Abstract
The reproductive cycle of captive and wild cownosed rays (Rhinoptera bonasus), is not well documented. A variety of values for duration of gestation, age of sexual maturity and inter-pregnancy intervals are mentioned in the literature.1 Rays, as other elasmobranchs, are ectothermic and, as such, are greatly affected by the temperature of their habitat and other environmental parameters, such as salinity, photoperiod and diet. These variables may account for different values reported.
Regular ultrasonography of female cownosed rays can accurately define the reproductive cycles of exhibit animals. It is quick, easy to do, and non-invasive. Ultrasonography can be coupled with plasma chemistry testing of various hormones to further define the animal's cycle.
R. bonasus is viviparous and carries its embryo in a uterus lined with villi, referred to as trophonemata. The embryo lives off its yolk sac in the early stages of the pregnancy but ingests an organically rich histotroph secreted by the villi during the remainder of the pregnancy. While cownosed rays do not have a placenta, there does seem to be a level of human chorionic gonadotropin (HCG) present in their blood when they are gravid. This hormone can be easily measured and employed to detect pregnancy at an early stage.
At Ripley's Aquarium of the Smokies in Gatlinburg, Tennessee, we conduct a monthly reproductive census of Rhinoptera. Using the ultrasound, a staging protocol has been developed that allows the staff to keep track of each individual's reproductive cycle. When rays are close to parturition they are moved to a nursery area. After birthing, they are returned to the exhibit. This monitoring program allows the animals to remain on display as much as possible but helps ensure the maximum survivability of pups. It also has provided information that helps define the cycles of this species in captivity. Captive rays have one pregnancy a year and exhibit a gestational diapause similar to that described in wild populations.2 A number of rays have been sampled and blood testing for HCG has been correlated to an animal's reproductive status as determined by ultrasound. Home pregnancy tests are capable of detecting HCG and confirming a ray's pregnancy about the same time it can be detected with the ultrasound, from one to two months post breeding. Because the results of this on-going program have proven useful from a practical as well as academic point of view, the program has been expanded to include a collection of Southern rays at the Ripley's Aquarium in Myrtle Beach.
Acknowledgements
The authors would like to thank the aquarists at the Ripley's aquariums for their professional support in handling the large numbers of animals involved in this project.
References
1. Smith JW, JV Merriner. 1985. Food Habits and Feeding Behavior of the Cownose Ray, Rhinoptera bonasus in the Lower Chesapeake Bay. Estuaries 8(3):305-310.
2. Smith JW, JV Merriner. 1987. Age and Growth, Movements and Distribution of the Cownose Ray, Rhinoptera bonasus, in Chesapeake Bay. Estuaries 10(2):153-164.