribs (pl. 4, fig. 3). 

There appears to be no literature on the mechanism 

of resorption in modern scaphopods. The scanty accounts of soft-part morphology suggest that the apical 

area is lined by a mantle like that at the aperture. 

Thus, this organism apparently is simultaneously building the tube at the aperture, while the animal is within 

the sediment, and absorbing the apex, which protrudes 

up into the water. 

The apices of the 6 smallest specimens, of 6 near the 

mean length, and of the 8 largest individuals in the 

Washington collection were measured. The shortest 

specimen that did not show any modification of the circular apical opening had a length along the convex (ventral) side of 1. 9 em; the largest having an unmodified 

apex was 4. 0 em. Commonly, an indentation in the margin of the apical aperture appears when a shell is about 

3.3 em long, but one specimen only 2.2 em long shows 

an incipient "V" -shaped notch. Specimens ranging from 

4.1 to 4.9 em in length show a prominent "V" or a slit. 

Resorption may be quite irregular; in many specimens, 

neither the apical area nor the slit is smooth. The 

longest slit observed was approximately 2.8 em long on 

a shell 5.0 em long; the slit has an average width of 0.6 

mm. In this group of 20 specimens, of the 9 specimens 

that have slits, 2 have the slit ventral, 3 have it on the 

lower right lateral, and 4 on the lower left lateral. 

A similar sequence and some individual variation 

among apical slits were found by Yancy (1973) in Permian Prodentalium. The large amount of individual 

variation in a modern species should be noted, for obviously one should not place much reliance on features of 

the slit in attempting to differentiate fossil species. 

The incipient "V" found at the apex of some specimens is different in inclination and in shape from a boring that lies on a line of fracture and now forms a half 

circle (pl. 3, fig. 1). One should be wary of confusing 

this feature with true apical characters of a scaphopod. 

In addition to the difference in shape, these "half holes" 

occur where the shell is far thicker than at the apical 

area. 

Few complete specimens of D. (Dentalium) 

obscurum are available, and these had a smaller average size than the sample of D. (Dentalium) laqueatum. 

However, we did observe a slit 2. 7 mm long in a shell 

4.9 em long. With regard to variation in rib number in 

this species: of the 29 specimens, 12 have 9 ribs; 1 has 

2 intercalated ribs among 9; 11 have 10 ribs; 3 have 1 

intercalated rib among 10; and 2 have more than 10 

ribs. 

DISTRIBUTION OF BORINGS IN FOSSIL 

AND PRESENT-DAY SCAPHOPODS OF 

EASTERN NORTH AMERICA 

To determine whether predation on scaphopods by 

naticid gastropods is a newly developed (Holocene) 

habit, we surveyed Cretaceous to Holocene collections, 

limiting our examination to those collections containing 

a large number of specimens. We have seen no evidence of predation on North American Paleozoic 

scaphopods. We have not studied any American Triassic or Jurassic samples; a few Triassic scaphopods from 

Austria and Italy examined by Yochelson in 1975 at the 

N aturhistorisches Museum, Vienna, did not show any 

borings. 

Examination of about 75 scaphopods in the Mesozoic 

biological collection of the National Museum of Natural 
DISTRIBUTION OF BORINGS IN FOSSIL AND PRESENT-DAY SCAPHOPODS OF EASTERN NORTH AMERICA 9 

History, Washington, D.C., did not show any evidence 

of predation. In the stratigraphic collections, borings in 

scaphopods were observed in the Ripley· Fonnation 

(Cretaceous) fauna. About 5 percent of the specimens of 

Cadulus obnustus (Conrad) at Coon Creek, Tenn., 

were bored, but the much larger Dentalium intercalatus (Wade) from this same locality does not show 

predation. A variety of pelecypods and gastropods from 

Coon Creek were also bored. In a collection from the 

Ripley Fonnation, 2.6 miles below Eufaula, Ga. (USGS 

28433), we did find 14 out of 105 specimens of Cadulus 

bored, and borings in 4 out of 71 of a finely ribbed 

species of Dentalium. Many of these Dentalium species 

are broken, and the percentage of predation may be 

slightly higher. 

The biological collection of Cenozoic scaphopods at 

the National Museum of Nat ural History contains about 

1,200 specimens. In 300 specimens from the Vicksburg 

Fonnation or Group, Oligocene, at several localities, D. 

(Dentalium) mississippiensis (Conrad), a moderately 

coarse ribbed species, showed 5--15 percent predation. 

Scattered smaller collections, totaling about 400 specimens, from a variety of localities in the Miocene of 

North Carolina and Virginia show a low percentage of 

predation. The only Miocene species significantly 

preyed upon were D. (Dentalium) carolinenese (Conrad) and D. (Dentalium) attenuatum (Say), both 

coarse-ribbed species. We estimate that 3 ,percent of 

the approximately 400 shells were bored, although as 

many as 25 percent of the specimens in an individual 

collection of these two species showed boreholes. In addition, 1,500 scaphopods in the stratigraphic collections 

from the Claiborne Group, Eocene, of Alabama were 

surveyed, but only one borehole was seen. 

Part of the Holocene biological collection at the N ationa} Museum of Natural History was surveyed. Approximately 3, 700 individuals were examined in some 

detail. In general, boreholes were limited to a very 

small percentage of the specimens. However, a few 

species did show abundant evidence of predation. Collections of D. (Dentalium) laqueatum, regardless of 

their geographical location or depth, commonly show 

predation. Similarly, D. ( Dentalium) taphrium Dall 

showed high levels of predation (almost 30 percent) at 

Cape Lookout, N.C. Collections from areas off Florida · 

showed more moderate levels of predation. D. (Dentalium) ceratum Henderson, found in generally deeper 

water, and D. (Dentalium) striolatum (Simpson), 

found in more northern locations and in very deep 

water, both showed still lower levels of predation, affecting about 10 percent of the specimens. All four 

species are coarse ribbed. 

In 1978, Y ochelson examined the collections of fossil 

scaphopods at the British Museum (Natural History) 

through the good offices of C. P. Palmer. He saw no 

evidence of borings in Paleozoic or Mesozoic specimens. 

The oldest boring observed in British scaphopods is in 

Entaliapsis brevis (Deshayes) from the Corbula bed of 

the Thanet Sands, Paleocene, at Bishopstone along the 

Thames Estuary west of Margate, Kent, in southeastern England. Finally, Yochelson in 1978 requested June 

Chatfield, curator of marine invertebrates, to search 

the collections of Holocene scaphopods at the National 

Museum of Wales, Cardiff, for borings in scaphopods. 

She confinned that predation by boring was pervasive 

but was at a very low level, except in coarse-ribbed 

species. 

IDENTITY OF THE PREDATOR 

The identity of the predator on 50 percent of the 

Dentalium in the Mississippi "mud lumps" is a matter 

of interest. Although various organisms bore for a variety of reasons, we are convinced that these holes result 

fro:m predation. Furthennore, we are satisfied that 

they were made by either a muriCid or a naticid gastropod. The morphology and position of the boreholes 

indicates that naticids were the predators. 

The edges of the borehole, almost without exception, 

are sharp. Carriker (1961) noted differences in the behavior of muricid and naticid gastropods that bore and 

also differences in the relative size of the accessory boring organ (ABO) that secretes a softening agent onto 

the shell. In both groups the ABO is critical, for when 

it is removed, the animal cannot bore (Carriker and 

Van Zandt, 1972a). The naticid ABO is placed mostly 

within the borehole. In contrast, the ABO of Urosalpinx and its relatives extends in a narrow area surrounding the hole (Carriker, 1969), and part of this 

chemically weakened area is then scraped off by the 

radula. As a consequence, the muricid hole is gently 

beveled, that is, inclined, on its upper slopes (Carriker, 

1961), whereas the naticid boring is not. 

Carriker (written commun., 1979) noted that the 

naticid ABO possesses a ring of mucous-secreting cells 

around the periphery of the gland, which might restrict 

the area of shell etching by ABO fluids. The holes in 

the scaphopods of the "mud-lump" fauna are steeply 

sloping, and the inner part of the hole has curved bowlshaped walls. Muricid gastropods commonly leave a 

shelf at the base of the hole once the radula has finally 

penetrated the shell (Carriker, 1970), whereas the 

naticids make a much neater hole. In effect, the naticid 

appears to complete the removal of the inner shell layer 

before dining. on its prey. 

N aticid holes commonly are nearly circular. Carriker 

and VanZandt (1972b) did find a very slight degree of 

ovalness in muricid holes, the long axis of the hole 

being alined parallel with the long axis of the foot of the 

predator. However, unless one looks with extreme 

care, both naticid and muricid holes would be 
10 PREDATION OF SUB-HOLOCENE SCAPHOPOD MOLLUSKS FROM SOUTHERN LOUISIANA 

categorized as "circular''; they appear drilled in the 

sense of a circular shaft penetrating the shell. The holes 

that we have investigated in the Dentalium shells are 

distinctly oval and are more strongly oval on the interior than on the exterior shell surface. Perhaps the uniform crosslamellar shell structure affects the direction 

of penetration. If so, we would expect to see a difference between the shape of boreholes on shells of 

pelecypods and gastropods that have thick crosslamellar structure and those that do not. Such a difference 

has never been reported, and we see no evidence of it 

in the borings in other mollusks of the "mud-lump" 

fauna. 

The simplest explanation for the oval shape is 

geometric. On most gastropod and pelecypod prey, the 

surface that is bored is essentially uniformly curved in 

all directions; thus, the ABO and radula are acting on 

a sphere. In contrast, the logarithmic curvature of the 

scaphopod shell is slight. In the longtudinal direction of 

a boring there is virtually no change in curvature; thus, 

the drilling is into a cylinder. Because the odontophore 

and radula in the mouth of the snail are not circular organs, they sweep laterally to approximate a circular 

shape. This would be more difficult and probably less 

efficient in penetrating the cylinder. The longitudinal 

ribs may also affect the shape of the hole, but their effect is more likely to be secondary. The area between 

them is flattened, and the ABO can be positioned easily. When a hole does impinge upon a rib, the margin 

of the hole immediately adjacent to the rib is very little 

affected. 

The physical relationship of prey to predator is not 

obvious, for no one has observed a scaphopod being 

bored. Because the long axis of the drill hole is alined 

with the shell length of the scaphopod tube, probably 

the gastropod was also alined in this general direction. 

The lateral margins of the foot could grasp the 

scaphopod tube. We speculate that the prey commonly 

attacked the scaphopod when it was in its life position; 

that is, with the apex slightly above the mud and the 

aperture and most of the tube below the substrate. The 

concave side of the tube is dorsal. 

We examin~d the apical area of the scaphopods in 

hopes of finding some staining or epizoan growth that 

might give data on life position; Tenery and Rowett 

(1979) have noted the presence of epizoans on the ventral apical area of a giant Pennsylvanian scaphopod. Unfortunately, we have found no features to suggest how 

deeply the aperture might have been buried. N evertheless, from what is known of the life habits of living 

scaphopods, most of the shell must have been below the 

sediment-water interface; we cannot document a single 

boring that may have been far enough toward the apex 

to have been above the sediment. In oyster predation 

by muricids (Carriker, 1955, p. 51), the prey oyster is 

above the substrate, and the valve is free of sediment. 

Some muricids capable of drilling occasionally do crawl 

on soft bottoms but seem never to drill while on such 

a bottom. The position of the boring relative to the 

within-sediment position of the scaphopod is nearly as 

strong support of naticid predation as is the morphology of the hole. In this connection, it is interesting to 

note that the gastropod Turritella, many species of 

which live as part of the shallow infauna, also shows a 

high frequency of predation at the "mud lumps." 

Four naticid species are known from the "mud lumps." 

The number of specimens and height of largest specimens in the Tulane collection is: N everita duplicata 

(Say), 7 specimens, largest, 25 mm; Natica marochiensis (Gmelin), 55, largest, 15 mm; N. menkeana Philippi, 

77, largest, 9 mm; and N. canrena (Linnaeus), 33, 

largest, 26 mm. None of the holes examined show morphologic differences. Either one species was responsible 

for all predation, or, more likely, all four presumed predators produced a borehole having essentially the same 

morphology. The larger borings seen in the Dentalium 

shells could have been the work of N everita duplicata 

and N atica can rena, the larger predators in the fauna, 

but apart from this, possible predator cannot be related 

to prey. Each species could have its own unique way of 

attacking Dentalium and thus its preferred position for 

boring. Perhaps some of the scatter in circumferential 

position of the borings is due to the preference of one 

species of predator for boring ventrally. During ontogeny, the predator could also have changed its preferred position for drilling, though this seems unlikely. 

§lEUC'JI'IION OIF JP>JF&JEJD)A 'JI'IION §II'JI'lE 

Although borings may occur anywhere along the shell 

circumference, the dorsal side is preferred. The more 

significant preference seems to be laterodorsal about interspaces 2, 3, and 6, 7, of D. (Dentalium) laqueatum; 

this same polarity seems to occur in all rib-number variants of the species. In this position, part, if not most, 

of the shell of the predator gastropod would be near or 

above the sediment-water interface. More borings also 

are found on the observer's right side of the aperture. 

Although the shell of the naticid would not be likely to 

come in contact with the scaphopod tube, the choice of 

bore site on the right side of the aperture would insure 

that the axis of the snail was inclined away from the 

tube, whereas if the predator positioned itself on the 

left side, its shell might be in contact with that of the 

scaphopod. 

The naticid could have crawled from apex to aperture, or it could have crawled upward along the tube. 

Movement in the latter direction would provide less 
SELECTION OF PREDATION SITE 11 

chance for the shell of the predator to come in contact 

with the more strongly curved apical area of the 

scaphopod. It would also provide a less curved surface, 

in a longitudinal direction, for the snail to drill. Large 

holes show asymmetry between the narrow ends of the 

oval. In most holes, the side toward the apex is more 

steeply inclined than the side toward the aperture. We 

have been unable to find any accounts of relationship of 

hole geometry to borehole position. The mechanics of 

naticid boring (Zeigelmeier, 1954; Fretter and Graham, 

1962, p. 243-250) suggest that the anteriorward slope of 

the hole should be steeper. In general, the shape and 

movement of the odontophore (Carriker and Van 

Zandt, 1972b) should promote some slight asymmetry 

of the hole, regardless of the geometry of the prey 

shell. Allowing for this, the asymmetry of the hole 

would suggest that the predator crawled along the tube 

toward the apex. 

In the present sea bottoms, naticids seemingly manipulate the valves of pelecypods to a preferred orientation before boring; observation is greatly hampered by 

the infaunal habit of the gastropods, and most of the information on life activities is anecdotal. If the predator 

is strong enough to move the prey, there need not be 

any correlation between life position of the prey and 

position of the borehole. One would expect that the 

long narrow tube of the scaphopod would be more difficult to manipulate in the sediment than a more globose 

pelecypod shell. Thus, the correlation between the 

borehole position and life position for scaphopods may 

be stronger than it is for pelecypods. 

Most of the borings are dorsal, as might be expected. 

However, some borings are found in the ventral quadrant. We have noted that the holes in the ventral area 

have a smaller average size than those in the dorsal 

area. One would expect that a small predator, which 

could not manipulate the shell, might find it more difficult to bore upward from the ventral side. However, if 

specimens were exposed during a storm, they would be 

more subject to predation, particularly by small animals 

that could not travel quite so readily through sediment. 

The large number of broken shells and some worn fragments at the "mud lumps" suggest that from time to 

time the bottom may have been stirred by strong 

waves or currents. 

A more speculative point is the relationship of prey 

size to borehole size. The mechanics of naticid predation 

have not been as exhaustively studied as have those of 

the commercially significant muricids (Carriker and 

VanZandt, 1972b). Among the muricids, the larger the 

snail, the larger the prey, and the larger the animal 

overall, the larger its ABO (Carriker and Van Zandt, 

1972a). Presumably, larger naticids bore larger holes 

than smaller ones. In general, the diameter of the 

scaphopod where bored is related to the size of the hole 

and hence perhaps to the size of the foot of the prey. 

This relationship is irrespective of the total length of 

the prey. We infer that the naticid crawled along the 

scaphopod tube until it reached a point where the foot 

was conveniently situated, possibly extending part way 

around the circumference. 

The number of individuals bored among scaphopods 

less than 20 mm long is significantly less than that 

among larger individuals. The smaller predatory gastropods probably preferred the small infaunal bivalves, 

which constitute the largest molluscan element of the 

"mud-lump" fauna, to small scaphopods. Several smallsized bivalve species common in the "mud lumps" have 

a high percentage of their valves bored. The low percentage of C adulus specimens bored may be a result of 

the small size of this genus. 

The longitudinal position of the borehole shows a 

much stronger locational preference by the predator 

than does the radial position. The position of borings 

averages nearly 45 percent of the distance between 

apex and aperture in the Washington collection and 54 

percent in the Tulane collection. We suggest that this 

positioning is closely related to the anatomy of the 

scaphopod. That the organism is capable of considerable 

retraction is demonstrated by its anchoring of the extended body and subsequent pulling of the shell forward as it burrows into the substrate (Dinamani, 1964a; 

Trueman, 1968). The wiping of the captacula on the infolded edge of the mantle is further evidence that the 

mantle edge is not attached near the aperture. We do 

not know of any anatomic studies of D. (Dentalium) 

laqueatum, but the anatomy of D. entalis Linnaeus, 

studied by Morton (1959), may be representative of the 

group. Morton has indicated that most of the organs 

are in the middle third of the tube. The apical area is 

lined with mantle tissue, but other organs are not present. 

The ability of the scaphopod to retract its soft parts 

a long distance within the tube also may explain why 

the predator gastropod does not simply reach into the 

aperture to eat the soft parts. Muricid gastropods have 

a long proboscis, whereas naticids have a short one; if 

the predator had been a muricid, it might have been 

able to reach the soft parts through the aperture. 

_Perhaps the feeding habits of the naticids are so 

stereotyped toward boring that they are unable to recognize a free meal, but more likely the shortness of the 

proboscis prevents them from attacking at the apertures. The short proboscis may also explain in part why 

the hole is finished neatly, rather than left with a projecting shelf as is characteristic of muricid predations. 

A few studies have been made of the position of 

naticid borings on prey, the latest being that by Berg 
12 PREDATION OF SUB-HOLOCENE SCAPHOPOD MOLLUSKS FROM SOUTHERN LOUISIANA 

(1976). Geometry of the prey is often cited as having an 

influence on selection of the drilling site. Because 

scaphopod shells of varying size show little variation in 

the position of the hole, we conclude that the longitudinal position of the boring is related to the presence of 

soft parts. If the boring were related to the geometry 

of the shell, the radial position of the hole would be as 

constant as the longitudinal position. Because there is 

such a close relationship of the borings to the position 

of the organs inside the tube, we speculate that somehow the predator can sense, perhaps by slight vibration 

of the shell caused by retraction of the mantle, movement of the enclosed soft parts and that it can thus locate a suitable site for drilling. This speculation should 

be tested with other naticid prey, as Berg (1975, p. 421) 

suggested that "internal anatomy seems less important 

in determining borehole position." 

OTHER CONSIDERATIONS 

Examination of part of the collection of present-day 

scaphopods in the National Museum of Nat ural History, 

Washington, D.C., shows that predation is not distributed at random. Occasional specimens may be bored, 

but the predation rate is seldom higher than 1 percent. 

However, collections of recent D. (Dentalium) 

laqueatum show a high degree of predation, which at 

some localities approximates that found in the "mud 

lumps." We have found a good likelihood of a moderate 

to high degree of predation in a sample consisting of 

coarsely ribbed species. In contrast, smooth or very 

finely ribbed forms are almost never bored. Louie 

Marincovich (oral commun., 1978) has provided a striking example of another possible relationship. He has 

noted that nearly 50 percent of Pleistocene D. neohexagonum Pilsbry and Sharp at San Pedro, Calif., are 

bored. The sediment in which this six-ribbed species occurs is relatively coarse sand; judging from the literature, it is similar to the shell pavement seen in the 

"mud lumps." 

We know of no studies of the relationship of ribbing 

in scaphopods to sediment size. Future studies might 

show a relationship between the coarsely ribbed taxa 

and a slightly coarser than normal sediment, relative to 

the finely ribbed or smooth form. A smooth tube should 

be best suited for penetration into sticky fine-grained 

sediment, whereas the external flanges of a strongly 

ribbed tube would aid in the penetration and anchoring 

of the scaphopod on a coarser grained, less well sorted 

sediment. Depth information is given with collections of 

modern scaphopods, but data are lacking regarding sediment size where such scaphopods have been dredged. 

The occurrence of naticid gastropods is at least related 

in part to sediment size of the substrate, as these 

predators cannot move through sticky sediments. 

Boreholes may form a crude "tracer" for patches of 

coarse-grained sediment. If so, an abundance of borings 

at an outcrop could give some data on the original size 

composition of the sediment. 

In a review of predation by gastropods through time, 

Sohl (1969, p. 725) noted that bored tests are known in 

"such diverse groups as the Foraminifera, Brachiopoda, 

Gastropoda, Pelecypoda, spirorbid polychaetes, 

echinoids, and ostracodes." To the best of our knowledge, the only report of gastropod predation on 

scaphopods was a brief mention of a single specimen 

from 283 fathoms that contained a naticid boring 

(Fankboner, 1970). This study documents that at some 

localities scaphopods may form a food resource and may 

indeed be subjected to significant predation by naticid 

gastropods. 

The survey of fossil scaphopods indicates that they 

are not a newly acquired prey of naticids. We have not 

considered Cadulus and other members of the 

Siphonodentalidae in any detail in part because the 

small size of the individuals makes them more difficult 

to examine. Possibly in the Cretaceous, where we have 

observed a few bored Cadulus, this form was attacked 

because it is in the same general small size range as ostracodes, which also were bored. 

Morphologic change within the Scaphopoda is not 

well known, but Paleozoic forms are all smooth or finely 

ribbed. Early Mesozoic forms are even less well known, 

but coarse-ribbed scaphopods do occur. in the Cretaceous at about the same time that boring naticid gastropods appeared (Sohl, 1969). There is no obvious 

coevolution between prey and predator. One might 

think that, in the face of predation, the adaptation of 

relatively few coarse ribs would have died out rather 

than persisting. It may be worthwhile to examine the 

Mesozoic and Cenozoic record of the class from the 

standpoint of functional morphology to see when the 

coarse-ribbed forms first appeared and whether they 

have diversified through time. 

One may summarize by noting that this study, like so 

many other scientific inquiries, answers certain questions but poses more interesting ones. 

REFERENCES CITED 

Abbott, R. T., 1974, American seashells (2d ed.); New York, Van 

Nostrand-Reinhold, 663 p. ' 

Berg, C. J., Jr., 1975, A comparison of adaptive strategies of predation among naticid gastropods [abs.]: Biological Bulletin, v. 149, 

p. 420-421. 

___ 1976, Ontogeny of predatory behavior in marine shails 

(Prosobranchia: Naticidae): Nautilus, v. 90, no. 1, p. 14. 

Carriker, M. R., 1955, Critical review of biology and control of oyster 
REFERENCES CITED 13 

drills Urosalpinx and Eupleura: U.S. Fish and Wildlife Service 

Special Scientific Report-Fisheries 148, 150 p. 

____ 1961, Comparative functional morphology of boring 

mechanisms in gastropods: American Zoologist, v. 1, no. 2, p. 

2~266. 

____ 1969, Excavation of boreholes by the gastropod Urosalpinx-an analysis by light and scanning electron microscopy: 

American Zoologist, v. 9, p. 917-933. 

____ 1970, The shell penetrating mechanism of the "oyster 

drill" Urosalpinx, in [Proceedings of Conference on Shell Fish 

Culture, Suffolk County Community College, April ~9, 1968]: 

[Long Island, N.Y., Regional Marine Resources Council], p. 61-

68. 

Carriker, M. R., and VanZandt, Dirk, 1972a, Regeneration of the accessory boring organ of muricid gastropods after excision: American Microscopical Society Transactions, v. 91, no. 4, p. 455-466. 

____ 1972b, Predatory behavior of a shell-boring muricid gastropod, in Winn, H. E., and Olla, B. L., eds., Behavior of marine 

animals: New York, Plenum Press, v. 1, p. 157-244. 

Carriker, M. R., Williams, L. G., and VanZandt, Dirk, 1978, Preliminary characterization of the secretion of the accessory boring 

organ of the shell-penetrating muricid gastropod Urosalpinx 

cinerea: Malacologia, v. 17, no. 1, p. 125--142. 

Dinamani, P., 1964a, Burrowing behavior of Dentalium: Biological 

Bulletin, v. 126, no. 1, p. 28--32. 

____ 1964b, Feeding in Dentalium conspicuum: Malacological 

Society of London Proceedings, v. 36, pt. 1, p. 1--5. 

Fankboner, P. V., 1970, Naticid predation on Dentalium complexum 

Dall (Dentalidae, Scaphopoda) [abs.], in The biology of molluscs; 

a collection of abstracts from the National Science Foundation 

graduate research training program: Hawaii Institute of Marine 

Biology Technical Report 18, p. 5. 

Fretter, Vera, and Graham, Alastair, 1962, British prosobranch molluscs--Their functional anatomy and ecology: London, Ray Society, 755 p. 

Haas, Winfried, 1972,. Micro- and ultrastructure of Recent and fossil 

Scaphopoda: International Geological Congress, 24th, Montreal, 

1972, [Report], Section 7, p. 15-19. 

Henderson, J. B., 1920, A monograph of the east American 

scaphopod mollusks: U.S. National Museum Bulletin 111, 177 p. 

Morgan, J. P., Colman, J. M., and Gagliano, S. M., 1968, Mudlumps-Diapiric structures in Mississippi delta sediments, in 

Diapirism and diapirs-A symposium: American Association of 

Petroleum Geologists Memoir 8, p. 145-161. 

Morton, J. E., 1959, The habits and feeding of Dentalium entalis: 

Marine Biological Association of the United Kingdom Journal, v. 

38, p. 225-238. 

Palmer, C. P., 1975, A new Jurassic scaphopod from the Oxford Clay 

of Buckinghamshire: Palaeontology, v. 18, pt. 2, p. 377-383. 

Sohl, N. F., 1969, The fossil record of shell boring by snails: American Zoologist, v. 9, no. 3, ed. 2, p. 725-734. 

Tenery, J. H., and Rowett, C. L., 1979, New specimens of Prodentalium raymondi Young (Scaphopoda) from the Pennsylvanian of 

Texas: Journal of Paleontology, v. 53, no. 1, p. 164-168. 

Trueman, E. R., 1968, The burrowing behavior of Dentalium 

(Scaphopoda): Journal of Zoology (London), v. 154, pt. 1, p. 19-

27. 

Warmke, G. G., and Abbott, R. T., 1961, Caribbean seashells: Narberth, Pa., Livingston Publishing Co., 346 p. 

Yancey, T. E., 1973, Apical characters of Prodentalium from the 

Permian of Malaya: Malaysian Journal of Science, v. 2(B), p. 145-

148. 

Ziegelmeier, Frich, 1.954, Beobachtungen tiber den Nahrungserwerb 

bei der Naticide liunatia nitida Donovan (Gastropoda, Prosobranchia): Helgolande-r Wissenschaftliche Meeresuntersuchungen, v. 5, no. 1, p. 1-33. 
r' 
PLATES 1-4 

Contact photographs of the plates in this report are available, at cost, from the U.S. Geological 

Survey Library, Federal Center, Denver, CO 80225. 
PLATE 1 

Figures 1-4. General views of D. (Dentalium). 

1. Dorsal-side view of a seven-sided variant of D. (Dentalium) laqueatum Verrill, 4.5 em in chord length; the borehole has 

a maximum length of 4.9 mm. x 5, USNM 780591a. 

2. Left-side view of a six-sided variant, D. (Dentalium) laqueatum Verrill, that has a seventh intercalated rib, shown to 

the left appearing some distance below the broken apical area. x 5, USNM 780591b. 

3. Left-side view of D. (Dentalium) obscurum Dall, a nine-sided variant, 3.1 em in chord length; the borehole has a 

maximum length of 5.8 mm. x 5, USNM 780952a. 

4. Part of a six-sided variant, D. (Dentalium) laqueatum, 4.5 em in chord length, showing a complete boring, an incomplete 

boring, and, to the left, some damage by a boring sponge. x 15, USNM 780591c. 
GEOLOGICAL ~l VEY 

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DEl\TTALJ[ TM (DENT ALI UM) 

P!WFE~S!O:\AL PAPER 12:-i2 PLATE 1 

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PLATE 2 

Figures 1--6. Scanning electron microscope photographs of a boring in a six-sided variant of D. (Dentalium) laqueatum Verrill, USNM 

780591d. 

1. Vertical view of "half hole" specimen, broken at the point of drilling. x 60. 

2. Vertical view of the boring, showing the fine reticulate surface ornament extending to the margin of the boring without 

any etched zone. x 150. 

3. Enlargement of the upper surface seen in figure 2, showing the relationship of crosslamellar structure to the ornament 

and emphasizing the lack of an etched zone. x 270. 

4. Slightly oblique view, showing the steep but gently concave curved wall of the boring. x 150. 

5. Detail at the inner margin of the boring. x 400. 

6. Detail of shell structure of the fracture surface at the juncture of outer and inner shell layers shown in figure 4. x 1000. 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 1282 PLATE 2 

DENTALIUM (DENTALlUM) 
PLATE 3 

Figures 1--6. Broken and repaired shells and apertural views of D. (Dentalium) laqueatum Verrill. 

1. A "half hole" between two ribs of a seven-sided variant. x 15, USNM 780591e. 

2. Apertural view of a seven-sided variant broken at the apex; compare with the thickened ribs shown in figure 1. x 15, 

USNM 780591f. 

3. Apertural view of a large seven-sided variant with reduced ribs, 4.9 em in chord length. x 15, USNM 780591g. 

4. Left-side view of a seven-sided variant, with a single break, 3. 7 em in chord length. x 5, USNM 780591h. 

5. Right-side view of seven-sided variant with two breaks, 3.2 em in chord length. x 5, USNM 780591i. 

6. Slightly oblique ventral view of a seven-sided variant with four breaks, 5.0 em in chord length; this is the only specimen 

that deviates from logarithmic curvature. x 5, USNM 780591j. 
GEOLOGICAL SUH\'EY 

I'ROFESSJOl\AI. I'A PEH l:!k:! PLATE :l 

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DENTALIC.W (DENTAL!['MJ 
PLATE 4 

Figures 1-3. Ontogenetic change in the apex of D. (Dentalium) laqueatum Verrill. All specimens x 15. 

1. A small keyhole in a seven-sided variant, 3.6 em in chord length; USNM 780591k. 

2. A V -shaped reentrant in a six-sided variant, 5.1 em in chord length; USNM 7805911. 

3. A prominent slit in a six-sided variant, 5.3 em in chord length; USNM 780591m. 
GEOLOGICAL Tl-\VEY I'ROF'ESSIONAL PAPER 121>2 PLATE .J 

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DE"A'TALICM (DE"A'TALJ[TM)