Abstract
The reproductive cycle of the Australian sharpnose shark, Rhizoprionodon taylori, includes a temporary suspension of development at the commencement of embryogenesis termed embryonic diapause. This study investigated levels of 17β-estradiol (E2), testosterone (T) and progesterone (P4) in plasma samples of mature wild female R. taylori captured throughout the reproductive cycle and correlated them with internal morphological changes. Levels of T were elevated through most of the embryonic diapause period, suggesting a role of this hormone in the maintenance of this condition. Increasing plasma T concentrations from late diapause to early active development were associated with a possible role of androgens in the termination of embryonic diapause. As in other elasmobranchs, a concomitant increase of E2 with ovarian follicle size indicated a direct role of this hormone in regulating vitellogenesis, while a peak in P4 suggested this hormone is associated with preovulation and ovulation. Additionally, significant correlations between photoperiod or water temperature and maximum follicular diameter and hepatosomatic index suggest that these abiotic factors may also play a role triggering and regulating the synchrony and timing of reproductive events.
Highlights
The ability to evolve and adapt reproductive strategies in response to changes in environmental conditions helps ensure the evolutionary success of any species [1]
Soon after parturition (January), diapausing ova consisting of bright-yellow dense yolk masses concealed in brown egg cases measuring 23.960.3 mm were observed in the uterus from mid February to mid September
Independent of the litter size, a single atretic vitellogenic follicles (AF) was present in the ovary throughout the diapausing period
Summary
The ability to evolve and adapt reproductive strategies in response to changes in environmental conditions helps ensure the evolutionary success of any species [1]. It has been postulated that environmental seasonality allows for the adjustment of reproductive events to occur over the most favourable period of the year maximizing survival and reproductive success [2,3,4]. In order to cope with unfavorable conditions and reduce energy waste, species have evolved several reproductive mechanisms (e.g. delayed fertilization, reduced gestation period), including embryonic diapause [2]. For live-bearing species delaying gestation allows parturition to occur when the environment is most likely to favour the survival of young [7,8]. It allows females to restore their energetic reserves after events of ovulation, mating, gestation and birth, without compromising the success of the following reproductive period [9]
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