Origin of the trochophora larva

Claus Nielsen

doi:10.1098/rsos.180042

Abstract

The trochophora larva, which is so well known from the marine plankton, is central to our understanding of the evolution of a large branch of the bilaterians. Two theories for this larval type have been prevalent, the trochaea theory and the theory proposed by Ivanova-Kazas. The embryology, or more precisely the cell-lineage, of these larvae seems to be central for our understanding of their origin, but important details have been missing. According to the trochaea theory, a circumblastoporal ring of blastomeres differentiates to follow the convoluted shape of the conspicuous ciliary bands of the larvae, with prototroch and metatroch around the mouth, forming a filtering system, and telotroch around the anus. According to the Ivanova-Kazas theory, the blastomeres with the ciliary bands develop through specialization of rings of cells of the general ciliation in a lecithotrophic larva. Now, a new cell-lineage study of the gastropod Crepidula has shown that the ring of cells at the edge of the blastopore develops into the band of cells carrying prototroch and metatroch, characteristic of the trochophora. This gives strong support to the trochaea theory.

Highlights

Ever since Hatschek [1] introduced the trochophora concept, this larval type has played a prominent role in phylogenetic discussions
A recent study of the embryology of the snail Crepidula fornicata, using blastomere labelling [15], has filled some of the most important gaps in our knowledge about the embryology, and this new information makes it timely to review the characters and concepts used to infer the origin of this characteristic larval type
2d metatroch cells gastrotroch cells approximate position of future proctodaeum prototroch mouth stomodaeum of 3a–d cells fused blastoporal edge of 2dcells area telotroch perianal ring of 3a and 3b anus. Spiralian groups, such as annelids, molluscs, entoprocts, nemerteans and platyhelminths, show a characteristic cleavage pattern known as spiral, and both ‘classical’ and modern cell-lineage studies are available for a number of species with trochophora larvae [14,15,16]

Summary

Introduction

Ever since Hatschek [1] introduced the trochophora concept, this larval type has played a prominent role in phylogenetic discussions. The most characteristic feature of the typical trochophora larva is the ciliary bands, which comprise prototroch, metatroch and telotroch, formed by compound cilia. The origin of the trochophora larva with its characteristic ciliary bands has been discussed by only a few recent authors. A recent study of the embryology of the snail Crepidula fornicata, using blastomere labelling [15], has filled some of the most important gaps in our knowledge about the embryology, and this new information makes it timely to review the characters and concepts used to infer the origin of this characteristic larval type

The trochaea theory

Ivanova-Kazas’ theory

Special types of trochophora larvae

Conclusion

Evolutionary significance of the larvae of

26. Richter S et al 2010 Invertebrate neurophylogeny