SICB Division of Invertebrate Zoology (DIZ)

DIZ Researchers Database Entry

Svetlana Maslakova

Evolution of Larval Development in Nemerteans
Nemerteans (Phylum Nemertea) 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 specific founder cells called the trochoblasts. Prototroch is the primary locomotory organ of many larvae and is also often involved in feeding. Nemerteans possess a wide diversity of larval body plans, but until now no nemertean had been shown to possess a trochophore larva or a prototroch, which hampers comparisons between the larval development of nemerteans and other trochozoans. Members of the nemertean clade Pilidiophora develop via a highly specialized planktonic pilidium larva, which undergoes catastrophic metamorphosis, while members of the sister clade to Pilidiophora - the order Hoplonemertea, and the basal palaeonemerteans have uniformly ciliated "planuliform" larvae, which develop into juvenile without a drastic change of the body plan. The answer to which type of development - with or without pilidium - is primitive for the nemerteans has implications for the evolution and loss of novel complex larval forms (such as pilidium). In collaboration with Mark Martindale (University of Hawaii) and Jon Norenburg (Smithsonian Institution) we used intracellular labeling markers, cLSM and SEM to demonstrate that uniformly ciliated larva of a palaeonemertean Carinoma tremaphoros possesses a hidden prototroch which is derived from the same cell lineage as prototrochs of other Trochozoans. This finding refutes the homology between the pilidium and the so-called larval ectoderm previously reported from some palaeonemertean larvae the main support for idea that pilidial development is ancestral for the phylum and instead suggests that pilidium is an elaboration on the trochophore groundplan, uniquely derived within nemerteans. Additionally, the inferred ancestral presence of a trochophore in nemerteans fills the gap in the distribution of trochophore larvae among the Trochozoa and allows a meaningful comparison between larval development of nemerteans and other trochozoans.