EMBRYOGENESIS, MORPHOLOGY AND ULTRASTRUCTURE OF THE PERICALYMMA LARVA OF ACILA CASTRENSIS (BIVALVIA : PROTOBRANCHIA : NUCULOIDA)

The pericalymma larva of the protobranch bivalve Acila 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, archaeogastropods, 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. Additional key words: bivalve evolution, developmental form, larval organ, test, velum The pericalymma larva of protobranch bivalves differs from the typical veliger larvae of most marine lamellibranchs in being barrel-shaped and cloaked with an outer, cellular, locomotory test or calyrnma. The test entirely covers the developing larva, including the shell, and is cast off at metamorphosis at which time the new juvenile commences a benthic existence. The term pericalymma was originally applied to a larval form considered primitive among the annelids, molluscs, and sipunculans (Salvini-Plawen 1972, 1973, 1980). We have adopted the term in a narrower sense, as others have, to refer to the larval form of protobranch bivalves and aplacophorans (Jablonski & a Author for correspondence. E-mail: jzardus @ lynx.neu.edu Currently on leave at the National Science Foundation, Arlington, Virginia 22230, USA Lutz 1983; Gustafson & Reid 1986, 1988a, 1988b; Gustafson 1987; Gustafson & Lutz 1992; Morse & Zardus 1997). Though distinct from other bivalve developmental forms, pericalymma morphology and embryogenesis are not uniform within the subclass Protobranchia, which includes the orders Nuculoida and Solemyoida and 11 or 12 families (Allen & Hannah 1986; Maxwell 1988). The full range of form and development in this subclass remains largely unknown, as few species have been examined. Previous descriptions consist of early accounts of 3 nuculoids (Drew 1897, 1899a, 1899b, 1901) and recent ultrastructural investigations of 2 solemyoids (Gustafson & Reid 1986, 1988a, 1988b; Gustafson 1987; Gustafson & Lutz 1992). Only 3 of these studies follow development from the single-cell stage (Drew 1901; Gustafson & Reid 1986; Gustafson & Lutz 1992).