Meeting Abstract

31.5  Tuesday, Jan. 5  Formation and fate of the musculatue in larvae of the nudibranch Phestilla sibogae HADFIELD, M. G. *; CROLL, R. P.; University of Hawaii; Dalhousie University hadfield@hawaii.edu

Developmental programs for many marine invertebrates include the assembly of muscular systems appropriate to functions of swimming and feeding in pelagic larvae. At the end of larval life, metamorphosis brings about the loss of larval specific structures and emergence of juvenile-specific structures. Metamorphosing larvae of marine gastropods undergo destruction of the velum, a swimming and feeding organ specific for pelagic life, together with its intrinsic nerves and muscles. In shell-bearing snails, much of the larval musculature is retained to allow the juvenile snail to retract into its shell and move on the benthic substratum. However, nudibranch gastropods, such as Phestilla sibogae, lose their shells at metamorphosis, rendering the larval muscles that were attached to the shell functionless and raising questions about the fate of most larval muscles. To investigate muscle development and loss in larvae of P. sibogae, they were relaxed and fixed at successive stages, labeled with rhodamine-phalloidin, which binds to muscle F-actin, and examined with a laser-scanning confocal microscope. 3-dimensional reconstructions of the confocal images of larvae after shell loss revealed nearly complete in situ dissolution first of the right (larval) retractor followed shortly thereafter by the left (pedal) retractor. As the retractor muscles dissolved, oblique and circular muscle fibers in the body wall and longitudinal fibers in the foot became more numerous and prominent to provide movement to the small slug. Simultaneously, the buccal musculature enlarged to support post-metamorphic predatory feeding.