Abstract

Valuable information on maturity and reproductive status can be obtained by the measurement of the concentrations of steroid hormones in the serum of captive carcharhinid sharks. The sharks at Sea World of Florida, Orlando, Florida, include mature females of several species of placental sharks. Serial serum samples from four mature female Carcharhinus plumbeus, one maturing female C. leucas, and one mature and one nearly mature female Negaprion brevirostris were used to determine serum concentrations of 17ß-oestradiol, progesterone, testosterone and dihydrotestosterone. The data from these mature placental sharks were then compared with concentrations measured in immature female and male conspecifics and in mature female Eugomphodus taurus (family Odontaspididae). Evidence of repetitive cyclical patterns was obtained from a mature female N. brevirostris. This shark was monitored from June 1988 to July 1990 by means of more than 50 samples that were often obtained at weekly intervals. Concentrations of 17ß-oestradiol in the serum of this shark varied in a cyclical fashion, with the highest values obtained in Year 1 (1988-89) being during April (the normal mating season) and those in Year 2 (1989-90) being during June. Concomitant elevations of testosterone concentrations were observed either simultaneously or immediately before or after maximal oestradiol concentrations were noted. Progesterone concentrations were low except for definitive spikes that immediately preceded the maximal concentrations of testosterone and 17ß-oestradiol. These periodicities suggest, for placental sharks, that consistently rising oestradiol concentrations may set in motion preovulatory events, that testosterone may be important for the initiation of subsequent ovulatory events, and that transitory elevations of progesterone concentrations may have specific short-term roles. These cyclical changes in the concentrations of steroid hormones observed in captive elasmobranchs support observations of hormone concentrations during reproduction in wild carcharhinids.