Observations on growth rate and allometry in the seasonal predatory killifish Nothobranchius ocellatus (Teleostei: Cyprinodontiformes)

Abstract

During the course of a taxonomic study involving Nothobranchius ocellatus, a fast-growing seasonal killifish of predatory behaviour, important morphometric differences were observed between the original measurements of the lost holotype and the neotype and additional available material of the species. In a laboratory study, the total length (TL) of selected specimens was measured from hatching and, during the sub-adult to sexually mature young adult phase, from 37 to 84 days age, subjected to an additional suite of detailed morphometric measurements. Growth rate was relatively rapid and linear at 1.3–1.4 mm/day through the first phase to sexual maturity at 7–8 weeks, followed by a marked flattening of the growth curve with, from 14 weeks, rates of only about 1.0 mm/week in males and 0.6 mm/week in females. Under captive conditions, maximum TL for adult male specimens ranged from 101 to 116 mm and 88 to 102 mm for females. Analyses of the morphometric character measurements made during the 37–84 day development phase showed allometric shape changes, primarily seen in body depth, head length and several other measures of head features, proportions of caudal peduncle, and length of fin bases. These changes are most prominently seen in males and changes in head proportions are important in relation to the predatory behaviour of the species. Morphometric differences between the immature holotype and the newly available material, including the neotype, are consistent with allometric shape variation, particularly in males of this species. Considering the seasonal life cycle common to all Nothobranchius species, and similar growth patterns, it is likely that allometric growth has broad relevance within the genus, suggesting it is a factor that should be taken into account in taxonomic endeavours, especially when the number of study specimens is limited.