Morphological, histological and behavioral features indicate thatMycedium elephantotus, a zooxanthellate scleractinian species without tentacles, is well adapted for utilizing suspended organic matter for nutrition. The colonies are composed of vertically growing fan-like plates and can reach diameters of more than 1 m in depths below 20 m. The external body surface is coated with a mucus layer (cuticle) which enables the acquisition and accumulation of suspended organic material. The mucus-entangled particles pass to the mouth openings by gravitational transport assisted by water movement. In experiments the corals were able to discriminate between suspended food and mineral particles. Both types of particles were rapidly entangled in fine mucus nets or filaments. Mineral particles were never ingested and instead tumbled down the inclined skeletal plates. In contrast, food particles were actively incorporated when the mucus filaments accidentally touched the stomodaea during the downward gliding. The food-enriched mucus filaments were either transported by ciliary activity into the coelenteron or were sucked into the body cavities by decreasing pressure in the coelenteron caused by contraction of longitudinal, mesenterial muscles. The discriminative reactions to mineral or food particles are probably based on the release of different types of mucus. Nematocysts are infrequent in the oral epidermis, indicating that the capture of living prey plays a subordinate role in nutrition. The mesenterial filaments, in contrast, are densely packed with large nematocysts. Storage products were piled up within the tissues of gastral pockets. The adaptations ofMycedium elephantotus for using suspended food particles may explain the particularly high abundance of this species between ca. 20 and 40 m depth on a steeply inclined fore-reef slope in the Gulf of Aqaba (Red Sea). The evidence indicating the importance of heterotrophic fueling toM. elephantotus is supported by carbonate production rates which are, in contrast to that of many other zooxanthellate scleractinian species, almost constant at depths between 5 and 40 m and which are uneffected by varying light regimes over the year, suggesting that the reduced phototrophic contribution by the zooxanthellae is compensated by mucus suspension feeding.
Scleractinia (Hard Corals)
Israel - Red Sea
SCUBA (open-circuit or unspecified)