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<I>Cephalodiscus</I> (Hemichordata: Pterobranchia)

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<I>Cephalodiscus</I> (Hemichordata: Pterobranchia) A NEW SPECIES OF CEPHALOD/SODS (HEMICHORDATA: PTEROBRANCHIA), THE FIRST RECORD FROM THE TROPICAL WESTERN ATLANTIC' FREDERICK M. BAYER lnstitute of Ma,rine Science, University of Miami ABSTRACT The first representative of the cephalodiscid pterobranchs from the tropical Western Atlantic, Cephalodiscus (Cephalodiscus) atlanticus, n. sp., is recorded from the vicinity of Cay Sal Bank in the Straits of Florida. The new species is described, illustrated, and compared with two related species from tropical East Indian waters. INTRODUCTION Pursuing its long tradition of significant faunal investigations in American waters, the u.s. Fish and Wildlife Service continues to amass rich collections of marine animals from poorly known areas in the tropical Western Atlantic, primarily through the operations of the exploratory vessel OREGON under the direction of Harvey R. Bullis. One of the most interesting recent discoveries in the OREGON collec- tions is a species of Cephalodiscus (Hemichordata: Pterobranchia) from the Straits of Florida. I am very grateful to Mr. Bullis for the opportunity to study and report upon this noteworthy find. The Pterobranchia are small hemichordates with or without gill slits, with two or more tentaculated arms borne by the mesosome, and with recurved digestive tract, that live as aggregations or colonies within an externally secreted encasement of unknown chemical nature, but not chitinous (Hyman, 1959). Ridewood (1918) and John (1931) have given systematic sum- maries of the class Pterobranchia, and Johnston and Muirhead (1951) have compiled a list of the species collected by various Antarctic expeditions, summarized their geographic and bathymetric distribution, and commented upon the fossils attributed to the Ptero- branchia. General reviews of this class have been published by Dawydoff ( 1948), van der Horst and Helmcke (1956), and Hyman (1959), to which the reader should refer for concise accounts of anatomy, biology, classification, systematic position, and distribution. IContribution No. 391 from The Marine Laboratory, Univers'ity of Miami. 1962] Bayer: Tropical Atlantic Cephalodiscus 307 DISTRIBUTION The pterobranchs are primarily an Antarctic and Subantarctic group, of which only the genera Rhabdopleura (Order Rhabdopleur- ida) and Atubaria (Order Cephalodiscida), and a few species of Cephalodiscus have been reported from tropical waters. No species has heretofore been detected in the Caribbean region. Typically Antarctic species frequently recurring in widely separated collections and possibly of circumpolar distribution are Cephalodiscus hodgsoni Ridewood (=C. aequatus and C. inaequatus Andersson); C. nigrescens Lankester; C. densus Andersson (=C. rarus Andersson and C. anderssoni Gravier) ; and C. solidus Andersson. Cephalodiscus dodecalophus McIntosh occurs in Subantarctic localities. Species known from temperate areas in the southern hemisphere are Cephalodiscus evansi Ridewood from New Zealand; C. australiensis Johnston & Muirhead from southwest Australia; and C. gilchristi from South Africa. The SIBOGAExpedition obtained two species of Cephalodiscus in tropical waters in the East Indies, C. gracilis Harmer and C. sibogae Harmer. Also from the tropics, an unidentified species has been reported from the coast of Indo-China (Dawydoff, 1944, 1953), and C. indicus Schepotieff is known from the vicinity of Ceylon. Cepha- lodiscus levinseni Harmer, from the Straits of Korea off the west coast of Japan, is the only species of the genus previously recorded from temperate regions in the northern hemisphere. The discovery, in Caribbean waters, of a tropical species of Cephalodiscus apparently related to one already known from the East Indies is of considerable interest. It extends the known geographic range of the genus into the tropical parts of the Atlantic Ocean, and suggests that other species may await discovery there. Moreover, the close similarity of species in the East and West Indies, and their dissimilarity from all of the known species of Antarctic regions, seems further to confirm the faunal relationship of the tropical Western Atlantic with the Indo-Pacific, already implied by the distribution of other animal groups. Bathymetrically, most species of Cephalodiscus have been taken at depths between 50 and 650 meters, but the original specimen of C. gracilis Harmer was collected intertidally in a reef habitat off the east coast of Borneo. The presence of a related species in the West Indies 308 Bulletin of Marine Science of the Gulf and Caribbean [J 2(2) suggests that pterobranchs should be sought in the reef habitats of this area as well. Genus Cephalodiscus McIntosh Cephalodiscus McIntosh, 1882: 348. (Type species, Cephalodiscus dode- calophus McIntosh, 1882, by monotypy.) Definition.-"Coenoecium consisting of a massive, irregularly-branch- ed, fucoid secretion resembling chitine, hispid with long spines of the same tissue, and honeycombed throughout by irregular apertures, channels, and spaces, in which the separate and independent polypides occur singly or in groups. "Lophophore richly plumose, with an enormous buccal shield and large oral lamella, the mouth opening between the two. Anus on the anterior dorsal prominence, behind the plumes. Two large eyes abut- ting on the ovaries. The homologue of the funiculus is short and quite free, its tip serving for the development of buds." (McIntosh, 1882, p. 348.) Remarks.-Because of the general similarity of the zooids of the various species of Cephalodiscus, the discrimination of species depends upon the characters of the coenecial structures that they produce. McIntosh's original description, quoted above, was based upon a single species and consequently does not take into account the diverse variations of coenecial architecture developed by the several species that have since come to light. The new species described below falls within the typical subgenus, Cephalodiscus s.s., which is defined as follows (John, 1931, p. 257): "Colony branching. Each ostium leading into a cavity which is continuous through the colony, and is occupied in common by the zooids and their buds. The walls of the coenecial cavity usually of irregular thickness and sometimes with inwardly projecting bars and ridges." Cephalodiscus (Cephalodiscus) atlanticus, new species Figs. 1-6 Material.--Several branches of coenecium, apparently from the same colony, trawled off Cay Sal Bank in the Straits of Florida, 24°03'N, 0 80 30'W, 150 fathoms, by the motor vessel OREGON at sta. 1349, July 18, 1955. Holotype, U.S. National Museum cat. no. 11612. Description.-The type consists of several fragments, apparently parts 1962] Bayer: Tropical Atlantic Cephalodiscus 309 2 3 ~ fum 10 mm. i.Omm. FIGURES1-6. Cephalodiscus atlantic us, new species. Fig. I.-Branch of coene- cium enlarged according to 10 mm scale. Figs. 2-3.-Lateral branchlets with single and multiple ostia, enlarged according to 1.0 mm scale between the figures. Fig. 4.-Part of main stem and one branch, showing midrib free of coenecial wall; enlarged according to 1.0 mm scale below Fig. 4. Fig. 5.- Terminal ostium, enlarged to same scale as Fig. 4. Fig. 6.-Cross section of main stem showing midrib immersed in coenecial wall, enlarged according to 1.0 mm scale below. Lettering: fum, lumen; mid, midrib; ost, ostia; prm, peristomial membrane; prs, peristomial spine. 310 Bulletin of Marine Science of the Gulf and Caribbean [12(2) of a single colony, of which the longest measures 27 mm. The coenecium is slender, profusely arborescent (Fig. 1), and contains a continuous tubular space opening to the exterior by way of the lateral branchlets. The coenecial substance is of a pale amber color. The coenecial walls consist of two layers as in other members of the genus, and the component lamellae are clearly visible in transmitted light. Inclusions of foreign matter, such as foraminiferal tests and molluscan shell fragments, are present but not abundant except in one branch that arises from a membranous expansion which may represent a base of attachment. The main branches are about 1 mm in diameter, tubular, and strengthened along one side by a stout midrib, which subdivides and extends as a support along each branch. Ostia for the egress of zooids do not occur directly in the walls of the branches, but always are placed at the ends of short lateral branchlets, each stiffened by an extension of the midrib which is further produced as a slender marginal spine (or peristomial filament) 4-6 mm in length. The shortest lateral branchlets may be either cylindrical or somewhat expanded distally, and the lip may be simple and thin or thickened as a distinct rim. The longer lateral branch lets become trumpetlike (Fig. 3) and develop as many as five marginal filaments, partly by subdivision of the midrib, partly by outgrowth of the ostial margin. In some of them, the flared end is compressed and the aperture constricted into two or more ostia (Fig. 2). In some cases, the original direction of the branch is maintained and the secondary ostia are left behind in a lateral position; in other instances, both terminal ostia continue to elongate, thus producing a bifurcation of the branch. The midrib is very prominent and usually situated between the two layers of the .coenecial wall (Fig. 6). However, at the broken end of one of the larger branches, the midrib stands quite free in the coenecial cavity; at a nearby branch axil it connects with the coenecial wall in three places (Fig. 4). It appears that such a condition could be brought about by the expansion of the ostial margin around the marginal filament in such a way that the latter stands up like a spadix within a spathe formed by the periostial wall. The free midrib might then reattach to the coenecial wall at any point in upward growth, but most likely at points of bifurcation. Unfortunately, the specimen is so poorly preserved (it seems to have been partially dried out at some time) that nothing can be said 1962] Bayer: Tropical Atlantic Cephalodiscus 311 about the zooids, not even the number of tentacles. No ova could be found in the coenecial cavity, and sex of the individuals was not determined.
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