OBSERVATIONS ON THE REPRODUCTIVE BIOLOGY OF THE 

 COWNOSE RAY, RHINOPTERA BONASUS, IN CHESAPEAKE BAY 12 



Joseph W. Smith and John V. Merriner 3 



ABSTRACT 



Cownose rays, Rhinopterabonasus, are abundant in Chesapeake Bay during summer. We made obser- 

 vations 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 (DW) of 75-85 cm. Males judged as mature averaged about 90 cm DW. Macroscopic 

 inspection of the oviducts suggested that females began to mature at 85-92 cm DW. Females judged 

 as mature averaged 96 cm DW. Only the left reproductive tract in female cownose rays appeared func- 

 tional and only one embryo per gravid female was observed. A total of 67 embryos ranging 18-440 mm 

 DW 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 DW. 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 DW), but histotrophic secretions of uterine villi provided nutrition for the young through 

 the remainder of gestation. 



The cownose ray, Rhinoptera bonasus, a large 

 myliobatoid ray, which attains a maximum weight 

 of 23 kg, is abundant in Chesapeake Bay during 

 summer (Schwartz 1965; Musick 1972) where it 

 preys heavily on commercially important shellfish 

 (Merriner and Smith 1979). Because of the severe 

 damage to shellfish beds and the paucity of infor- 

 mation on the biology of the cownose ray, the 

 Virginia Institute of Marine Science began a study 

 on the life history of the cownose ray in 1976. Prior 

 to our work, information on the cownose ray's 

 reproductive biology was primarily limited to obser- 

 vations of single gravid females, usually included in 

 more general literature (Gudger 1910; Bigelow and 

 Schroeder 1953; Joseph 1961; Hoese 1962; Bearden 

 1965; Orth 1975), and size at maturity was unknown 

 (Bigelow and Schroeder 1953). Schwartz's (1967) 

 brief abstract represented the most complete state- 

 ment on the species' reproductive cycle. Here, we 

 report on the reproductive biology of the cownose 

 ray, specifically on 1) the estimated size at matur- 



1 Based on part of a thesis submitted by the senior author in par- 

 tial fulfillment of the degree of Masters of Arts at the College of 

 William and Mary, Williamsburg, VA 23185. 



Contribution No. 1305 from the Virginia Institute of Marine 

 Science and the College of William and Mary School of Marine 

 Science, Gloucester Point, VA 23062. 



3 The College of William and Mary, School of Marine Science, 

 Gloucester Point, VA 23062; present address: Southeast Fisheries 

 Center Beaufort Laboratory, National Marine Fisheries Service, 

 NOAA, Beaufort, NC 28516-9722. 



ity for both sexes, 2) the definition of the gestation 

 period, and 3) the description of the embryonic 

 development. 



MATERIALS AND METHODS 



Most cownose rays were taken from pound nets 

 in the lower Chesapeake Bay during three summers, 

 1976-78, but some rays came from haul seines used 

 in spring along the Virginia-North Carolina coast, 

 and from gill nets and rod and reel catches. Disc 

 width (DW = distance between tips of the pectoral 

 fins) was measured in mm on a measuring board. 

 References to specimen size (including embryos) 

 hereafter are disc width measurements. 



We judged male cownose rays sexually mature 

 if 1) the clasper rhipidion was fully developed and 

 easily spread and 2) clasper cartilages were well 

 calcified (rigid). We measured clasper length as the 

 distance from the junction of the clasper and pelvic 

 fin to the distal end of the clasper. Criteria modi- 

 fied from Smith (1975) were used to determine the 

 following stages of sexual maturity for females: 



1) immature - ovaries thin and flaccid; uterus thin 

 and elongate, lining appears rugous. 



2) maturing - ovary slightly developed, yellowish 

 ova visible, ova <1 cm diameter; uterus somewhat 

 dilated, trophonemata (uterine villi) small, general- 

 ly <0.5 cm long. 



Manuscript accepted May 1986. 



FISHERY BULLETIN: VOL. 84, NO. 4, 1986. 



871 



