Development of the musculature in the limpet Patella (Mollusca, Patellogastropoda)

Whole-mount technique using fluorescent-labelled phalloidin for actin stain ing and confocal laser scanning microscopy as well as semi-thin serial sect ioning, scanning and transmission electron microscopy were applied to inves tigate the ontogeny of the various muscular systems during larval developme nt in the limpets Patella vulgata L. and P. caerulen L. In contrast to earl ier studies, which described a single or two larval shell muscles, the pret orsional trochophore-like larva shows no less than four different muscle sy stems, namely the asymmetrical main head/foot larval retractor muscle, an a ccessory larval retractor with distinct insertion area, a circular prototro ch/velar system, and a plexus-like pedal muscle system. In both Patella spe cies only posttorsional larvae are able to retract into the shell and to cl ose the aperture by means of the operculum. Shortly after torsion the two a dult shell muscles originate independently in lateral positions, starting w ith two fine muscle fibres which insert at the operculum and laterally at t he shell. During late larval development the main larval retractor and the accessory larval retractor become reduced and the velar muscle system is sh ed. In contrast. the paired adult shell muscles and the pedal muscle plexus increase in volume, and a new mantle musculature, the tentacular muscle sy stem, and the buccal musculature arise. Because the adult shell muscles are entirely independent from the various larval muscular systems, several cur rent hypotheses on the ontogeny and phylogeny of the early gastropod muscle system have to be reconsidered.