Zebrafish Danio rerio

Zebrafish is a small South Asian cyprinid - FishBase gives 3.8 cm standard length, 5.5 years in captivity and maturity at about 2.5 cm - that matters to veterinary medicine not as a food animal but as the most heavily used laboratory fish. European husbandry guidance houses it at 24 to 29 degrees Celsius, with standard growth conditions at 28.5, pH 6.5 to 8, conductivity 150 to 1,700 microsiemens per centimetre, a 14-hour light day and four to ten adults per litre, breeding at two to four months. Under EU Directive 2010/63 larvae become protected animals from 120 hours post-fertilisation, so welfare monitoring and colony health surveillance are legal duties rather than good practice. Note that FishBase's stated 18 to 24 degrees sits below the laboratory standard; the two figures should not be averaged. Two pathogens dominate facility health programmes. Pseudoloma neurophilia, a microsporidian of the midbrain, spinal cord and nerve roots, was detected in 19 to 74 per cent of facilities submitting to the Zebrafish International Resource Center; most fish appear normal but show emaciation, spinal curvature, anxiety-like behaviour and reduced fecundity, and because it passes in ovarian fluid and inside eggs, surface disinfection of embryos does not stop it. Mycobacterium species arrive from tank biofilms.