ULTRASTRUCTURE OF THE SPIROCYST TUBULE IN BLACK CORALS (COELENTERATA,ANTIPATHARIA) AND ITS TAXONOMIC IMPLICATIONS

Spirocyst tubules in five shallow-water antipatharian species Antipath es pennacea, Antipathes sp. cf. salix, A. fiordensis, A. galapagensis, and Cirrhipathes luetkeni) contain an electron-opaque, helically arra nged structure, composed of four distinct and separate bundles of micr ofibril-like material. Each bundle appears to be solid, rather than ho llow, and is enveloped by a sheath of lesser electron-opacity that is continuous with the pleats. Tubules from a more distantly related, dee p-water antipatharian, are similarly configured but are composed of on ly three distinct bundles, separated from one another by electron-opaq ue sheath material. The bundles are electron-lucent; their substructur e was not preserved because this specimen was prepared for microscopy after preservation in alcohol for 27 yr, apparently without initial fi xation. Most other details of the spirocyst tubule and capsule can be determined from this museum specimen. The arrangement within the tubul e of multiple bundles, clearly defined by sheaths, is structurally dis tinct from other zoantharian spirocysts, and appears to be unique to t he Antipatharia. The tubule structure of the anemone Aiptasia sp., exa mined for comparison, is consistent with that of spirocysts from other actiniarians and zoantharians generally, including the appearance of electron-opaque rodlets within the tubule lumen. No substructural info rmation was obtained from Aiptasia sp. rodlets. It is unclear whether the microfibrillar bundles of antipatharian spirocysts differ from tho se of other zoantharians by being more amenable to resolution, or if t heir tubule substructure is fundamentally distinct from those of more typical zoantharians. In terms of overall structure, Aiptasia sp. spir ocyst tubules contain a single bundle of rodlets without a clearly def ined sheath, in contrast to antipatharians which have three or four bu ndles, each clearly defined. These structural differences may be usefu l in determining relationships both within the Antipatharia, and betwe en antipatharians and allied orders.