MASLAKOVA, S; MARTINDALE, M; NORENBURG, J; George Washington Univ., Washington DC; University of Hawaii, Honolulu, HI; NMNH, Smithsonian Institution, Washington DC: Hidden trochophore in nemertean development revealed by confocal microscopy and cell lineage analysis

Nemerteans belong to the protostome clade the Trochozoa, which includes annelids, mollusks, sipunculids, echiurids and entoprocts. All trochozoan phyla, except nemerteans have been shown to possess a trochophore larva characterized by the prototroch, a pre-oral belt of specialized ciliated cells derived from the trochoblasts. Members of the nemertean clade Pilidiophora develop via a pilidium larva, which undergoes catastrophic metamorphosis, while members of the sister clade to Pilidiophora (Hoplonemertea) and the basal palaeonemerteans have so-called planuliform larvae, which develop into juvenile without a drastic change of the body plan. However, several species with planuliform larvae had been shown to possess a transitory larval ectoderm which had been homologized with pilidium, with the implication that pilidial development is ancestral for nemerteans. Here we present data on development of a palaeonemertean Carinoma tremaphoros, which possesses a planuliform planktonic larva. Using intracellular labeling markers and confocal microscopy we demonstrate that what appeared to be a larval ectoderm in this species is in fact a vestigial prototroch, derived from the same cell lineage as prototroch of other trochozoans. Lack of uniform ciliation obscures presence of the prototroch in C. tremaphoros (hence the hidden trochophore), however, trochoblast specialization is clearly manifested in their permanent cleavage arrest and ultimate degenerative fate. This refutes the homology between the pilidium and palaeonemertean larval ectoderm and suggests that pilidial development is a derived condition in nemerteans.