The Bryozoa are a mainly marine phylum of oligomerous coelomate invertebrates with a long fossil history stretching back to the Lower Ordovician. They form colonies that are frequently a conspicuous part of the sessile epifauna in many marine habitats, from the intertidal to depths of 8000 m. Bryozoans are active suspension feeders, ingesting living and non-living particles from the surrounding medium. Many species are structurally significant in seafloor biotopes, forming bushy and coral-like growths that are ecologically important as habitats for a wide range of other organisms.
Apart from the obscure phylogenetic affinities of Bryozoa that are a long-term source of scientific interest and speculation, there are a remarkable array of reproductive and developmental patterns within the phylum, ranging from planktotrophy to lecithotrophy and extra-embryonic nutrition. This diversity affords opportunities for comparative and experimental studies, and gives insight into the role of larval dispersal and hence gene flow in evolutionary diversification. The variety of reproductive patterns and larval types offers a wide scope for speculation on the evolutionary trends of sexual reproduction of marine invertebrates in general. Owing to a rich fossil record, where bryozoans with different strategies can be recognized, we can explore the relationship of recent data to the past, and make comparisons of evolutionary fate with those of other similar marine organisms.
Research on the sexual reproduction of marine bryozoans has attracted zoologists since the beginning of the 19th century. Existing reviews on the topic show that since this time considerable information has been accumulated. However, there is no single historical review that represents the key names and works published since the 19th century, and early 20th century knowledge has never really been evaluated properly. Many early scholars analyzed and reassessed existing data, but most of these works, especially short papers, have been often neglected if not forgotten. One can also find reviews in some old and more recent monographs and textbooks, but little attention has been paid to these as most of them were written in German or French. An attempt to review both old and recent literature concerning brooding structures and oviposition in Cheilostomata, and fertilization in Bryozoa has recently made by Ostrovsky (see this volume). Similarly, in this review we aim to chronicle the main steps in the history of the research of sexual reproduction (origin of the germ cells, gonado- and gametogenesis, fertilization, oviposition and some aspects of brooding) in marine gymnolaemate bryozoans with particular emphasis placed on the observation and recording of different structures and development of the modern understanding of the specific processes involved. It should be stressed that, apart from many data and ideas that have been completely forgotten, later authors sometimes incorrectly interpreted the hypotheses or conclusions of the previous researchers. Sometimes, mistaken traditional opinions survived for many years despite the emergence of new facts and contradicting data as has happened, for instance, in the case of bryozoan fertilization. We aim to highlight these contradictions where appropriate. In an effort towards making the review as comprehensive as possible, we have analyzed many obscure papers and listed small descriptive details in all species studied. This gave us the opportunity to resurrect many forgotten names and facts simultaneously, thus the review represents an integrated picture of the available literature on bryozoan sexual reproduction and associated taxonomic diversity. It should be noted however, that some difficulties were encountered in trying to trace a small number of short papers and some incidental reports of the reproductive organs in several works.