2.6 Nutrition and Food Connections of Reef Fauna 



For many common species of the infauna, and particularly for the 

 corals, a diversity of modes of feeding is characteristic, including 

 symbiotic autotrophy. Among the animals which have plant symbionts in 

 addition to the corals are the actinia Anthopleura , the soft corals 

 Zooanthus , the colonial actinia Palythoa , the gastropod mollusks 

 ( TridachTa , Placobranchus ) , and the bivalve mollusks ( Tridacna , 

 Hippopus ) . The Tridacna , which dre quite large (length up to 1 m, 

 weight up to 250 kg), frequently play a significant role in the 

 productivity and accumulation of calcareous material on the reefs. 



The symbiotic algae include the dinoflagel lates (Zooxanthellae) , 

 the green algae (Zoochl orel lae) and individual chloroplasts of free- 

 living siphon algae, consumed by their animal hosts ( Tridachia ) . These 

 chloroplasts accumulate and function in special mucous glands of the 

 mollusks (Trench, 1971). 



In addition to the ability to use solar energy for autotophic 

 nutrition, most coelenterates (corals, hydroids, actinia) as well as the 

 bivalve mollusks, sponges and Ascidia , are able to effectively utilize 

 dissolved organic matter in sea water (Sorokin, 1973a; Sorokin, 

 Wyshkwarzew, 1972). The consumption of dissolved organic matter begins 

 at a concentration of some tens of pg/£ (Fig. 4), whereas the 

 concentration of low-molecular organic matter in the water above a 

 eutrophic coral community is 0.2-1 mg C/£, with the total content in the 

 water of 3-5 mg C/£. Its liberation by the benthic community in the 

 process of photosynthesis could be as high as 0.3-0.6 mg C/£ per day 

 (Khaylov, 1971). Consequently, we can assume that the dissolved organic 

 matter of sea water, like the plankton and seston, participates in the 

 transfer of energy between reef biotopes, bonding them into a unified 

 ecosystem. 



Another important source of nutrition for benthic filter feeders on 

 the reef is the bacterioplankton. Studies by the radiocarbon method 

 have shown (Sorokin, 1973e; Sorokin, 1973b, c) that among the filter 

 feeders which feed intensively on the bacterioplankton are the sponges, 

 bivalve mollusks, ascidians, polychaetes (serpul 1 ides, sabellides), 

 corals, hydroids and zoantharians ( Palythoa ) . The most active consumers 

 of bacterioplankton were found to be the sponges, polychaetes and 

 bivalve mollusks, for which, at a concentration of bacterioplankton near 

 the natural level of around 1-2«10° cl/ml , the filtration rate reaches 

 600 £/g of organic carbon of the filterers' body per day. The 

 filtration apparatus of these animals is so highly perfected that they 

 can consume bacteria with a biomass of less than 10 mg/m-^--50-100 times 

 lower than the natural concentration of bacterioplankton in the water 

 over a reef and 3-5 times lower than that of the surface waters of the 

 neritic zone of the ocean. Consequently, they can utilize 

 bacterioplankton from the oceanic water passing over the reef and thus 

 include this external source of energy in the metabolism of the reef. 



The daily ration of the active filter feeders of the reef, feeding 

 on bacterioplankton at its natural concentration, is 5-50% of their body 

 weight, sufficient to compensate for the expenditures for metabolism. 

 For the sediment feeders (madreporarians , zoantharians, hydroids) and 



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