Diversity of brood chambers in calloporid bryozoans (Ggymnolaemata, Cheilostomata): comparative anatomy and evolutionary trends

摘要

Comparative anatomical studies of 12 species from 10 genera (Callopora, Tegella, Amphiblestrum, Parellisitio. Corbulella, Crassimarginatella, Valdemunitella, Bryocalyx, Concertina, Cauloramphus) belonging to one of the largest and most diverse bryozoan taxa, the Calloporidae, and one species from the genus Akatopora belonging to the related taxon Antroporidae, were undertaken to elucidate the morphological diversity of brooding structures and to recognize main trends in their evolution. Most of the species studied possess ovicells (specialized brooding receptaeles; formed by the distal and maternal (egg-producing) autozooids. The distal zooid can be an autozooid, a vicarious avicularium or a kenozooid. The calcined protective Jiood (ooecium) is an outgrowth from the distal zooid. Hypcrstomial or prominent ovicells are most common. They were found in species of the genera Callopora, Tegella, Amphiblestrum, Parellisina, Corbulella, Bryocalyx and Concertina. Subimmersed ovicells were found in Valdemunitella, and immersed ovicells in Crassimarginatella and Akatopora. Cauloramphus has an internal brooding sac and a vestigial kenozooidal ooecium, budded by the maternal zooid. Based on the structure of the brooding organs, the following evolutionary trends can be recognized within the group: (1) reduction of the distal (ooecium-producing) zooid, (2) immersion of the brooding cavity correlated with a reduction of the ooecium and ooecial vesicle and with changes in the ovicell closure and the structure ofthe brood chamber floor, (3) reduction of the calcification of the ectooecium, and (4) transition from bilobate to entire ooecium. The trend towards immersion of the brooding cavity could have evolved repeatedly within the Calloporidae. Transition frombilobate to entire ooecium is characteristic of the related taxon Cribrilinidae, showing a good example of parallel evolution of the ooecium in two closely related clades. Possible causes for the transformations described are discussed.