Observations On The Reproductive-Biology Of The Cownose Ray, Rhinoptera-Bonasus, In Chesapeake Bay

Abstract

Cownose rays, Rkinoptera,bO?IIUI1l8, are abundant in Chesapeake Bay during summer.We made observations on the reproductive biology of specimens collected primarily from commercial pound nets and haul seines from May through October 1976-78.Clasper development suggested that males began to mature at disc widths (OW) of 75-85 em.Males judged as mature averaged about 90 em OW.Macroscopic inspection of the oviducts suggested that females began to mature at 85-92 cm OW.Females judged as mature averaged 96 em OW.Only the left reproductive tract in female cownose rays appeared functional and only one embryo per gravid female was observed.A total of 67 embryos ranging 18-440 mm OW were collected and the sex ratio of the embryos was 1:1.Gravid females carried three-quarter term embryos in May and parturition occurred in late June and July.Full-term embryos averaged about 40 cm OW.Gestation of another group of embryos began by August.Growth of these embryos was rapid and they were relatively large when cownose rays left the Chesapeake Bay in October.Cownose rays exhibited aplacental viviparity.Yolk reserves supplied the initial energy demands of the embryos (up to about 20 cm OW), but histotrophic secretions of uterine villi provided nutrition for the young through the remainder of gestation.