A Setigerous Collar in Membranipora chesapeakensis n. sp. (Bryozoa): Implications for the Evolution of Cheilostomes from Ctenostomes

Abstract

Membranipora chesapeakensis n. sp. was collected on wood substrate in the Chesapeake Bay, Maryland. Colonies are erect, vermiform or ribbon-shaped to bilaminar, with two back-to-back layers sharing one basal cuticle. There is a minute cryptocyst and a beaded gymnocyst; zoids have 14 tentacles and lack knobs or other decorations in their corners. Closest affinities are with M. tenuis Desor, sensu lato. A collar is present, the first reported among membraniporid cheilostomes. Membraniporids are morphologically similar to the earliest fossil cheilostomes and to the encrusting ctenostomes from which most cheilostomes are thought to have evolved. A probably generally present in ctenostomes, has hitherto been found only in two aberrant cheilostome genera. The presence of a collar in a species of Membranipora suggests that collars may also have been present in early cheilostomes and therefore supports the hypothesis that most or all cheilostomes evolved from ctenostomes. Additional key words: Ectoprocta, Polyzoa, Cheilostomata, Ctenostomata, Membraniporidae, Chesapeake, Maryland The setigerous collar, or pleated collar, is an acellular secretion present in some bryozoan taxa. In retracted zoids* the collar projects distally from the diaphragm into the vestibule. When the lophophore is extended, the collar projects from the neck of the autozoid like the starch-stiffened formal collars popular in Elizabethan England. We know of no published suggestion as to its function. Setigerous collars have been found to be present in all ctenostome bryozoans in which they have been sought. However, they are known from only two cheilostome genera, Aetea and Scruparia. The collar of Aetea is only a short ring of teeth; Scruparia has a relatively short vestibule and proportionally diminished collar. Both genera have erect colonies and are regarded as aberrant cheilostomes (Prenant & Bobin 1966, pp. 78, 94; Jebram 1992). Although they did not rule out convergent evolution of the these authors consider these genera outside the main line of cheilostome evolution and possibly indicative of a polyphyletic origin of cheilostomes. Furthermore, both Aetea and Scruparia brood * The more common spelling of this word, zooid, used by Huxley (1851, p. 579), can be roughly translated animallike (thing). We prefer the shorter alternative spelling zoid, derived from Bryozoid, (Reichert 1870, p. 248), roughly translated from latinized German, as a (thing) derived from an animal. lecithotrophic larvae in specialized organs. Primitive cheilostomes are thought to have possessed planktotrophic cyphonautes larvae because cyphonautes occur among both cheilostomes and ctenostomes (Reed 1991, p. 176), because no brooding has been observed among the earliest fossil cheilostomes (Taylor 1988), and because those early cheilostomes resemble species of Electra, Membranipora, and Conopeum; all of these genera possess at least some members with cyphonautes larvae (Reed 1991, p. 176). This paper reports the discovery of new species of Membranipora with a well-developed collar. As Membranipora holds an undisputed position near the base of the main line of cheilostome evolution, this discovery supports the hypothesis that most or all cheilostomes evolved from a ctenostome ancestor with a collar (e.g., Banta 1975; Taylor 1988; Jebram 1992).