Meeting Abstract

91.3  Thursday, Jan. 7  Embryonic and post-embryonic development of the polyclad flatworm Maritigrella crozieri, and the homology of lophotrochozoan larval characters RAWLINSON, Kate A; Smithsonian Marine Station rawlinsonk@si.edu

There is a patchy distribution of ciliated planktonic (“larval”) stages across the Lophotrochozoa, and these stages share many morphological characters. This broad distribution of larval characters can be interpreted either as the result of convergent evolution, or as the retention of primitive lophotrochozoan larval features. Polyclad flatworms exhibit a continuum of developmental modes, with direct development at one extreme, and indirect development via a trochophore-like planktonic larval stage at the other. Here I present embryological and larval anatomical data from the indirect developing polyclad Maritrigrella crozieri, and consider these data within a comparative lophotrochozoan context. The planktonic stage of M. crozieri possesses a transient ciliary band and apical organ, and these show significant developmental differences relative to those of trochophore larvae. Unlike the prototroch of some mollusks and annelid larva, the ciliary band of M. crozieri lacks a specific ciliary band muscle. The dense ciliary band nerve net of M. crozieri bares more resemblance to the extensive nerve net of larval polyplacophorans and ectoprocts than to the prototroch nerve rings of larval polychaetes, bivalves and nemerteans. M. crozieri larvae posses a well-differentiated neuropile, above which sits an apical plate. This differs considerably from the larval apical organs of many lophotrochozoan taxa. Overall, polyclad larval myo- and neuroanatomy more closely resembles that of direct developing and juvenile polyclads than that of other trochophore larvae. This raises the possibility that the polyclad planktonic life stage is not homologous with the trochophore, though further embryological and developmental genetic studies are needed to test the homology of lophotrochzoan larval characters at the morphological and/or developmental genetic levels.