Archives: Fall 2000/Winter 2001 Table of Contents
A Shell is Worth a Thousand Maps
by Tracey Crago, WHOI Sea Grant

They may not have Boston accents, but soft-shell clams spawned in Beantown waters retain traces of that water in their shells and, thanks to state-of-the-art tools and the work of WHOI researchers, it is now detectable.

A collaborative project involving WHOI biologists and geochemists has resulted in a promising method for identifying the source habitat for soft-shell clams–right down to the specific geographic location of their spawning grounds. As with many of nature’s wonders, the answer to where clams–and presumably other bivalves–originate has been there all along, in their shells.

With WHOI Sea Grant support, biologists Lauren Mullineaux and Susan Mills recently completed a round of controlled experiments in which hatchery-spawned soft-shell clam embryos (Mya arenaria) are incubated in clean seawater (the control) and seawater spiked with trace elements. Within 36 hours, the larval shells are formed. The shells are then collected and analyzed for evidence of trace metals, including lead, cobalt, silver, and cadmium.

That analysis takes several weeks to prepare for and is made possible using two tools: the Inductively Coupled Plasma Mass Spectrometer (ICP/MS for short), and the ion microprobe. Each has a specialty: the ICP/MS can measure bulk samples and gives results for different elements at the same time, whereas the ion microprobe can more precisely detect the location in the shell of each element, though it can only look for one element at a time.

According to Mullineaux, it was Stan Hart’s use of these instruments to study records of environmental change in adult bivalves and corals that inspired her to try using them to look at larval and juvenile bivalve shells. Hart, a geochemist, is a senior scientist in WHOI’s geology and geophysics department.

Because the ICP/MS requires 20—40 larval shells per sample, each sample takes Mills approximately 10 days to prepare. Once the larval shell has been deposited, the larvae must be filtered, air dried, and bleached to remove any tissue. The shells are then refiltered and dissolved in acid and, with assistance from geochemist Jerzy Blusztajn, run on the ICP/MS. To date, results indicate that concentrations of selected trace elements–lead and cobalt–are elevated in larval shells spiked with those elements. However, other elements–silver and cadmium–have yielded no distinctive results. One explanation, says Mullineaux, is that the clams simply do not pick up these elements in their larval and juvenile shells.

Circles represent varying levels of lead concentrations in sediments along the Massachusetts coastline, as mapped by the U.S. Geological Survey in 2000. In a nice correlation, Mullineaux and Mills found that clams collected from Neponset Harbor (see arrow), showed lead levels 10 times higher than in clams collected in Barnstable Harbor.

The other analysis tool, the ion microprobe, has proven very valuable for looking at field-collected juvenile clams, and allowed researchers to compare clams from a known polluted area, Neponset Harbor, just south of Boston, to clams from a relatively clean harbor, Barnstable Harbor on Cape Cod. Mullineaux and Mills saw lead levels 10 times higher in Neponset clams than those from Barnstable, and a cobalt reading twice as high in Neponset as Barnstable. As a bonus, their results dovetailed nicely with a U.S. Geological Survey map of trace element distributions in sediments along the Massachusetts coastline.

Preparing samples for the ion microprobe requires that the shells be embedded in epoxy. This in itself is time consuming, because the tiny shells get lost and break easily. Once mounted, the epoxy is sectioned and polished. Focusing micro-beams of ions on the larval shell, the microprobe gathers very sensitive trace-element analyses for each sample. Though they have used the ion microprobe primarily for analysis of juvenile shells, Mullineaux and Mills also used it to analyze some of the spiking experiment larvae for lead and cobalt uptake; results were similar to those from the ICP/MS analysis.

In addition to the Neponset and Barnstable clams, researchers plan to sample juvenile clams from New Bedford, Mass., a Superfund site known for its heavy metal contamination. They expect to find high levels of various elements, due to contamination from manufacturing plants near the harbor and also because the harbor’s hurricane barrier tends to retain the metals within the harbor that might otherwise be flushed out.
Mullineaux and Mills will soon begin another important component of the project: estimating the maternal contribution of trace elements in the larval shell. They hope to determine whether the mothers are passing on the trace elements to the larvae, or if the larvae are soaking up the trace metals from their environment. Of course, it could be a combination of the two. Mills says that while they think that the mothers contribute a fair amount of trace elements to the shell, it will be worth the effort. "Even though the mother and the larvae are from the same environment," she says, "it will be good to be able to show the maternal contribution to analyze the results of the spiking experiments." To determine the maternal contribution, they will collect "ripe," or ready to spawn adult clams from Neponset and Barnstable. They will then repeat the spiking experiment using the field-collected clams and repeat the analyses with both the ICP/MS and the ion microprobe.

If in fact the larval shell of soft-shell clams incorporates trace elements from a specific geographic location, it will serve as a "fingerprint," and that fingerprint could be used as a marker. This has important implications for researchers in several related areas. "The ability to identify the source habitat of larvae will give researchers a powerful, direct tool for tracking larval dispersal in coastal waters," says Mullineaux, "and for understanding the interaction of hydrodynamics and behavior in the dispersal process." But researchers aren’t the only beneficiaries; coastal managers and regulators, take note. Mullineaux believes that the tool could also be used to "track the export of pollutant-impacted organisms and predict the potential effects on neighboring communities."

So tourists beware: accent or not, Cape Cod clams will know when their Boston counterparts come to town.

Image credit: Massachusetts coastline, USGS Gulf of Maine Database; graph, Lauren Mullineaux et al., WHOI