Abstract
BackgroundThe shipworm Lyrodus pedicellatus is a wood-boring bivalve with an unusual vermiform body. Although its larvae are brooded, they retain the general appearance of a typical bivalve veliger-type larva. Here, we describe myogenesis of L. pedicellatus revealed by filamentous actin labelling and discuss the data in a comparative framework in order to test for homologous structures that might be part of the bivalve (larval) muscular ground pattern.ResultsFive major muscle systems were identified: a velum retractor, foot retractor, larval retractor, a distinct mantle musculature and an adductor system. For a short period of larval life, an additional ventral larval retractor is present. Early in development, a velum muscle ring and an oral velum musculature emerge. In late stages the lateral and dorsal mantle musculature, paired finger-shaped muscles, an accessory adductor and a pedal plexus are formed. Similar to other bivalve larvae, L. pedicellatus exhibits three velum retractor muscles, but in contrast to other species, one of them disappears in early stages of L. pedicellatus. The remaining two velum retractors are considerably remodelled during late larval development and are most likely incorporated into the elaborate mantle musculature of the adult.ConclusionsTo our knowledge, this is the first account of any larval retractor system that might contribute to the adult bodyplan of a (conchiferan) mollusk. A comparative analysis shows that a pedal plexus, adductors, a larval velum ring, velum retractors and a ventral larval retractor are commonly found among bivalve larvae, and thus most likely belong to the ground pattern of the bivalve larval musculature.
Highlights
The shipworm Lyrodus pedicellatus is a wood-boring bivalve with an unusual vermiform body
Larvae were relaxed in 7.5% MgCl2, fixed in 4% paraformaldehyde in 0.1M phosphate buffer (PB) and stored in 0.5M PB +0.1% NaN3 at 4C?
A pair of dorsal velum retractors attaches to the area of the developing shell, extends into the velum and runs along the inner surface of the anterior adductor (Figures 2A and 3A, B)
Summary
The shipworm Lyrodus pedicellatus is a wood-boring bivalve with an unusual vermiform body. Its larvae are brooded, they retain the general appearance of a typical bivalve veliger-type larva. Laterally compressed lophotrochozoans with two shell valves that are hinged dorsally and interconnected by a ligament, which acts antagonistically against the adductor muscles. Representatives of the Teredinidae and some Pholadidae (Heterodonta) burrow into and feed on wood and thereby as adults greatly deviate from the typical bivalve anatomy and mode of life. The valves are interconnected by the adductor muscles and not by the hinge as is the case in other bivalves. While the adult L. pedicellatus has a highly derived body, its laterstage larvae bear significant gross morphological resemblance to trochophore and veliger larvae of most other
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