Potential of bivalves' secondary settlement differs with species: a comparison between cockle (Cerastoderma edule) and clam (Ruditapes philippinarum) juvenile resuspension |
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Authors: | X de Montaudouin |
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Institution: | (1) Laboratoire d'Océanographie Biologique, Centre d'Océanographie et de Biologie marine, URA 197, 2 Rue du Professeur Jolyet, F-33120 Arcachon, France, FR |
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Abstract: | Juvenile bivalves may be dispersed by entering a bysso-pelagic phase where they drift through the water mass aided by a long
thread. The ability to resuspend and control the specific weight in two bivalve species, the cockle Cerastoderma edule (L.) and the Japanese clam Ruditapes philippinarum (Adams and Reeves), was documented with juveniles through flume and still-water experiments. Cockle juveniles initially placed
on an unsuitable substratum were exposed to two shear velocities (u
*). At the end of the experiment, 42 (±15)% (for u
*=0.51 cm s−1) and 79 (±9)% (for u
*=0.99 cm s−1) of individuals were retrieved from the sand area which represents only 8% of the total flume surface. Most juveniles (70.5%)
with shell lengths <2.5 mm migrated from the unsuitable Plexiglas substratum to the sand array by resuspension in the water
column. The percentage was lower (21.5%) for larger individuals. The same experimental design was applied to clams, which
immediately adhered to the Plexiglas substratum and remained attach to it. Sinking rates of live and dead specimens of both
species were measured in a 1 m long transparent PVC tube. Cockle fall velocities showed severe deceleration, probably due
to byssus secretion (up to 15-fold slower than dead cockles), sometimes interrupted by brutal acceleration probably due to
byssal rupture. Cockles were able to reduce their sinking rate for shell lengths up to 4.25 mm. By contrast, clam sinking
rates were constant, and similar to dead clam sinking rates. Specific weights of all experimental juveniles were calculated
in relation to their lengths, and their passive motion into the boundary layer was theoretically assessed with Shields curve.
In short, C. edule and R. philippinarum can both exhibit dense populations in the field with a good capacity to colonize, although juveniles display different abilities
to resuspend in the water column.
Received: 27 January 1997 / Accepted: 13 February 1997 |
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