Flow fields around ciliated larvae: effects of natural and artificial tethers

The effects of tethering planktonic organisms on the swimming and feeding currents they produce were examined in 2 different ways. Larvae of the bivalve Crassostrea gigas and the gastropod Calliostoma ligatum were tethered in a miniature flow tank in still water and in water flowing at the approximate swimming speed of the organisms. The currents produced by the velar cilia of these larvae were filmed at 200 frames S-', using algal cells as markers to visualize fluld movement under these 2 conditions. Larvae of the polychaete Mesochaetopterus taylor~ were filmed at 200 frames S-' while swimming and while tethered to a microscope slide by then own mucus. The tethered blvalve larvae in stlll water and the tethered polychaete larvae generated flow fields in which particles followed curved trajectones. In contrast particles followed straighter trajectones around freely swimming polychaete larvae and the bivalve larva tethered in flowing water. For one C. gigas larva, angular velocities of prototrochal cilia were not significantly different under the 2 conditions. Calculations indicate that in still water this larva moved ca 20 O/O less water through the velar cilia per unit time than when in flowing water. For the polychaete, the cilia of tethered larvae beat more slowly than those of freely swimming larvae. For all 3 experimental animals, shear of water around cilia, estimated from clliary angular velocities and particle velocities, was greater for tethered larvae in still water than for those in flowing water or for freely swimming larvae. These changes in flow field, flux through the ciliated layer, and shear of water around cilia all have quantitative effects on rates of feeding calculated from observations on tethered organisms, but effects depend on the mechanism of particle capture utilized. There is no evidence that mechanisms of particle capture are changed as a result of tethering. Because buoyancy and body drag act as partial tethers, results of this study indicate how flow fields can be changed around small, elaborately shaped, planktonic organisms, and at the scale of feeding appendages. Hydrodynamic changes that occur simply as a result of being free swimming or tethered may also have influenced the evolution of body shape and feeding mechanisms of organisms that changed from free living to sessile (or vice versa).