Ultrastructure of the spirocyst tubule in black corals (Coelenterata: Antipatharia) and its taxonomic implications

Abstract

Spirocyst tubules in five shallow-water antipatharian species Antipathes pennacea, Antipathes sp. cf. salix, A. fiordensis, A. galapagensis, and Cirrhipathes luetkeni contain an electron-opaque, helically arranged structure, composed of four distinct and separate bundles of microfibril-like material. Each bundle appears to be solid, rather than hollow, and is enveloped by a sheath of lesser electron-opacity that is continuous with the pleats. Tubules from a more distantly related, deep-water antipatharian, are similarly configured but are composed of only three distinct bundles, separated from one another by electron-opaque sheath material. The bundles are electron-lucent; their substructure was not preserved because this specimen was prepared for microscopy after preservation in alcohol for 27 yr, apparently without initial fixation. Most other details of the spirocyst tubule and capsule can be determined from this museum specimen. The arrangement within the tubule of multiple bundles, clearly defined by sheaths, is structurally distinct from other zoantharian spirocysts, and appears to be unique to the Antipatharia. The tubule structure of the anemone Aiptasia sp., examined 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 information was obtained from Aiptasia sp. rodlets. It is unclear whether the microfibrillar bundles of antipatharian spirocysts differ from those of other zoantharians by being more amenable to resolution, or if their tubule substructure is fundamentally distinct from those of more typical zoantharians. In terms of overall structure, Aiptasia sp. spirocyst tubules contain a single bundle of rodlets without a clearly defined sheath, in contrast to antipatharians which have three or four bundles, each clearly defined. These structural differences may be useful in determining relationships both within the Antipatharia, and between antipatharians and allied orders.