NEDVED, B.T.*; HADFIELD, M.G.: Fate of larval muscles during metamorphosis of Hydroides elegans

Metamorphosis of marine invertebrate larvae occurs rapidly and is triggered by the binding of specific external cues with appropriate larval receptors. This binding activates a cascade of events that culminate with the loss or reorganization of larval tissues and the appearance of adult structures. Larvae of the serpulid polychaete Hydroides elegans undergo metamorphosis after contact of the larval sense organs with bacterial biofilms. Metamorphosis of H. elegans is characterized by the secretion of a proteinaceous primary tube; restriction of the collar region; loss of the trochal cilia; elongation of pygidial region; and development of the branchial crown. While the time course and the external morphogenic events of this process have been determined, relatively little is known about the fate of internal structures (i.e. larval musculature) during metamorphosis. In this study, we used a TRITC-phalloidin conjugate to track differentiation of muscle fibers during larval development of H. elegans, and determined the fate of these fibers during metamorphosis and early juvenile development. At metamorphic competence, the major components of the larval musculature consist of 2 pairs of longitudinal muscles and a pair of circular fibers. The longitudinal muscles originate in the 3 ^rd larval segment, terminate in the larval episphere, and persist through early metamorphic stages. The circular fibers are located slightly anterior of the prototroch, and may assist in the constriction of the collar region during metamorphosis.