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COI Funded Project: Use of Trace Elements in the Larval Shell as a Health Marker of Bivalve Dispersal

Project Duration: 6/1/97-5/31/98
Key Words: bivalve larvae, benthos, chemical fingerprint, dispersal

Progress Report

Objectives and Methods:
Larvae of coastal bivalve species spend as long as 60 days in the plankton, and during this time they can be advected substantial distances in coastal currents. Thus, it is very difficult to determine the origin of bivalve larvae that settle into any particular benthic habitat. This information, however, is critical for understanding how larvae disperse from one habitat to another and how dispersal processes affect recruitment, population dynamics and gene flow.

The main objective of this project was to identify the suite of trace elements that are incorporated from the fluid environment into the initial shell of a larval bivalve, and can provide a unique, location-specific fingerprint of its source habitat. To do this, we used bivalve larvae reared in the laboratory and analyzed the trace elements with the Finnigan Inductively Coupled Plasma Mass Spectrometer (ICP/MS) in the WHOI Chemistry Department The ICP/MS is highly sensitive, making possible, for the first time, analysis of the extremely small amount of material contained in an initial bivalve shell replicates was modified in subsequent trials depending on the variation among replicates.

The soft-shell clam, Mya arenaria, was chosen for these studies because it is locally common, economically important, and ties in with our ongoing field studies of dispersal. We spawned M. arenaria using classical techniques and transfered aliquots of fertilized embryos into beakers of clean, 0.45mm filtered seawater from Vineyard Sound. Each beaker was spiked with one of a suite of trace elements (Ag, Cd, Co, Cu, Ni, Pb, V), in one of three concentrations (low, moderate, and high, as determined from concentrations typically reported from local coastal habitats). Additional beakers with unspiked seawater were used as controls. The beakers were maintained at constant-temperature on an orbital shaker and stirred slowly until the initial larval shell was deposited (approximately 24-36 hours). After depositing the initial shell, larvae from each treatment were filtered through a Millipore apparatus onto an Osmonics nylon filter and air dried. Individual larvae were cleaned of tissue by bleaching in a sodium hypochlorite solution, which also removes any adsorbed trace metals. The shells were refiltered, dissolved in 0.3 ml weak HNO3, and aspirated directly into the ICP/MS with no further manipulation. All the trace elements used had been shown to produce linear calibration curves on the ICP/MS, and were known to vary geographically in coastal sediments.

Results:
Trace element concentrations in the larval shells from one representative experiment show that uptake in the spiked beakers tended to be notably higher than controls for Pb and Co, but only slightly higher for Cd and Ag. Survivorship was high and consistent across treatments, suggesting that none of the spiked concentrations was lethal. Variation among replicates, however, was very high and none of the differences was significant (Mann-Whitney U test, P < 0.05). Nevertheless, we are pleased with these results because they demonstrate two important features of our experimental design - larvae do indeed incorporate the trace elements into their shells, and the ICP/MS is sufficiently sensitive to detect trace elements in the very small shells.
Long-Term Goals
A long-range goal of this project is to pinpoint the specific source location of the larval migrants. This objective requires a more comprehensive catalogue of source-habitat fingerprints, obtained by rearing larvae in seawater collected from multiple sites, and by developing the capability to predict the larval-shell fingerprint directly from seawater analyses. The resulting tool will be useful for researchers investigating larval dispersal, population dynamics and gene flow of bivalves, in addition to shellfish managers and aquaculturists. By analyzing the trace-element composition of a bivalve recruit or juvenile, the researcher will be able to pinpoint the specific source location of recruits. This capability will allow managers to assess the relative contributions of local versus remote populations to annual recruitment, and evaluate the resiliency of bivalve populations to local impacts such as natural predation, shellfishing and pollution.

Two subsequent grants have been funded based on the results of this project:

Office of Naval Research
Use of Trace Elements in the Larval Shall as a Marker of Bivalve Dispersal
S.R. Hart - funded for 1999-2000 ($113,937)

Sea Grant
'Bivalve dispersal as indicated by shell trace element composition'
S.R. Hart - funded for 2000-2002 ($60,000)

Originally published: January 1, 1997