Correlates of choroid rete development with the metabolic potential of various tropical reef fish and the effect of strenuous exercise on visual performance

Abstract

It is hypothesised that the transport of oxygen to the retinal cells of fish with Root effect haemoglobins (Hb) is impaired by strenuous exercise due to a proton load that drastically reduces arterial haemoglobin–oxygen affinity. Routinely active reef fishes have enhanced oxygen transport and anaerobic (i.e. blood lactate loading) potentials relative to inactive species. Surprisingly, the development of the choroid rete mirabile (employed as an oxygen concentrating apparatus in the eye) is directly correlated with post-exercise lactate loads rather than with the magnitude of the Root effect and suggests that an increased development is adaptive for fish with high anaerobic potentials. The hypothesis that visual performance is reduced by strenuous exercise was tested in Lutjanus carponotatus using the optomotor response. Moderate blood lactate loads (2 mmol l −1 blood lactate) and red cell swelling responses were induced by exercise, but the optomotor response threshold (180 min of arc) was maintained. A moderate metabolic disturbance does not therefore appear to be a liability for the visual performance of a tropical fish in possession of Root effect haemoglobins.