The potential influence of photoperiod and temperature on the male reproductive physiology of the narrownose smooth hound shark, Mustelus schmitti.

Environmental conditions have a significant influence on the physiological processes that regulate the reproduction of fish. However, there is less information on this subject for chondrichthyans, and particularly for males. This study describes the spermatogenic cycle of the narrownose smooth hound shark Mustelus schmitti related to sex steroid plasma levels and the potential influences of photoperiod and temperature as reproductive cues. A total of 70 males were caught bi-monthly for over a year in the coastal waters of Argentina. Testicular samples were taken for histological characterization and blood samples were taken for the measurement of testosterone (T) and progesterone (P₄) plasma levels from each male. Bottom temperature was recorded at each sampling point and the daily photoperiod was estimated from Julian Day Calendar. Histological assessment revealed that the complete cycle of spermatogenesis in M. schmitti lasts just over a year, with the onset of the new spermatogenic cycle (spermatocyte formation) overlapping with the end (spermiation and filling of the seminiferous ducts) of the previous cycle. This particular phase was associated with the peak in T plasma levels, which was found to be significantly explained by the increase in photoperiod in spring. The rise in temperature above 15–16 °C at the end of spring was significantly associated with a reduction in T plasma levels. Additionally, an elevation in P₄ plasma levels was observed after the photoperiod increase phase. Males would be ready to mate from mid-spring, following female parturition and just before ovulation/pregnancy. [ABSTRACT FROM AUTHOR]