Fall 2000/Winter 2001 Table
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 natures
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 Harts 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 WHOIs
geology and geophysics department.
Because the ICP/MS requires 2040 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.
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
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 harbors 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 arent 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