Reproductive biology and larval morphology of the marine plotosid Cnidoglanis macrocephalus (Teleostei) in a seasonally closed Australian estuary

Abstract

Monthly trends shown by gonadosomatic indices, the prevalence of the different gonadal stages, and the size distribution of the oocytes, indicate that the large marine and commercially important plotosid Cnidoglanis macrocephalus spawns in Wilson Inlet between October and January. The conclusion that spawning occurs within this seasonally closed estuary was confirmed by the presence of males in large nests and by the capture of newly-hatched, yolk sac larvae from one of those nests. The fact that C. macrocephalus, which is also widely distributed in coastal marine waters throughout much of southern Australia, can spawn within Wilson Inlet would be of particular value to this species in those periods when closure of the estuary would preclude a seawards spawning migration. Sexual maturity is size dependent, with spawning rarely occurring before fish have reached a total length of 425 mm. Sexual maturity was attained by a few fish at the end of their second year, by several at the end of their third year and by most, if not all fish, at the end of their fourth year. Comparisons with data for the more northern and permanently open Swan Estuary indicate that C. macrocephalus also spawns within that system and that the spawning time of this species is related to water temperature. The adult male guards the larvae under its pelvic fins in burrows. The larvae increased in total length from 29 mm just after hatching to 43 mm in the 17–18 days after capture, during which time their yolk sac was resorbed. Details are given of the morphology, morphometrics, meristics and pigmentation of larval C. macrocephalus. In comparison with the larvae of three other plotosid genera, the larva of C. macrocephalus is far larger in size and more developed at hatching and takes a shorter time to transform into a juvenile.