A Setigerous Collar in Membranipora Chesapeakensis N
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Invertebrate Biology 114(1): 83-88. © 1995 American Microscopical Society, Inc. A setigerous collar in Membranipora chesapeakensis n. sp. (Bryozoa): implications for the evolution of cheilostomes from ctenostomes William C. Banta, Federico M. Perez, and Scott Santagata Department of Biology, American University, Washington, D.C. 20016, USA Abstract. Membranipora chesapeakensis n. sp. was collected on wood substrate in the Ches apeake 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 gym nocyst; zoids have 14 tentacles and lack knobs or other decorations in their corners. Closest affinities are with M. tenuis Desor, sensu lato. A setigerous collar is present, the first reported among membraniporid cheilostomes. Membraniporids are morphologically similar to the ear liest fossil cheilostomes and to the encrusting ctenostomes from which most cheilostomes are thought to have evolved. A setigerous collar, probably generally present in ctenostomes, has hitherto been found only in two aberrant cheilostome genera. The presence of a setigerous collar in a species of Membranipora suggests that setigerous 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 lecithotrophic larvae in specialized organs. Primitive acellular secretion present in some bryozoan taxa. In cheilostomes are thought to have possessed plankto retracted zoids* the collar projects distally from the trophic cyphonautes larvae because cyphonautes occur diaphragm into the vestibule. When the lophophore is among both cheilostomes and ctenostomes (Reed extended, the collar projects from the neck of the au 1991, p. 176), because no brooding has been observed tozoid like the starch-stiffened formal collars popular among the earliest fossil cheilostomes (Taylor 1988), in Elizabethan England. We know of no published sug and because those early cheilostomes resemble species gestion as to its function. Setigerous collars have been of Electra, Membranipora, and Conopeum; all of these found to be present in all ctenostome bryozoans in genera possess at least some members with cyphon which they have been sought. However, they are autes larvae (Reed 1991, p. 176). known from only two cheilostome genera, Aetea and This paper reports the discovery of new species of Scruparia. The collar of Aetea is only a short ring of Membranipora with a well-developed setigerous col teeth; Scruparia has a relatively short vestibule and lar. As Membranipora holds an undisputed position proportionally diminished collar. Both genera have near the base of the main line of cheilostome evolu erect colonies and are regarded as aberrant cheilo tion, this discovery supports the hypothesis that most stomes (Prenant & Bobin 1966, pp. 78, 94; Jebram or all cheilostomes evolved from a ctenostome ances 1992). Although they did not rule out convergent evo tor with a setigerous collar (e.g., Banta 1975; Taylor lution of the collar, these authors consider these genera 1988; Jebram 1992). outside the main line of cheilostome evolution and possibly indicative of a polyphyletic origin of cheilo Methods stomes. Furthermore, both Aetea and Scruparia brood Colonies were fixed and stored in 70% ethanol. Some material was stained overnight in alum cochi * The more common spelling of this word, zooid, used by Huxley (1851, p. 579), can be roughly translated "animal neal, dehydrated through an ethanol series to propyl like (thing)." We prefer the shorter alternative spelling ene glycol and vacuum-embedded in Epotuf resin for zoid, derived from "Bryozoid," (Reichert 1870, p. 248), thin-ground sections containing calcareous tissues in roughly translated from latinized German, as a "(thing) place next to soft tissues (Nye et al. 1972). Other alum derived from an animal." cochineal-stained material was glycerinated by placing 84 Banta, Perez, & Santagata Figs. 1-4. Membranipora chesapeakensis n. sp. Fig. 1. Holotype colony photographed wet in 70% ethanol. Scale bar, 4.0 mm. Fig. 2. Paratype. NaGel-treated autozoids stained in congo red and coated in ammonium chloride; distal at top. Beaded gymnocyst (g); narrow, unadorned cryptocyst (c). Scale bar. 200 f.Lm. Fig. 3. Paratype, Transverse epotuf section of a Bryozoan setigerous collar 85 specimens in dilute glycerine and allowing the solution four or more zoid rows (Fig. 1). The vermiform por to evaporate in a petri dish for several days, then tion gradually expands laterally as it elongates, adding mounted in glycerine jelly. Slides were then ringed in new zoid rows and widening gradually into a bilaminar nail polish. Colonies soaked overnight in NaDCl to ribbon or elongate frond with 20 or more rows of remove all organic material disintegrated into frag monomorphic zoids on a side. Sections demonstrate ments, which were dyed with 1% aqueous congo red that the lateral colony margins are also occupied by and coated with sublimed ammonium chloride for light ordinary autozoids (Fig. 3); we saw no specialized photomacrography (Kier et al. 1965). kenozoids or other heterozoids. There are no stolons or other obvious provisions for attachment; the erect Taxonomic Account colony parts presumably grew from an encrusting base that has become lost or obscured. Colonies are lightly Family Membraniporidae Busk 1854 calcified and disintegrate into groups of a few zoids Membranipora deBlainville 1830 when treated in NaDCl. Membranipora chesapeakensis new species Autozoids are rectangular, arranged in quincunx, Type specimens. Holotype, Smithsonian Institu lightly calcified. The basal wall of each autozoid is tion, U.S. National Museum of Natural History, bryo adherent to that of one or more zoids on the opposite zoanType USNM 15223, preserved in 70% ethanol, side of the colony. Sections demonstrate that the basal Fig. 1), is a 0.1 g fragment of a mass of paratype cuticle is shared (Fig. 4); we found no evidence of USNM 15224 weighing 12.3 g, preserved in 70% eth foreign material or visible line of demarcation in the anol. Paratype specimen USNM 15225 is a dry slide basal cuticle. Inspection of the growing tip of the col from the same sample. Small colony fragments of ony confirms that the basal wall of each zoid develops paratype, labeled "Specimen 21," are deposited in the simultaneously with its basally opposite partner. This invertebrate reference collection at the Smithsonian indicates that the basal wall is developmentally and Environmental Research Center, Edgewater, Maryland; morphologically shared and implies developmental co a small amount of paratype, including sectioned ma ordination between the zoids on opposite sides of the terial, is retained in the senior author's collection. colony; it cannot be a case of one colony growing Type locality. A 12.4 g mass, from which holotype haphazardly on the back of another erect frond. and paratype specimens were selected, was growing Zoids on opposite sides of the colony are offset so on a small tree taken by otter trawl at 4-6 m from a that the lateral wall separating two zoids on one face mud bottom at the mouth of the Rhode River (Mary runs beneath the midline of a row of zoids on the land) between Cheston Point and Dutchman Point, opposite face (Fig. 3). The basal two-thirds of each Chesapeake Bay, in May 1993. Bottom salinity at the transverse wall is a simple calcareous partition perfo time of collection was 4%0. The collector was Timothy rated by communication pores; each pore is occupied C. Steelman of the Smithsonian Environmental Re by a refractile cincture and at least two specialized search Center, Edgewater, Maryland. cells that send lobes through the cincture from the Diagnosis. Colony tan, erect; branches vermiform, proximal to the distal side of the pore (Fig. 4, p). These ribbon-shaped, or frondose. Brackish water. Autozoids cells are herein named "bobin cells" after their dis rectangular, monomorphic, arranged in quincunx, coverer, the late Dr. Genevieve Bobin. The frontal third lightly calcified. Cryptocyst narrow and smooth; gym of each transverse wall resembles a lateral wall: a layer nocyst narrow and finely beaded. No spines, stolons, of cuticle continuous with the frontal membrane of the ovicells, or avicularia; 14 tentacles. A setigerous collar distal zoid and the vestibular floor of the proximal zoid fills most of the vestibule. is sandwiched between two calcified layers (Fig. 4, i). Description. Preserved colonies are tan and erect, This doubled wall apparently results from incorpora intermixed with muddy tubes of an errant polychaete, tion of intercalary cuticle into the transverse wall as identified tentatively as Hedista diversicolor. Each col the wall grows vertically from the original, simple, in ony branch originates from the distolateral margin of terior transverse wall. Lateral walls of each autozoid a flattened bilaminar frond as a vermiform cylinder of are provided with up to four pore plates 25-50 J-Lm in frondose branch containing 16 zoids, showing the relative positions of zoids and apparently unmodified marginal autozoids (m). The circle (x) is a grinding artifact. Scale bar, 500 f.1m. Fig. 4. Paratype. Median sagittal thin-ground epotuf section to show the setigerous collar. Frontal membrane