Corallimorphidae do not possess zooxantheilae and must acquire energy through feeding. As a result genera in these families have the widest variety and greatest numbers of nematocysts. The sticky tentacles of Corynactis spp. and especially Pseudocorynactis spp. are usually extended at night and are capable of capturing food items including zooplankton, small crustaceans and small fish (den Hartog, 1980). Nectactis and Sideractis, are deep water coral-limorphs speculated to feed on detritus and jellyfish (cien Hartog, 1987). Deep-dwelling Corallimorphus spp. also feed on a wide variety of small live foods and have very sticky tentacles.
Zooxanthellate families include the Ricordeidae and the Discoso-matidae. The major difference between the two families is that tentacles of the members of the Discosomatidae do not possess spirocyst nematocysts (except Amplexidiscus which may have small numbers of them), generally felt to be indicative of their ability to capture prey (den Hartog, 1980). As in the Corallimorphidae, Ricordeaspp. have a full compliment of nematocyst types but their spirocysts are quite small and much less numerous. As a result they are still capable of catching small prey items and the occasional feeding is encouraged in aquaria (see chapter nine). However, the presence of zooxantheilae probably accounts for a large portion of their nutritional requirements (den Hartog. 1980).
Despite the lack of spirocysts there is a wide degree of feeding behaviour observed in the Discosomatidae. It is generally agreed that given the large amounts of zooxantheilae found in the genera of this family, they are primarily autotrophic in nature (den Hartog, 1980, Hamner and Dunn, 1980; Miles, 1991). They lack nematocysts on their outer surface, but have them concentrated internally and around the mouth, and since they lack prey capturing spirocysts, den Hartog (1980) proposed that feeding played a very insignificant role in their nutrition, though uptake of dissolved nutrients directly across the body column could not be ruled out. However, subsequent studies by Hamner and Dunn (1980) and Miles (1991) have shown that some genera are in fact quite active feeders. For example Amplexidiscus fenestrafer (Elephant Ear polyps) feeds by enveloping its prey within a sac it creates with its body wall. The sides of the polyp move upwards and the top edge draws closed just like the purse strings on a pouch, preventing the prey from escaping. This entire process can occur as quickly as three seconds! The closing rate of Rhodactis howesii and Discosoma (=Actinodiscus) fu ngiform is is much slower; ten seconds to one minute. The upper margin of the body
Crowding is evident in this group of Discosoma sp. Discosoma spp. can rapidly cover rocks and glass in aquaria via asexual proliferation and rapid growth. J. C. Delbeek wall of Amplexidiscus contains marginal discal tentacles that do possess ectodermal nematocysts and these presumably function to prevent prey from escaping through the rapidly closing opening of the sac (Hamner and Dunn, 1980). Once the top is closed the mouth is opened and the prey is ingested. Similar behaviour was observed in Rhodactis howesii and Discosoma (=Actinodiscus) fungiformis, but in these species the mesenterial filaments were
extruded into the cavity and the large numbers of nematocysts (homotrichs) contained in these are thought to subdue the prey (Hamner and Dunn, 1980). However, A. fenestrafer does not extrude its mesenterial filaments and how the prey is subdued is not known. A mucus is excreted from the gastrovascular cavity into the sac and it is possible that some sort of toxin within this mucus subdues the prey. Fish left in this mucus for ten minutes or more generally do not recover even if moved to another aquarium with fresh seawater (Hamner and Dunn, 1980).
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