SOME STUDIES ON THE BIOLOGY OF THE CLOWN ANEMONEFISH, AMPHIPRION OCELLARIS CUVIER

Abstract

Some aspects of the biology of the clown anemonefish, Amphiprion ocellaris Cuvier were investigated. These included taxonomy, reproduction, behaviour, tolerance, diseases and parasites and fish-anemone relationship. Four species of the genus Amphiprion occur in the coral reefs around Singapore. Taxonomic studies revealed that the species under study was A. ocellaris and not A. percula as was reported before. The length-weight relationship provided a useful means for converting length data to weights. The relationship for A. ocellaris was found to be W = 0.0000293 L3.0965 , where "W" is the weight of the fish in grams and "L", the standard length in millimetres. It was, however, not possible to assess the length of the fish at first maturity from data on the relationship between relative condition factor, Kn and length. No sexual dimorphism was observed in A. ooellaris. Preliminary studies (by dissecting out the gonads) seemed indicate a preponderance of females in the population. The standard length at which female fish first attained sexual maturity was found to be around 43.6 mm. The fecundity of A. ocellaris was relatively low; it was found to vary from 210 to 696. Territorial and aggressive behaviour were commonly observed. When several fish were kept together, a social hierarchy or ranking order was established with the biggest and strongest individual as the dominant fish. A. ocellaris was found to be fairly tolerant to sudden changes of certain environmental factors such as pH, temperature and salinity. The 24-Hour TLM values for pH, temperature and salinity were 4.58, 34.67¡C and 4.40 ¡/oo respectively. When compared with direct transfer, gradual acclimation improved the survival of the fish in low salinities, but only slightly. The addition of 50 p.p.m. of calcium chloride to the ambient water during gradual acclimation did not enhance the tolerance of the fish to low salinities. Diseases and parasites were the major cause of fish mortality in this investigation. A. ocellaris was found to be susceptible to bacterial, protozoan, helminth and fungal infections. A study of these diseases and their treatment was made. The presence of host anemone in the aquarium neither enhanced growth and survival nor reduced the susceptibility of A. ocellaris to protozoan diseases. However, intra-specific aggression was found to be markedly reduced among fish kept with their host anemones.