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