Swimming responses of larvae of three mactrid bivalves to different salinity gradients / Bernardita Campos Maia.

Incluye anexos.

Master of Arts. College of William and Mary. School of Marine Science. 1988.

Bibliografía: hojas 95 - 109.

Larval swimming behavior in response to varied salinity conditions is an important component of active mesoscale dispersal mechanisms. Vertical distribution in salinity gradients of 0,5, 10 and 15°/oo in magnitude was examined for straight-hinge, umbo and pediveliger larvae of the mactrid bivalves Spisula solidissima, Mulinia lateralis and Rangia cuneata. Vertical swimming and sinking velocities were measured at constant temperature and light, and variable salinity depending on the species. Laboratory cultured larvae were used for the experiments. Larvae concentrated in the discontinuity independently of the species, stage of development and larval brood. Spisula solidissima larvae initially swimming in a 30°/ oo salinity layer crossed a discontinuity of 5° / oo and 10°/ oo discontinuity magnitude but nota 15°/ oo salinity discontinuity. Mulinia lateralis larvae initially swimming in the 25°/ oo salinity layer also crossed a discontinuity of 5°/ oo salinity magnitude but they did not swim through a discontinuity of 10 and 15°/ oo salinity magnitude. Rangia cuneata larvae initially swimming in a 10°/ oo salinity layer crossed even a discontinuity of 15°/ oo sallinity magnitude but they mostly concentrated in the 10°/ oo salinity layer and in the discontinuity. Vertical velocity changed with larval stage with the peak in the umbo stage, independently of the species . Upward vertical velocity in S. solidissima larvae ranged from 0.18 to 0.49 mm s-1 and increased with an increase of salinity. Upward vertical velocity in M. lateralis larvae ranged from 0.25 to 0.50 mm s -1 but no consistent pattern in relation to salinity was observed. Upward vertical velocity in R. cuneata larvae ranged from 0.18 to 0.53 mm s -1 . The lowest velocity was measured in the highest salinity, but in pediveliger larvae, the opposite was observed. Downward vertical velocities were similar to the upward velocities values . No significant differences in downward vertical velocity were detected in relation to salinity. Passive sinking was more frequent than active downward swimming in umbo and pediveliger larvae. Sinking velocity increased with larval size in S. solidissima and M. lateralis larvae; however, R. cuneata straight-hinge larvae sank faster than umbo and pediveliger larvae. The velar morphology was different in the three species. Although the outer ciliary band had short blunted tips irrespective of the species, the inner ciliary band and the central ciliary tuft bore long cilia with paddles in S. solidissima, discs in M. lateralis but regular blunted cilia in R. cuneata . The implications of the observed larval behavior and morphological characteristics of the velum on larval depth regulation and dispersal are discussed.