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Introduction

The micrabaciids are known from the Early Cretaceous (about 135 million years ago) to the Recent. Although not commonly collected, they are found in all ocean basins, including off continental Antarctica, at depths of 49 to 5000 m (Keller, 1977), making them the second deepest-living coral family. Micrabaciids are exclusively azooxanthellate, and all are solitary in growth form, laying unattached on soft substrates. The largest known specimen is 51 mm in calicular diameter (Letepsammia, see Cairns and Zibrowius, 1997), but most species are less than half this diameter. Thirteen living species are known (Cairns et al., 1999) and an additional 22 species have been described as fossils. One species (Letepsammia formosissima) is known to host ascothoracidan crustacean parasites (Grygier and Zibrowius, 1985).

  

Squires (1967) and Owens (1984a, b) discussed the adaptations of the species of this family to the deep-water environment. According to Squires, as species gradually invaded deeper water, the polyp increased in size and the skeleton decreased in size, resulting in a progressive increase in the polyp:corallum ratio. Indeed, many deep-water Leptopenus appear to be primarily polyp with an inconspicuous, fragile, usually fragmented corallum embedded in its tissue. With increasing depth, the skeleton also reduces its number of septal trabeculae and increases its septal and thecal porosity, all of which lead to a small, very fragile, porous skeleton. The skeleton of a cleaned Leptopenus or Letepsammia is so porous that, when held to a light source, one can see through their bases. Owens (1984b) suggested that the micrabaciids were preadapted for deep-sea life by having ancestors that had auto-mobile coralla, a character that also benefits from having a light corallum.

  

Descriptions and illustrations of many of the Recent species are given by Cairns (1989). A discussion of the family and its Recent genera are also provided in that paper. Cairns (1989: 13) provides a key to genera.

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