SENSORY DEVELOPMENT OF THE ANTARCTIC SILVERFISH PLEURAGRAMMA-ANTARCTICUM - A TEST FOR THE ONTOGENIC SHIFT HYPOTHESIS

In 1996 Montgomery proposed an ontogenetic shift in the use of visual and non-visual senses in Antarctic notothenioid fishes, with visual do minance in larval fishes giving way to son-visual senses in adults. On e prediction of the hypothesis is timing differences in the developmen t of the respective sensory systems, with the visual system expected t o develop earlier than the other systems. The volume of certain brain centres can be determined from fixed material and should correlate wit h sensory development. This study determined the relative volumes of v isual and lateral line brain areas. and relative eye size as a functio n of fish length in Pleuragramma antarcticum. The relative volume of o ptic tectum was largest in larval fish. exhibiting a negative allometr y with growth. The eminentia granularis, and crista cerebellaris (late ral line associated areas) were not recognisable in the smallest larva e: the!: became differentiated at standard lengths of 10-20 mm and the ir relative volumes continued to increase over the size range of fish studied (up to 150 mm standard length). Relative eve diameter decrease d dramatically over the size ranee 5-25 mm and then increased such tha t relative eve diameter doubled over the size range 25-30 mm. A simila r. but less extreme. pattern was seen over the size range 30-60 min. A bove 60 mm relative eye diameter increased slightly with size. Our int erpretation is that eye growth and somatic growth are on separate traj ectories, and the breaks in the relative eye diameter curve result fro m overwinter periods when somatic growth is static. but the eve contin ues to grow. These results provide support for the ontogenetic shift h ypothesis. and indicate that the timing of the shift probably occurs a fter the second winter.