School formation and concurrent developmental changes in carangid fish with reference to dietary conditions

Synopsis

The ontogeny of schooling behaviour was studied in comparison to the development of sensory and swimming organs and taxis in carangid fish. Striped jack, Pseudocaranx dentex, larvae showed strong phototaxis at 3 days after hatching (3.5 mm in TL) when they developed pigmentation in the retina. Rheotaxis and optokinetic responses were apparent at 4.0–6.0 mm TL as larvae completed development of the basic structure of their eyes. A major inflection of allometric growth occurred at 9 mm, and fin ray formation was completed at these stages. Schooling behaviour, represented by one IT of inter-individual distance and parallel orientation, only appeared at 16 mm TL, and just prior to this behaviour, fish showed mutual attraction through vision at 12 mm TL. Canalization of buccal lateral lines was complete at 18 mm TL, whereas that of trunk lateral lines started at 23 mm TL and was complete at 30 mm TL. With these results, we assumed that critical factors of the ontogeny of schooling behaviour in carangid fish include not only the development of sensory or swimming organs, but also other factors such as development of the central nervous system. To show this, we reared another carangid species, the yellowtail Seriola quinqueradiata, with dietary depletion of DHA (docosahexaenoic acid), which is indispensable for the development of the central nervous system. Although DHA-free fish showed optokinetic response, they did not show schooling behaviour when they attained their schooling size. Tracer experiments using radioisotope labelled DHA showed that DHA is incorporated into the brain, spinal cord, and retina of juvenile fish. Under natural conditions, carangid fish larvae should intake enough DHA through diet to develop schooling behaviour; the fluctuation of dietary quality in zooplankton might therefore influence the development of indispensable antipredatory behaviour. Morphological changes of striped jack occurred in two steps; first at 9–12 mm (fin formation and inflection of allometric growth) and then second at 20–30 mm (scale and lateral line formation), and these changes corresponded with the development of schooling and recruitment to coastal waters, respectively. Since the onset of schooling is the first step of active antipredatory behaviour, we considered that 12 mm TL is the size at which they attain the juvenile period.