Elliott and Cook (1989) showed that the Caribbean corallimorpharian Discosoma sanctithomae rel ies on zooxanthellae for its nutritional requirements during the day but at night the polyps exhibit morphological changes designed to capture prey items. In the evening, a semi-transparent membrane along the outer edge of the polyp expands, greatly increasing the surface area of the polyp. When prey contact the discal tentacles this outer membrane folds inwards, capturing the prey against the mouth within a few seconds. Within forty five minutes to an hour the prey is ingested; again no mesenterial filaments are extruded. Undigested remains are excreted one to two days later. As in Amplexi-discus, the marginal tentacles possess abundant nematocysts which may aid in preventing escape or subduing the prey item.
In our experience, most Discosomatidae do not require direct feeding to do well in aquaria as they often receive enough food from stray fish food, or from planktonic organisms at night. In some cases such as with Amplexidiscus and Rbodactis, the occasional direct feeding is quite beneficial. Feeding also increases growth and the rate of asexual reproduction. A general rule with mushroom anemones is that if they have large bumps or tentacle-shaped protrusions, they may accept additional feedings. Smooth surfaced forms, however, generally obtain enough nutrition from their zooxanthellae or via mucus and ciliary feeding on bacteria, detritus and small zooplankton (see chapter nine for more detailed information on feeding the individual species).
Few studies have been conducted on growth rates in corallimorphar-ians. As Chen et al (1995b) pointed out, it is possible that size is controlled by environmental factors such as temperature and desiccation, as it is in some sea anemones. In our aquaria, it is not unusual for colonies of Discosoma and Rbodactis spp. to spread quickly, while others such as Ricordea spread much more slowly. Since asexual reproduction represents the primary means by which corallimorphs spread in our aquaria it stands to reason that factors affecting this will determine the rate of colony and polyp growth. Chen et al. (1995a) demonstrated a significant positive relationship between temperature and the rate of asexual reproduction. It is possible that other factors such as nutritional status and perhaps light intensity will also be shown to play an important role in colony growth rates.
Aggressive and Defensive Mechanisms
Corallimorpharians share space on the reef with a multitude of other organisms. As a result, they have developed strategies for protecting
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