Comparative Anatomical Study of Internal Brooding in Three Anascan Bryozoans (Cheilostomata) and Its Taxonomic and Evolutionary Implications Andrew N. Ostrovsky, 1 Andrei V. Grischenko, 2 Paul D. Taylor, 3 * Phil Bock, 4 and Shunsuke F. Mawatari 2 1 Department of Invertebrate Zoology, Faculty of Biology & Soil Science, St. Petersburg State University, St. Petersburg 199034, Russia 2 Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060 0810, Japan 3 Department of Palaeontology, Natural History Museum, London SW7 5BD, UK 4 School of Ecology & Environment, Deakin University, Burwood, VIC 3125, Australia ABSTRACT The anatomical structure of internal sacs for embryonic incubation was studied using SEM and light microscopy in three cheilostome bryozoans— Nematoflustra flagellata (Waters, 1904), Gontarella sp., and Biflustra perfragilis MacGillivray, 1881. In all these species the brood sac is located in the distal half of the maternal (egg-producing) autozooid, being a conspicuous invagination of the body wall. It consists of the main chamber and a passage (neck) to the outside that opens independently of the introvert. There are several groups of muscles attached to the thin walls of the brood sac and possibly expanding it during oviposition and larval re- lease. Polypide recycling begins after oviposition in Gon- tarella sp., and the new polypide bud is formed by the beginning of incubation. Similarly, polypides in brooding zooids degenerate in N. flagellata and, sometimes, in B. perfragilis. In the evolution of brood chambers in the Cheilostomata, such internal sacs for embryonic incuba- tion are considered a final step, being the result of immer- sion of the brooding cavity into the maternal zooid and reduction of the protecting fold (ooecium). Possible rea- sons for this transformation are discussed, and the hy- pothesis of Santagata and Banta (1996) that internal brooding evolved prior to incubation in ovicells is rejected. J. Morphol. 267:739 –749, 2006. © 2006 Wiley-Liss, Inc. KEY WORDS: parental care; ovicells; internal brood sacs; evolution; Cheilostomata; Bryozoa The origin of parental care in the bryozoan order Cheilostomata is considered one of the key evolu- tionary innovations contributing to their success in benthic marine communities from the mid- Cretaceous to the present day (Taylor, 1988). Two facts that favor this supposition are: 1) despite an- tedating brooders, nonbrooders are represented by very few fossil and Recent species, and most cheilos- tomes instead hold their embryos in special embry- onic chambers called ovicells; and 2) the earliest known fossil ovicells coincide in time with the onset of an explosive radiation of cheilostomes in the Late Albian. Incubation in the coelom (more precisely, in the ovary) is restricted to one cheilostome family (Epis- tomiidae). Only a few species, belonging to the prim- itive genera Aetea (Aeteidae), Eucratea (Eucratei- dae) and Leiosalpinx (Leiosalpingidae), brood their embryos externally in a membranous sac. The over- whelming majority of cheilostomes brood embryos in chambers with calcified walls (ovicells), in which the brood cavity is external with respect to the visceral coelom. In species with internal brood sacs formed by noncalcified zooidal walls, the brood cavity is also external with respect to the coelomic cavity of the maternal (egg-producing and brooding) zooid and yet is completely immersed in the maternal zooid and therefore often difficult to recognize (reviewed in Hyman, 1959; Stro ¨m, 1977; Reed, 1991; Os- trovsky, 2006a). Internal brooding is known in both anascan (Sub- order Flustrina) and ascophoran (Suborder Asco- phora) cheilostomes. Among anascans it has been reported in species assigned to the Flustridae, Can- didae, Steginoporellidae, Poricellariidae, Chlidoni- idae, and a few other families (see below), and among ascophorans, in species of Adeonidae, Ad- eonellidae, Watersiporidae, Cryptosulidae, and Eu- Contract grant sponsor: Alexander von Humboldt Foundation, Ger- many, and the Lise Meitner Foundation, Austria (postdoctoral fellow- ships to A.N.O.); Contract grant sponsor: 21st Century Center of Excellence (COE) Program on “Neo-Science of Natural History” (Pro- gram Leader: H. Okada, Co-ordinator Dr. P. Gautam) at Hokkaido University financed by the Ministry of Education, Culture, Sports, Science, and Technology, Japan (research fellowship); Contract grant sponsor: JSPS (fellowship to P.D.T.). *Correspondence to: Dr. P.D. Taylor, Department of Palaeontology, Natural History Museum, Cromwell Road, London SW7 5BD, UK. E-mail: pdt@nhm.ac.uk Published online 8 March 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jmor.10438 JOURNAL OF MORPHOLOGY 267:739 –749 (2006) © 2006 WILEY-LISS, INC.