Development of the larval muscle system in the mussel Mytilus trossulus (Mollusca, Bivalvia).

In the present study we examined muscle development throughout the complete larval cycle of the bivalve mollusc, Mytilus trossulus. An immunofluorescence technique and laser scanning confocal microscopy were used in order to study the organization of the muscle proteins (myosin, paramyosin, twitchin, and actin) and some neurotransmitters. The appearance of the muscle bundles lagged behind their nervous supply: the neuronal elements developed slightly earlier (by 2 h) than the muscle cells. The pioneer muscle cells forming a prototroch muscle ring were observed in a completed trochophore. We documented a well-organized muscle system that consisted of the muscle ring transforming into three pairs of velar striated retractors in the early veliger. The striations were positive for all muscle proteins tested. Distribution of FMRFamide and serotonin (5-HT) immunocytochemical staining relative to the muscle ring differed significantly: 5-HT-immunoreactive cells were situated in the center of the striated muscle ring, while Phe-Met-Arg-Phe-NH2 neuropeptide FMRFamid immunoreactive fibers were located in a distal part of this ring. Our data showed clearly that the muscle proteins and the neurotransmitters were co-expressed in a coordinated fashion in a continuum during the early stages of the mussel development. Our study provides the first strong evidence that mussel larval metamorphosis is accompanied by a massive reorganization of striated muscles, followed by the development of smooth muscles capable of catch-contraction.