Natural Habitat: In the Solomon Islands we found R. yuma in a calm, clear, green-water lagoon at about 15 metres (45 ft.) growing on intact Pavona cactus skeletons; part of a reef slope built almost entirely by Pavona. This species is most commonly imported from Indonesia and Singapore.
Aquarium Care: Moderate to bright light. Slight water currents or calm. It will lose its dark pigment (bleach) and shrink if the light is too dim, or shrivel up and emit mesenterial filaments if the light is too strong (or if it is stung). Feed occasionally with blackworms.
Family Discosomatidae Duchassaing and Michelotti, 1864
Metarhodactis was erected by Carl-gren (1943) to describe a special corallimorph, M. boninensis. Photographs of this species in the original description look like Discosoma or especially like small Amplexidiscus. The description notes that this corallimorph has no sphincter and no marginal tentacles, though the margin is crenelate (like Amplexidiscus). It has tentacles on the oral disc, branched at the inner part of the disc and simple toward the margin. The presence of a special type of nema-tocyst distinguishes it: it has long hoplotelic p-mastigophors, similar to those of Corallimorphidae, otherwise not known in Discosomatidae.
As mentioned in chapter three, den Hartog (1980) proposed that all remaining genera of corallimorphs be included in this one family. Due to the lack of reliable taxonomic characters to differentiate between the genera, he further proposed that they all be classified as Discosoma (except for Amplexidiscus). To aid in identification of the various species we have opted to retain the four main genera: Amplexidiscus, Discosoma, Rbodactis, and Metarhodactis.
Members of the Discosomatidae possess the following traits: the body tends to be soft (Discosoma) to very rigid (Rbodactis cf. mussoides); tentacles range from inconspicuous protuberances (Discosoma) to well developed and branched outgrowths (.Rbodactis); both marginal and discal tentacles may be present and are non-retractile; marginal tentacles, when present, are tiny, acute or finger-shaped appendages, often with acrospheres (sometimes well-developed, see chapter three); mesogloea usually thick to very thick; sphincter very weak to absent; spirocysts are absent with the exception of Amplexidiscus which has minimal amounts; nematocyst batteries mainly containing homotrichs, can develop in marginal tentacles if present, or in acrospheres when present (see chapter 3); and the ectoderm has very few nemato-cysts (den Hartog, 1980). Finally, all the species within this family possess zooxanthellae but benefit from occasional feeding.
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