Sensory development of the Antarctic silverfish Pleuragramma antarcticum : a test for the ontogenetic shift hypothesis

In 1996 Montgomery proposed an ontogenetic shift in the use of visual and non-visual senses in Antarctic notothenioid fishes, with visual dominance in larval fishes giving way to non-visual senses in adults. One prediction of the hypothesis is timing differences in the development of the respective sensory systems, with the visual system expected to develop earlier than the other systems. The volume of certain brain centres can be determined from fixed material and should correlate with sensory development. This study determined the relative volumes of visual and lateral line brain areas, and relative eye size as a function of fish length in Pleuragramma antarcticum.The relative volume of optic tectum was largest in larval fish, exhibiting a negative allometry with growth. The eminentia granularis, and crista cerebellaris (lateral line associated areas) were not recognisable in the smallest larvae; they became differentiated at standard lengths of 10–20 mm and their relative volumes continued to increase over the size range of fish studied (up to 150 mm standard length). Relative eye diameter decreased dramatically over the size range 5–25 mm and then increased such that relative eye diameter doubled over the size range 25–30 mm. A similar, but less extreme, pattern was seen over the size range 30–60 mm. Above 60 mm relative eye diameter increased slightly with size. Our interpretation is that eye growth and somatic growth are on separate trajectories, and the breaks in the relative eye diameter curve result from overwinter periods when somatic growth is static, but the eye continues to grow. These results provide support for the ontogenetic shift hypothesis, and indicate that the timing of the shift probably occurs after the second winter.