U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2004–3141

February 2005

Shells as Recorders of Environmental Change—A Study in Florida Bay

Printed on recycled paper

Introduction

The South Florida region is currently undergoing a massive restoration effort guided by the Comprehensive Everglades Restoration Plan 

(CERP) written by a multi-agency group of Federal, State and local governments. One of the primary goals of the CERP is to restore the quantity, quality, and timing of freshwater delivery throughout South Florida 

to a more natural state. The natural flow of freshwater in the South 

Florida ecosystem was altered throughout the 20th century by the construction of a series of canals and water-control structures. These systems 

affected the estuaries of South Florida, including Florida Bay (located 

within Everglades National Park), by altering the natural seasonal mixing 

of fresh and saline waters. In addition, construction of the Flagler Railroad and, later, U.S. Highway 1 through the Florida Keys restricted the 

exchange of water between the Atlantic Ocean and Florida Bay. In order 

to establish realistic goals and performance measures for restoration, it 

is essential to first determine how the natural system functioned prior 

to significant human alteration. The goal of this research is to develop 

a method to identify historical patterns of salinity change throughout 

the bay, perhaps at the seasonal scale, from the remains of animals 

preserved in shallow-water sediment cores.

Setting

Florida Bay (fig. 1) is a very shallow, subtropical estuary that is 

subdivided by a complex mosaic of mudbanks and mangrove keys 

(islands) that constrain the movement of tides and currents. Variations 

in salinity and temperature within the bay are largely controlled by 

the annual cycle of wet and dry seasons. The cooler winter months of 

November to April constitute the dry season (average daily precipitation is approximately 0.075 inches). During the hot, wet summer 

months of May through October, precipitation increases three- to fourfold (average daily precipitation is 0.25 to 0.30 inches). 

The natural precipitation cycle, though annually variable, drives the 

timing and amount of natural freshwater available to the South Florida 

ecosystem. Low regional rainfall during the winter reduces the freshwater input, both as direct rainfall and as reduced runoff from the terrestrial Everglades, into the bay. This decrease in freshwater raises the 

salinity in the bay, especially along the northern margin where terrestrial 

runoff from the Everglades enters the bay. During the wet season, the 

abundance of regional rainfall, both as direct precipitation into the bay 

and as increased outflow from the terrestrial Everglades, increases the 

freshwater input to the bay and decreases the salinity. The result of these 

seasonal changes is that salinity can vary significantly in northern and 

central Florida Bay. These changes create a rigorous environment in 

which only very adaptable organisms can survive.

Marine Animals as Data Recorders

Information on the regional variation and seasonal changes in the 

water chemistry of Florida Bay is reflected in the variations in the shell 

chemistry (trace elements and stable isotopes) of marine organisms. 

As the animals grow by adding sequential layers of calcium carbonate, 

they are recording information about the water in which they grow. By 

learning how to “read” the record contained in the shells, scientists can 

derive information about both modern and historical changes in salinity, temperature, and freshwater sources.

In order to interpret the shell chemistry data, the following questions must be answered: (1) What are the spatial and temporal patterns 

of the animal’s growth and how are these patterns related to seasonal 

change? (2) Does the temporal growth record coincide with seasonal 

variations in salinity, temperature, or some other environmental variable? (3) Can the chemical variations of individual shell layers be 

accurately measured at a microscopic scale? (4) Are the variations in 

the trace-element concentration of the shell layers the response to a 

biological function of the animal that is independent of changes in the 

water mass, or are the variations representing changes in salinity and 

temperature while the shell is secreted? (5) If the trace-element variations of the shell correlate to salinity and temperature, then can these 

variations be calibrated and used as a proxy to represent changes in the 

water mass through time?

For this study, the marine bivalve Chione cancellata (commonly 

known as the Cross-barred Venus) was selected because it met these 

important criteria necessary to serve as a good “data recorder” of salinity change: (1) the species is widely distributed and locally abundant 

throughout Florida Bay; (2) although it prefers marine conditions, it is 

tolerant of a range of salinities from 20 to >40 parts per thousand (ppt); 

(3) it is comparatively long-lived (estimates of 5-7 years for the oldest 

specimens); and (4) its thick, ornamented shell facilitates specimen 

preparation, identification of seasonal growth, and accurate tracing of 

growth layers for geochemical analysis. 

Once the basic questions have been answered from studies of living Chione cancellata in both Florida Bay and the laboratory setting, 

a calibration data set can be established that will be used to compare 

the animal’s shell chemistry with the chemistry of the water in which 

it grew. The calibration data can then be applied to the analysis of 

shells taken from isotopically dated sediment cores collected in Florida 

Bay and other South Florida estuaries as part of the U.S. Geological Survey’s Ecosystem History projects. The results will provide the 

regional and temporal patterns of change in salinity over the last 100 to 

500 years, which is necessary to set realistic and obtainable performance measures for restoration.

Field Work

In February 2001, scientists placed two habitat enclosures in Whipray Basin, an isolated basin in central Florida Bay (figs. 1, 2). The location was established because of the close proximity to an Everglades 

National Park water monitoring station that records daily temperature, 

salinity, and precipitation. One hundred Chione cancellata specimens, 

collected locally, were labeled, digitally photographed, and placed in 

each habitat.

Everglades National

Park

Whipray Basin

Florida Bay Gulf of

Mexico Atlantic Ocean

Water Depth

0 5 10 Miles

> 3 feet

< 3 feet FLORIDA

Study

Area

Figure 1. Satellite image of southernmost Florida showing Whipray 

Basin, where two growth habitats were placed in order to study Chione 

cancellata. Most of Florida Bay lies within Everglades National Park 

(blue dashed line). Image from Jones and others (2001).
Since February 2001, the specimens have been retrieved on a regular winter and summer schedule and have occasionally been retrieved 

in the spring and fall to provide additional data. Each specimen, 

whether dead or alive, was compared to its previous photograph to 

determine when and how much growth had occurred. All live specimens that exhibited growth were digitally rephotographed prior to their 

replacement in the field habitat. Dead and lost specimens were replaced 

with new labeled and photographed individuals to maintain a statistically significant population of approximately 100 individuals.

When do Chione cancellata grow?

Preliminary data from field growth studies indicate that Chione cancellata from Florida Bay can provide the necessary seasonal growth record 

over time for shell-chemistry analysis. Data analyzed over three years 

indicate that the habitat population grew in response to the seasonal water 

temperature cycle (fig. 3). The preliminary data indicate that during those 

months when the water mass is cool to cold (fall, winter, and early spring), 

over 80 percent (typically over 95 percent) of the population did not exhibit 

growth; however, data from the late spring and summer indicate that 40 

to 47 percent of the individuals had measurable valve growth. During 

this period, both water temperature and salinity increased from the winter 

minimums to late spring and summer maximums.

Data also indicated that juvenile and young adult individuals represented the largest part of the population that exhibited growth (fig. 4). 

Image analysis of digital photographs of the specimens with confirmed 

growth indicated that juveniles accounted for 25 percent of the population and young adults accounted for 20 percent of the population; 

however, “old-age” individuals accounted for less than 1 percent of 

this population. In fact, some “old-age” specimens in the study did not 

grow for 2 to 3 years. In general, small specimens showed more continuous, sequential annual growth, while larger individuals grew more 

episodically in 2- to 3-year increments. 

These combined data indicate that as Florida Bay water temperatures 

and salinities increase from winter minimums to late spring and summer 

maximums (fig. 5B), 40 to 50 percent of the Chione cancellata population 

of Florida Bay show growth. Of this population, a majority of juvenile 

and sub-adult individuals will secrete biogenic calcium carbonate, which 

records the physical and chemical changes in the water mass.

How do Chione cancellata grow?

Chione cancellata have a morphology similar to many other molluscan bivalves. The first and most obvious common feature is the 

two-part valve structure, which is characterized by a thin, relatively 

featureless inner layer of aragonite, and a thicker, outer prismatic layer 

of subparallel, anastomosing calcite crystals (fig. 6). The second common feature, which is more difficult to detect, is the presence of parallel laminae secreted at the shell margin, which represents incremental 

growth. The laminae are approximately 1 μm (micrometer) thick in the 

inner aragonite layer and thicken to 5 to 7 μm in the outer calcite layer. 

Throughout the Chione cancellata’s life history, changes in the 

physical environment (seasonal temperature, salinity, food supply, and 

turbidity), together with the individual’s age, affect the timing and rate 

of shell growth. The outer prismatic layers show successive cycles 

of growth in the form of groupings of laminae (fig. 6). These groups 

of laminae are set apart by both distinct and subtle punctuations, or 

“breaks,” that represent cessation of calcium-carbonate secretion; therefore, a missing period of time in the shell’s “data recorder” is created. 

Three different scales of cyclical groupings, or bundles of laminae, can 

be detected that probably represent daily, intraseasonal, and seasonal 

growth history.

How do scientists detect change in shell layers?

Microbeam instruments, such as the electron microprobe (EM), are 

designed to measure the composition of extremely small amounts of 

material. The small probe diameter (10 μm) and close spatial sampling resolution (5-50 μm) of these instruments make them ideal for 

measuring the compositional changes at or near the micrometer-scale 

thickness of the shell-growth laminae. The EM performs both quantitative and semiquantitative analyses by detecting characteristic X-rays 

Figure 2. Placement of specimens in Whipray Basin habitat.

0

10

20

30

40

50

60

70

80

90

100

Aug.

2001

Feb.

2002

May

2002

Oct.

2002

Feb.

2003

Dec.

2003

Aug.

2003

Aug.

2002

Growth

Habitat re-collection dates and number of individuals (n)

No Growth

Percent of population, Whipray habitats

n=45 n=253 n=189 n=93 n=66 n=176 n=140 n=86

0

200,000

400,000

600,000

800,000

Number of specimens measured

Original specimen size

New growth measured

after six months

1 5 10 15 20 25 30 35 40

Size of specimen (in pixels)

Figure 3. Histogram illustrating percentage of growth versus no growth 

in Chione cancellata populations in Whipray Basin habitats. The population shows a response to the seasonal climate cycle. Between 40 

and 47 percent of the individuals retrieved during the summer showed 

measurable growth, compared to 1 to 12 percent during the fall and 

winter, with the exception of February 2002.

Figure 4. Histogram showing the preliminary growth analysis of Chione 

cancellata at Whipray Basin from February 2001 to August 2001. All 

specimens were digitally photographed at the same distance and magnification. The histogram shows the size distribution in pixels (determined 

from image-processing techniques) of 40 specimens photographed both 

before and after six months. New shell-margin growth principally occurs 

in smaller juvenile to young adult individuals, while very little growth is 

observed in the larger, mature to “old-age” individuals.
M

g/C

a

Sr/C

a

2/03/01

8/20/01

2/07/02

5/02/02

WRA036.1

WRA036.2

WRA036.3

WRA036.4

Distance from margin (mm) Margin of shell

6 5 4 3 2 1 0

6 5 4 3 2 1 0

WRA036.1

167 samples

WRA036.2

178 samples

WRA036.3

132 samples

WRA036.4

177 samples

WRA036.5

27 samples

WRA036.6

87 samples

WRA036.1

167 samples

WRA036.2

178 samples

WRA036.3

132 samples

WRA036.4

177 samples

WRA036.5

27 samples

WRA036.6

87 samples

2/03/01 8/20/01 2/07/02 5/02/02

2/03/01 8/20/01 2/07/02

5/02/02

Toward umbo

1 mm

Mg/Ca total counts

Sr/Ca total counts

0.004

0.006

0.008

0.010

0.010

0.015

0.020

0.025

#WRA036

Temperature, oC

Salinity, ppt

Temperature (bottom, oC)

Salinity (ppt)

2001 2002

10

20

30

40

50

m

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Jan Feb Mar Apr May June

m

Everglades National Park

Whipray Basin water monitoring station

Growth 2/7/02

to 5/2/02

Growth 8/20/01

to 2/7/02

Growth 2/3/01

to 8/20/01

2/3/01

8/20/01

2/7/02

5/2/02

B

C

D

A

WRA036.6

WRA036.5

Figure 5. A, Digital photograph of Chione cancellata specimen #WRA036 

showing growth record of the valve from February 3, 2001, to May 2, 

2002. B, Graph showing Whipray Basin water-monitoring record of 

temperature and salinity for dates of growth. C, Electron-microprobe 

(EM) trace-element analysis of this specimen; top graph plots ratio 

of magnesium to calcium; lower graph plots the ratio of strontium to 

calcium. D, Photograph of longitudinal section through the specimen 

showing valve morphology and EM line transects. The 10-μm sample 

spacing provides a high-resolution record of trace-element variation 

through sequential growth record of the specimen. The Mg/Ca counts, 

and to a lesser extent Sr/Ca, show significant variations in concentration 

related to shell morphology. From February to August 2001, both episodic 

shell growth and cyclical variability of Mg/Ca appear to correspond to 

intraseasonal water temperature and salinity cyclicity as water mass 

moves from winter minimums to late spring and summer maximums. 

Between August 2001 and February 2002 (fall and winter), when water 

temperatures and salinity are declining to minimums, trace-element 

variation and shell morphology show little variability. However, as water 

temperatures and salinity again increase through late winter and spring 

of 2002 (February to May), trace-element concentration also increases.
emitted from selected elements of interest in the shell material. As the 

concentration of an element increases in the shell layer, a proportional 

number (or count) of X-rays also increases. 

The middle and lowest graphs on figure 5 illustrate the results of six EM 

line analyses of a highly polished longitudinal section of Chione cancellata

specimen #WRA036 (fig. 5A) from Whipray Basin. The transects, which 

parallel the axis of growth, have a spatial sampling resolution of 10 μm and 

a 10-μm probe diameter, which provides 100 compositional analyses per 

millimeter. This sampling resolution allows scientists to derive seasonal and 

intraseasonal patterns of compositional change from the shell.

What do scientists measure in the shell?

All living organisms either actively or passively sequester trace 

elements from their environment. Marine organisms, like Chione cancellata, incorporate trace elements from the water in which they live 

into the calcium-carbonate structure of the shell. Magnesium (Mg) and 

strontium (Sr) have many physical properties in common with calcium 

(Ca), and thus can easily substitute for calcium in the shell structure. 

The concentration of magnesium and strontium in the world’s oceans 

has remained stable for thousands of years. Both elements also are stable 

relative to the calcium that is a component of sea salt (CaCl); therefore, 

as the amount of salt in the water varies, the amount of magnesium and 

strontium varies. Previous studies (Dwyer and Cronin, 2001, and references therein) have demonstrated that the ratios of Mg/Ca and Sr/Ca in 

organisms with calcium-carbonate shells can be used to calculate the 

salinity of the water in which they grew. In this study, those ratios were 

measured in the EM transects across the shell as shown in figure 5C. 

Considerable questions still exist about the effect of water temperature 

and biological processes on the uptake and secretion of magnesium, 

calcium, and other elements. These questions still need to be answered. 

What do we know so far?

The following points summarize the results to date:

1. Marine organisms (such as Chione cancellata) that secrete 

biogenic calcium-carbonate record information about the water in 

which they live.

2. Close to 50 percent of the Chione cancellata population of 

Florida Bay exhibits measurable growth during the spring and 

summer seasons, which also is the period when salinity and temperature increase from winter minimums to summer maximums.

3. Juvenile and sub-adult Chione cancellata generally exhibit both 

the greatest amount and the most continuous rates of shell growth.

4. Sequentially secreted growth laminae of the Chione cancellata

shell are punctuated by discontinuities due to cessation in growth 

during seasonal and intraseasonal cool to cold water temperatures, 

which means information is missing from the shell “data recorder.”

5. High resolution electron-microprobe analyses of Chione cancellata shells, which have documented growth during specific time 

periods, produce accurate “maps” of changes in trace-element 

concentrations over a given period of time. 

6. The highly variable ratios of magnesium to calcium, and more 

subtly of strontium to calcium, appear to record both a first- and 

a second-order temporal cycle of seasonal and intraseasonal 

changes in water mass related to salinity and (or) temperature.

Will shell analyses be a useful tool for restoration?

Although additional investigation is needed, shell-chemistry 

analyses of Chione cancellata can be used to extract information about 

its environment; however, several cautions are in order. First, Chione 

cancellata do not grow continuously throughout the year; therefore, 

although they are not recording a complete annual history, the preliminary studies indicate that at least 50 percent will show growth during 

the summer maximums in salinity and temperature. Second, if only 

50 percent show growth at any one time, either the results will need 

to be statistically averaged when analyzing specimens from cores, or 

some other method of dealing with the potential lack of growth during 

a given time period must be determined. Third, analyses should be 

limited to juveniles and sub-adult individuals, or to the juvenile and 

sub-adult portions of the shells of the older specimens. 

Whether temperature or salinity is the controlling factor on traceelement uptake is a question that still needs to be answered; however, 

for South Florida, this issue is not critical because increases and 

decreases in temperature and salinity generally parallel each other and 

this relationship can be demonstrated statistically. Hence, the cautious analysis of Chione cancellata specimens from isotopically dated 

sediment cores from Florida Bay and other South Florida estuaries will 

provide information about historical changes in the environment. The 

results of this study potentially could be applied to other species of 

Chione and possibly to other genera of the Veneridae family. 

By Robert G. Stamm and G. Lynn Wingard

References Cited

Dwyer, G.S., and Cronin, T.M., 2001, Ostracode shell chemistry as a 

paleosalinity proxy in Florida Bay, in Wardlaw, B.R., ed., Paleoecologic studies of Florida Bay: Bulletin of American Paleontology 

361, p. 249–276. 

Jones, J.W., Thomas, Jean-Claude, and Desmond, G.B., 2001, South 

Florida Everglades: U.S. Geological Survey Miscellaneous Investigations Series Map I–2742, scale 1:100,000, 2 sheets. [Also available 

online at http://sofia.usgs.gov/projects/remote_sens/sflsamap.html]

For further information, please contact:

100 μm

Prismatic

calcite

layer

Nacreous

aragonite

layer

Specimen

WR181

100 μm

Inter-annual growth band

Toward umbo Toward margin

Outer layer

Inner layer

Figure 6. Scanning-electron microscope photograph of a cross section 

of an etched Chione cancellata valve (specimen WR181) illustrating the 

two-part calcium-carbonate shell structure and the more subtle growth 

laminae sequentially secreted through time. In the outer layer, wavy 

calcite prisms are secreted perpendicular to parallel, nominal ~5-μmthick growth laminae; the laminae form 10- to 20-μm-thick couplets, 

which in turn form variably thick, light to dark, 60- to 120-μm-thick interannual growth bands.

Robert G. Stamm

U.S. Geological Survey

926A National Center

Reston, VA 20192

Telephone: 703-648-5271

email: rstamm@usgs.gov

G. Lynn Wingard

U.S. Geological Survey

926A National Center

Reston, VA 20192

Telephone: 703-648-5352

email: lwingard@usgs.gov