Meeting Abstract
The visual system of the cownose ray (Rhinoptera bonuses) demonstrates temperature sensitivity similar to that of cranially endothermic billfishes. Although physiological mechanisms that support cranial endothermy have not been identified in batoids, previous authors have described a pre-cerebral rete in some species of derived Myliobatids. The function of the pre-cerebral rete has been assumed to play a role in cranial thermoregulation. Upon gross and histological examination, we confirmed that R. bonuses brains possess an extensive vascular network, extending anteriorly to perfuse the olfactory structures, ventrally to the saccus vasculosus, and posteriorly to the thyroid gland. However, the network was composed of branching arteries that did not directly contact one another as is typical of countercurrent heat exchanging retia, which conduct heat between arterial and venous blood. Field-based temperature measurements from freshly landed rays were inconclusive and raise speculation that the pre-cerebral rete contributes to brain temperature regulation. Though temperatures across the body were elevated 1-3°C above ambient, these measurements are lower than those reported for other thermoregulating fishes. Elevated temperatures from cownose rays may reflect either the production of heat by the red muscles or the retention of that heat for thermoregulation. Ongoing field temperature measurement data will be supplemented by thermal manipulation experiments. Future work will model the extent to which R. bonuses may produce or retain heat, as well investigate the potential for elevated body temperatures to contribute to brain warming.
