saw no evidence of increased TUNEL positive staining during the premeiotic
stage as was observed in S. acanthias. This contrast is likely
due to the difference in reproductive strategies between the two
species. Unlike U. halleri, S. acanthias is a biennial breeder and
spermatogenesis occurs over a protracted two year period (McClusky,
2005). Throughout this period testes contain two distinct spermatogenic
cohorts, separated by a clear zone of degeneration (ZD) composed
of earlier stage spermatocysts entering apoptosis. The ZD
observed in round stingrays, similar to that observed in Atlantic
stingrays, is composed of mature spermatocysts and is associated with
seasonal testicular regression and sperm release