Zebra Danio (Brachydariio rerio, Hamilton-Buchanan) tropical fish are easily maintained and bred under laboratory conditions; procedures for maintenance and breeding are described. Intensive inbreeding of this fish gave rise to a high incidence of spontaneous skeletal deformities which appear to reflect a genetic defect rather than an environmental stimulus. This finding casts doubt on the suitability of the Zebra Danio for fish toxicity testing. I. INTRODUCTIONThe wide range of fish species used to assess the toxicity of environmental pollutants reflects the diverse requirements of regulatory authorities and the numerous practical considerations which influence selection of fish for toxicity tests. There is a growing awareness of the need for rationalisation of test species so that data from different laboratories can be compared and used to establish national and international standards for water quality and pollution control. Native species for fish are often chosen for acute toxicity tests because of their relevance to local environments, and consequently different species have been used in different countries. The difficulty can be resolved by establishing the relationship between the response to pollutants of native species and the fish chosen as a standard for toxicity tests. Tropical fish are likely to be most suitable for breeding studies because large numbers can be bred and maintained readily in the laboratory, and because their rapid maturity provides an opportunity for two or three generation studies to be completed within a year. This paper describes our experience of breeding the Zebra Danio (Brachpdunio rerio, Hamilton-Buchanan) for toxicity tests.This tropical species was chosen because of its convenient small size, ease of breeding in the laboratory, lack of special requirements of water hardness, temperature, and pH, and its relative resistance to common diseases. Later we found that spontaneous deformities arose when it was in-bred, making it unsuitable for toxicity studies.

Laboratory bred Zebra Danio (Brachydanio rerio, Hamilton-Buchanan) tropical fish are prone to skeletal deformities resembling scoliosis and lordosis. This condition appears related to diet and has been studied by breeding from different broods of fish for three generations. Two broods bred using two commercially available tropical flaked foods developed severe spinal curvature three to six weeks post-hatching. A third brood of fish fed exclusively on live food did not develop any deformity. The results demonstrate that a dietary factor is responsible for the deformities and that the Zebra Danio is especially susceptible to this factor as other species breed normally when fed the same commercial diet. Analysis of the diets showed no deficiency in either ascorbic acid (Vitamin C) or tryptophan, deficiency of either being associated with scoliosis and lordosis in fish. The analysis did show, however, a relationship between the lead content of the diet and the incidence of deformities. Lead has previously been implicated in skeletal deformities in second and third generation Brook trout, Salvelinus fontinalis.

A procedure is described for breeding Convict Cichlid (Cichlasoma nigrofasciatum) tropical fish under laboratory conditions. Three generations of Convict Cichlids were bred successfully using this procedure and there was no significant incidence of deformities from inbreeding to the F,, which contrasts with a high incidence of skeletal deformities observed when Zebra Danio tropical fish were inbred to the F8 generation. The Convict Cichlid is a suitable species for laboratory toxicity testing, particularly studies to assess effects on breeding performance.