The present data indicate that plasma T concentrations in males are
negatively correlated with ambient water temperature. At the onset of
the experiment males from both treatment and control tanks exhibited
plasma T concentrations similar to those of males at Seal Beach during
winter (36.3±3.7 ng/mL). Male stingrays subjected to water temperatures
typical of summer (~25 °C) showed a rapid decline in T
concentrations to levels similar to those in males caught at Seal Beach
during the quiescent phase. At the time of experimentation, treatment
rays were subjected to the short photoperiods of the degenerative phase,
the period when wild caught males exhibit the highest plasma T
concentrations. Because all captive rays were maintained at the same
photoperiod, increased water temperatures may act as an important
secondary cue for seasonal androgen regulation, and ultimately
reproduction in male round stingrays. In epaulette sharks seasonal
changes in plasma androgen concentrations were negatively correlated
with water temperature and showed a clear unimodal annual cycle
(Heupel et al., 1999). Interestingly, when held in captivity and not
exposed to seasonal water temperature changes, H. ocellatum will breed
year-round. In addition, geographically isolated populations of clearnose
skates breed at different times but similar water temperatures
(Rasmussen at al., 1999). While the exact mechanism of temperature
regulation of T concentrations is unknown, data from the present and
previous studies suggest it can be an important factor in elasmobranch
reproductive cycling and warrants further investigation.