Embryogenesis, Morphology and Ultrastructure of the Pericalymma Larva of Acila castrensis (Bivalvia: Protobranchia: Nuculoida)

The pericalymma larva of the protobranch bivalve Acita castrensis is described from sections and whole mounts using light and electron microscopy. This is one of few published descriptions of a protobranch larva and the first ultrastructural study of a member of the order Nuculoida. Comparisons with other protobranch pericalymmae are made with respect to size, morphology, ciliation, development mode, and larval period. Early embryogenesis follows a typical bivalve pattern and by day 5 results in a fully formed, barrel-shaped pericalymma with prominent apical tuft. The larva is planktic and lecithotrophic. It propels itself with three transverse bands of cilia and a patch of accessory locomotory cilia. Late-stage larvae become laterally compressed as the shell develops. Metamorphosis occurs at day 8 with shedding of the larval test. The pericalymma of A. castrensis most closely resembles that of Nucula proxima and N. turgida. They share features that are possibly unique to the Nuculidae including an ovoid form, 2 ab-apical openings in the test, a post-anal structure, no statocyst, and reduced musculature. The post-anal organ of A. castrensis is a unique larval organ possessing two types of cilia and a small cavity near the tip. The organ begins to form during mid-embryogenesis and most likely serves as a sensory device in site selection or as a locomotory director, but may perform other functions. The protobranch pericalymma may represent a modification of the veliger larva form, a precursor to the veliger form, or the veliger and pericalymma may have diverged from a common ancestral developmental form. The pericalymma also shares morphological similarities with the developmental forms of aplacophorans, archacogastropods, polyplacophorans, and scaphopods; all possess a test or velum with ciliary bands, and some possess a posterior protrusion. Further work is needed among these developmental forms to ascertain structural homologies and phylogenetic affinities.