Similarities in Form and Developmental Sequence for Three Larval Shell Muscles in Nudibranch Gastropods

Abstract

AbstractShell‐anchored muscles that extend into the cephalopodium of five species of planktotrophic nudibranch larvae were studied by ultrastructural examination of sequential larval developmental stages. All species, regardless of larval shell type (inflated or non‐inflated), showed a similar basic pattern of shell muscles. The larval retractor muscle (LRM) differentiates prior to hatching and its fibres insert on epithelia of the velum, apical plate, stomodeal region, or mantle fold. Many fibres also connect with subepithelial intrinsic muscles of the cephalopodium. Most but not all LRM fibres Project to left‐sided targets and are innervated from the left cerebral ganglion. Two pedal muscles, which are innervated from the pedal ganglia, differentiate during the post‐hatching larval stage and both insert primarily on pedal epithelium attached to the operculum. The left pedal muscle is anchored to the shell immediately adjacent to the attachment plaque of the LRM and consists of basal and distal tiers of muscle cells. The right pedal muscle arises on the ventral rim of the shell aperture and consists of a single tier of muscle cells. Ontogenic changes in larval retraction behaviour correlate with developmental change in the muscle effectors. Although some interspecific differences were noted, the presence of a common ground plan for larval shell muscles in these five species contrasts with previous indications of marked variability for nudibranch larval shell muscles.