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Diversity of Brood Chambers in Calloporid Bryozoans (Gymnolaemata, Cheilostomata): Comparative Anatomy and Evolutionary Trends

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Diversity of Brood Chambers in Calloporid Bryozoans (Gymnolaemata, Cheilostomata): Comparative Anatomy and Evolutionary Trends Zoomorphology (2009) 128:13–35 DOI 10.1007/s00435-008-0070-8 ORIGINAL PAPER Diversity of brood chambers in calloporid bryozoans (Gymnolaemata, Cheilostomata): comparative anatomy and evolutionary trends Andrew N. Ostrovsky · Claus Nielsen · Norbert Vávra · Ekaterina B. Yagunova Received: 10 December 2007 / Revised: 25 June 2008 / Accepted: 27 June 2008 / Published online: 8 August 2008 © Springer-Verlag 2008 Abstract Comparative anatomical studies of 12 species organs, the following evolutionary trends can be recognized from 10 genera (Callopora, Tegella, Amphiblestrum, Parel- within the group: (1) reduction of the distal (ooecium-pro- lisina, Corbulella, Crassimarginatella, Valdemunitella, ducing) zooid, (2) immersion of the brooding cavity corre- Bryocalyx, Concertina, Cauloramphus) belonging to one of lated with a reduction of the ooecium and ooecial vesicle the largest and most diverse bryozoan taxa, the Callopori- and with changes in the ovicell closure and the structure of dae, and one species from the genus Akatopora belonging the brood chamber Xoor, (3) reduction of the calciWcation to the related taxon Antroporidae, were undertaken to eluci- of the ectooecium, and (4) transition from bilobate to entire date the morphological diversity of brooding structures and ooecium. The trend towards immersion of the brooding to recognize main trends in their evolution. Most of the spe- cavity could have evolved repeatedly within the Callopori- cies studied possess ovicells (specialized brooding recepta- dae. Transition from bilobate to entire ooecium is charac- cles) formed by the distal and maternal (egg-producing) teristic of the related taxon Cribrilinidae, showing a good autozooids. The distal zooid can be an autozooid, a vicari- example of parallel evolution of the ooecium in two closely ous avicularium or a kenozooid. The calciWed protective related clades. Possible causes for the transformations hood (ooecium) is an outgrowth from the distal zooid. described are discussed. Hyperstomial or prominent ovicells are most common. They were found in species of the genera Callopora, Keywords Brooding · Ovicells · Internal sacs · Tegella, Amphiblestrum, Parellisina, Corbulella, Bryoca- Evolution · Cheilostomata lyx and Concertina. Subimmersed ovicells were found in Valdemunitella, and immersed ovicells in Crassimargina- tella and Akatopora. Cauloramphus has an internal brood- Introduction ing sac and a vestigial kenozooidal ooecium, budded by the maternal zooid. Based on the structure of the brooding Parental care is generally interpreted as an evolutionary novelty improving oVspring survival (see Adiyodi and Adiyodi 1989; Wray 1995 and references therein). The A. N. Ostrovsky · E. B. Yagunova Department of Invertebrate Zoology, Faculty of Biology most common method is brooding, with strong variation in and Soil Science, St. Petersburg State University, position and morphology of the brooding structures, often Universitetskaja nab. 7/9, 199034 St. Petersburg, Russia indicating diVerent routes of evolution. Except for a few broadcasting species, cheilostome bry- A. N. Ostrovsky (&) · N. Vávra Department of Palaeontology, Faculty of Earth Sciences, ozoans brood their embryos/larvae in special chambers. Geography and Astronomy, Geozentrum, Universität Wien, The variety of brooding methods and structures involved is Althanstrasse 14, 1090 Wien, Austria high, including external membranous sacs, protective calci- e-mail: [email protected] Wed chambers, epistegal space, and internal sacs (reviewed C. Nielsen in Hyman 1959; Ström 1977; Nielsen 1990; Reed 1991; Zoological Museum, University of Copenhagen, Mukai et al. 1997; Ostrovsky 2008a, b). Species of the taxon Universitetsparken 15, 2100 Copenhagen Ø, Denmark Epistomiidae are viviparous (Marcus 1941; Dyrynda 1981; 123 14 Zoomorphology (2009) 128:13–35 Dyrynda and King 1982); this has somewhat confusingly six calloporids of the genera Callopora, Tegella and Cor- been called intracoelomic brooding. The wide distribution bulella (as Crassimarginatella). Recently, the development and diversity of parental care clearly indicate that it has of the ovicell in the earliest known fossil calloporid Wilber- been of great importance for the evolutionary success of the topora, has been described (Ostrovsky and Taylor 2005b). clade. The diversity is obviously a result of independent Altogether, these studies seem to give a clear picture of evolution of brooding in several cheilostome lineages ovicell anatomy and development in the Calloporidae. (Taylor 1988), but only one study has speciWcally aimed at However, except Cauloramphus, only so-called hypersto- proving this (Ostrovsky and Taylor 2005a). mial or prominent ovicells have been studied, although the The most common brooding organ is the ovicell, a com- morphological diversity of the brood chambers is much plex protective structure consisting of (1) the calciWed dou- higher within this group (see, for instance, Cook 1968; Gor- ble-walled hood (ooecium) with an enclosed coelomic don 1984, 1986; Soule et al. 1995; Tilbrook 2006). Apart of cavity, and formed either by the distal or the maternal the diversity, the structure of Recent calloporid ovicells is zooid, (2) a space/cavity for embryonic incubation and, as a of particular interest because they show both ancestral and rule, (3) a closing structure (ooecial vesicle/plug or the non- derived characters (Ostrovsky and Schäfer 2003; Ostrovsky calciWed distal wall of the maternal zooid). Sometimes, it is et al. 2003; Ostrovsky and Taylor 2004, 2005a, b). also closed by the zooidal operculum. An ooecium pro- Here we present a comparative study of the various types duced by the distal zooid is an outgrowth of the zooidal of brood chambers in 12 species of 10 genera from the chei- wall, and its coelomic lumen communicates with a visceral lostome family Calloporidae. Additionally, we include data coelom through a slit or pore(s), normally plugged by non- on ovicell structure of a species of Akatopora from the specialized epithelial cells. An ooecium produced by the closely related group Antroporidae. The main aims of this maternal zooid is kenozooidal, with communication pores paper are (1) to describe the morphological diversity of plugged by the special pore-cell complexes (Ryland and brooding structures in the Calloporidae, (2) to show the Hayward 1977; Nielsen 1981, 1985; Ostrovsky 1998, 2002; main trends in their evolution, and (3) to relate the new data Ostrovsky and Schäfer 2003). The ooecial vesicle can be to the evolution of brooding in the earliest cheilostome retracted by special muscle bundles, thus opening the groups. entrance of the brooding cavity. Because the ovicell is formed by two zooids performing synchronized reproduc- tive activities (oogenesis and embryonic incubation) it rep- Materials and methods resents a colonial ‘brooding organ’ (see also Cheetham and Cook 1983). Colonies of Callopora craticula (Alder, 1857) and Tegella The Calloporidae sensu lato includes 75 nominal genera armifera (Hincks, 1880) were collected on 17.08.1996 by (Gordon 2007). It is the second largest and the earliest dredge and SCUBA from 3 to 4 m depth near Bezymjanniy taxon where ovicells evolved within the order Cheilosto- Island (66 18.23ЈN, 33 27.23ЈE), Chupa Inlet, Kandalarsha mata (Cheetham 1954, 1975; Cheetham et al. 2006), but the Bay, White Sea. T. armifera was collected by dredge on internal structure of their ovicells has been studied in detail 03.09.1995, 04.08.1996, 19.08.1996, and 25.06.1998 from in only few species of Tegella (Levinsen 1893, 1894), 3 to 10 m depth near Matrenin (66 18.36ЈN, 33 38.03ЈE) Amphiblestrum (Calvet 1900), and Callopora (Levinsen and Sredniy (66 17.12ЈN, 33 39.56ЈE) Islands, Chupa Inlet, 1909; Ostrovsky and Schäfer 2003; Ostrovsky et al. 2003). Kandalarsha Bay, White Sea. Cauloramphus spinifer Levinsen mainly studied cleaned skeletons, so the informa- (Johnston, 1832) was collected by dredge on 13.06.1998 tion about epithelia and tissues is often missing in his from 5 to 8 m depth near Sredniy Island (66 17.12ЈN, 33 works. The anatomy of the internal brood chambers has 39.56ЈE) Chupa Inlet, Kandalarsha Bay, White Sea (coll. been recently studied in eight species of the calloporid Dr. N. N. Shunatova). genus Cauloramphus (Ostrovsky et al. 2007). Colonies of Tegella unicornis (Fleming, 1828) were col- Levinsen (1893, 1894, 1909) was the Wrst author to pub- lected on 05.08.1997 by dredging from 16 to 29 m depth at lish illustrations of the developing ovicell, more speciWcally the Herthas Flak-reef (57 27.50ЈN,10 35.21ЈE), North Katt- of its calciWed parts, the ooecium, in four species of the egat, E of Skagen, Baltic Sea (coll. Dr A. N. Ostrovsky). genera Tegella and Callopora. Subsequently, ovicellogene- Colonies of Callopora dumerilii (Audouin, 1826) and sis has been studied in detail in representatives of the gen- Corbulella maderensis (Waters, 1898) were collected on era Corbulella and Tegella (Harmelin 1973a; Nielsen 12.06.1997 by SCUBA from 22 m depth near Riou Island 1985). A brief description of the unusual ovicells of Bryo- (43 10.36ЈN, 05 23.38ЈE), near Marseille, Mediterranean calyx cinnameus has been given by Cook and Bock (2000). Sea (coll. Dr. J.-G. Harmelin). Later, Ostrovsky et al. (2003) investigated the ultrastructure Colonies of Amphiblestrum inermis (Kluge, 1914) and of the ovicells walls and the development of the ovicell in Valdemunitella lata (Kluge, 1914) were collected on 123 Zoomorphology (2009) 128:13–35 15 05.05.2000 by dredge from
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