Depth and timing of
settlement of veligers from different populations of giant scallop, Placopecten
magellanicus (Gmelin), in thermally stratified mesocosms
C.M. Pearce, J.L. Manuel, S.M. Gallager, D.A.
Manning, R.K. O'Dor, E. Bourget-2004
Journal of Experimental Marine Biology and Ecology, 312(1): 187-214
Abstract:
The present study was conducted to evaluate the
relative roles that water column stratification intensity and possible
inter-population behaviour differences play in determining depth of larval
settlement of giant scallops, Placopecten magellanicus, in relation
to thermoclines. Differences in timing of settlement of larvae from various
populations were also examined. Two separate experiments were conducted in a
10.5-m-deep, 3.7-m-diameter, thermally stratified tank with larvae spawned
from scallops collected from several adult beds located in areas with
differing oceanographic regimes (Georges Bank, Mahone Bay, Passamaquoddy
Bay). Previous mesocosm experiments had shown that veligers from these
various populations differ in their vertical migration patterns. In the
first experiment (December 1992–February 1993), larvae from all three
populations were held in separate 9.5-m-deep tubes and exposed to a 1.5 °C
temperature differential established over a depth interval of 1 m. The
number of settled juveniles (spat) of each population collected at the end
of the experiment increased with depth and showed no peak at or above the
thermocline. This depth distribution of spat was most likely driven by
preferential larval settlement. In the second experiment (February–May
1994), larvae spawned from Georges Bank and Passamaquoddy Bay stocks were
held in 9.0-m-deep tubes and exposed to a 5 °C temperature differential
established over a depth interval of 1 m. The number of settled spat of both
populations was greater above this thermal boundary and increased with
decreasing depth. This depth distribution was most likely driven by
preferential settlement above the thermocline followed by upward
post-settlement migration. The results from the two experiments indicate
that larvae from various populations show similar trends in settlement
patterns in response to similar thermal stratifications. Stratification
intensity, however, does affect depth of larval settlement. In the second
experiment, both populations of larvae settled throughout the time interval
of collector deployment (larvae 32–82 days old). These results extend the
range of planktonic developmental times generally reported in the literature
and may be more indicative of natural planktonic development in the field.
While Georges Bank larvae settled in consistent numbers through time,
Passamaquoddy Bay larvae showed peaks in settlement at certain time periods,
indicating that pulses of larval settlement may occur even from an
individual spawning event.
(GIROQ, Département de Biologie, Université Laval,
Sainte-Foy, Québec, Canada G1K 7P4, e-mail: PearceC@pac.dfo-mpo.gc.ca)