SWIMMING OF JUVENILE SEA SCALLOPS, PLACOPECTEN-MAGELLANICUS (GMELIN) - A MINIMUM SIZE FOR EFFECTIVE SWIMMING

Three types of swimming (stepwise, variable and stable) of juvenile Pl acopecten magellanicus (Gmelin) observed in the laboratory are describ ed, and the effectiveness of the different swim types with regard to h orizontal, vertical and overall velocities are compared. The relations hip between relative velocity and Reynolds number seems to indicate a threshold in the hydrodynamic regime at a Reynolds number of about 3 x 10(3). Scallops passed this threshold between 12 and 16 mm shelf heig ht, and byssal attachment below this size may be an adaptive response to hydrodynamic regime. Above that threshold, scallops swam at a veloc ity of about 15 body lengths per second. Relative velocity decreased s teadily below the threshold to about 10 body lengths per second at a R eynolds number of 270. A derived model of scallop flight above the thr eshold indicated that scallops of these sizes swim with an angle of at tack of about 0.5 degrees. A negative correlation of angle of ascent a nd shell height was explained by the model and was due to the combined effect of swim type and the effect of scale. Juvenile sea scallops ma y use changes in velocity to change angle of ascent. Scallops appear t o exhibit two different types of claps, and undulating flight seen whe n the scallops are swimming horizontally appears to be due to the same mechanism as causes the stepwise swimming seen in very small juvenile s.