Reproduction and embryonic development of the red stingray Hemitrygon akajei from Ariake Bay, Japan

Abstract

The reproductive system of the red stingray Hemitrygon akajei was described from 1,418 specimens, 682 males and 736 females, sampled year-round (2003–2014) from Ariake Bay to provide reproductive life history information for conservation and management of the species. Females reach sexual maturity at a larger size than males with the size at 50% sexual maturity 522.2 mm disc width (DW) and 321.5 mm DW, respectively. Male stingrays had semen in the seminal vesicles year-round. Dental sexual dimorphism was aseasonal. The mating period is protracted, spanning 7 consecutive months October–April but ovulation occurred during May. This suggests female sperm storage, which has not been described for dasyatid stingrays. Females have a single functional ovary and uterus (left) and reproduce via aplacental viviparity with lipid histotrophy. Gestation required 3 months with parturition during late July and early August. Uterine eggs without macroscopic embryos were observed during the first half of gestation suggesting a short period of arrested development or diapause. Developmental cohorts based on morphological features were described for embryos and can be used to characterize embryo growth and development for other stingray species. Observations of foetal mortality (1.25%) and morphologically abnormal embryos (0.72%) were uncommon. Pregnancy rate was 90% and reproduction was annual and synchronous. Hemitrygon akajei fecundity ranged from 7 to 25 and increased with female size. Although H. akajei is a medium-sized dasyatid ray, it has the highest fecundity reported for any batoid species. Elasmobranchs life histories usually are associated with a low degree of productivity that results in rapid population decline with increased fishing pressure. Producing high numbers of small young is a successful reproductive strategy for H. akajei that may be partly responsible for their resilience to fishing pressure and continued abundance in Ariake Bay. This study is the first to describe reproduction of H. akajei, an economically important top predator inhabiting coastal ecosystems throughout Asia.