Sequential developmental programmes for retractor muscles of a caenogastropod: reappraisal of evolutionary homologues

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Abstract
Evolutionary changes in the development of shell-attached retractor muscles in gastropods are of fundamental importance to theories about the early evolution and subsequent diversification of this molluscan class. Development of the shell–attached retractor muscle (columellar muscle) in a caenogastropod has been studied at the ultrastructural level to test the hypothesis of homology with the post–torsional left retractor muscle (larval velar retractor) in vetigastropod larvae. The vetigastropod muscle has been implicated in the generation of ontogenetic torsion, a morphogenetic twist between body regions that is important to theories about early gastropod evolution. Two shell–attached retractor muscles develop sequentially in the caenogastropod, Polinices lewisii, which is a pattern that has been also identified in previous ultrastructural studies on a vetigastropod and several nudibranch gastropods. The pattern may be a basal and conserved characteristic of gastropods. I found that the first–formed retractor in larvae of P. lewisii is comparable to the larval velar retractor that exists at the time of ontogenetic torsion in the vetigastropod, Haliotis kamtschatkana. However, the post–metamorphic columellar muscle of P. lewisii is derived exclusively from part of the second–formed muscle, which is comparable to the second–formed pedal muscle system in the vetigastropod. I conclude that the post–metamorphic columellar muscle of P. lewisii, is not homologous to the larval velar retractor of the vetigastropod, H. kamtschatkana.

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