Unravelling the Evolution of Bryozoan Larvae

Abstract

The medium-sized lophotrochozoan phylum Bryozoa demonstrates a surprisingly wide range of larval forms. Few zygoparous species from the class Gymnolaemata possess long-lived planktotrophic larva (cyphonautes and paracyphonautes). The rest of gymnolaemates, and all species from classes Stenolaemata and Phylactolamata, incubate their embryos, whose development relies on egg’s yolk, extraembryonic nutrition (matrotrophy) or both, and have a brief free-swimming larval stage. Comparative morpho-functional analysis indicates that in bryozoans, similar to many other marine invertebrates, transitions from planktotrophic to endotrophic larvae were multiple and, obviously, were based on changes in oogenesis. Besides, the acquisition of a new larval type has always occurred in association with the evolution of embryonic incubation in Bryozoa. In myolaemates, the main trends in the evolution of endotrophy were reduction of the larval gut, loss of the larval protective cuticle/shell, invagination of the pallial epithelium of the episphere, and increase of the corona. Furthermore, larvae of stenolaemates lost their aboral and pyriform organs. Although being planktotrophic, the cyphonautes is a highly modified larval form, and cannot be considered as an ancestral type of bryozoan larvae. Phylactolaemates have a highly derived heterochronous development with a free-swimming stage that is, in fact, a chimera—either an ancestrula or a juvenile colony having a larval ciliary covering.