The development of seahorse vision: morphological and behavioural aspects

Seahorses are visually guided feeders that rely on a well-developed visual system to successfully predate on small shrimp and plankton. Despite their dependence on vision for feeding and survival, very little is known about seahorse vision and visual development. Many species of seahorse have a specialisation in their retina associated with acute vision, a convexiclivate, rod-free fovea located at the centre of gaze. In the first part of this thesis, the foveae of temperate Hippocampus abdominalis and tropical H. taeniopterus seahorses are characterised. Both species possess a rod-free convexiclivate fovea and the pattern of photoreceptor and ganglion cell distribution is similar. Despite these similarities, H. taeniopterus has higher cell densities on the foveal slope and better theoretical and behaviourally measured visual resolution compared to H. abdominalis. These data indicate that seahorses have a well-developed acute visual system, and this tropical seahorse species has higher visual resolution compared to this species of temperate seahorse. In the second part of the thesis, developmental changes in foveal morphology are analysed using three increasingly larger sized groups of H. taeniopterus seahorses. Morphologically, the depression of the foveal pit deepens as the fish grows. The area of rod-free zone also increases with fish size. The photoreceptor and ganglion cell densities increase at the foveal slope, although there is a decrease in cone and ganglion cell density at the foveal centre this is observed as the fish grows. Both the theoretical and the behavioural visual resolution improve with increasing fish size, with larger fish being able to detect the same sized prey at a greater distance. The behaviourally measured increased visual function correlates with changes in morphology with fish size. The last aspect of the thesis uses five H. taeniopterus seahorse groups of increasing size to analyse the morphological and behavioural development of the fovea and rod-free zone, with an emphasis on the contribution of cell death and proliferation. The behavioural visual resolution improves with size. Morphologically, the area of rod-free zone increases in size during development. BrdU labelling, a measure of cell proliferation, shows the absence of cell division in the ONL; however, there are dividing neuroretinal cells in the INL within the rod-free zone. The TUNEL assay confirms the lack of programmed large-scale photoreceptor cell death in and around the rod-free zone during development. Taken together, our data are not able to rule out the possibility of cone photoreceptor cell generation from INL stem cells within the rod-free zone, although cell death is not likely to be a major mechanism of foveal development. Future studies are designed to determine the underlying mechanism in the establishment of the adult rod-free zone