Abstract

The reproductive cycles of captive as well as wild Southern rays, Dasyatis americana, are well documented.^1,2 Little is known about the reproductive activities of the spotted eagle ray, Aetobatus narinari. A variety of values for duration of gestation, age of sexual maturity and inter-pregnancy intervals are mentioned in the literature for both species. Rays as well as other elasmobranchs are ectothermic, and as such are greatly affected by the temperature of their exhibit or habitat as well as other environmental parameters such as salinity, photoperiod, diet, etc. These variables may account for different values reported from a variety of institutions and researchers.

Regular ultrasonography of female rays of both species can accurately define the reproductive cycles of these exhibit animals. It is a quick, easy, and non-invasive procedure. Ultrasonography can be coupled with plasma chemistry testing of various hormones to further define the animal's cycle. D. americana and A. narinari are both viviparous and carry their multiple embryos in a uterus lined with villi referred to as trophonemata. The embryos live off of yolk sacs in the early stages of the pregnancy but ingest an organically rich histotroph secreted by the villi during the remainder of the pregnancy.

At Ripley's aquariums a monthly reproductive census of the collection's Southern and spotted eagle rays is performed. 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 may be 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 ensures the maximum survivability of pups. Ultrasonography provided information to help define the reproductive cycle of these species in captivity. For Southern rays gestation periods varied from 4 to 5 months. One ray in the collection had three multi-pupped litters in 10 months. Medroxyprogesterone (Depo-Provera), a parenteral human birth control medication, was administered to six Southern rays in an effort to curtail their reproductive effort. In mammals this drug alters the uterine lining and prevents implantation. It was not useful in disrupting pregnancies of Southern rays.

The spotted eagle ray in the Gatlinburg aquarium that produced pups on 9/27/09 has been monitored with the ultrasound. Little information is available concerning the reproductive cycle of Aetobatus, but this reproductively active ray has provided us with a starting point to help further define their reproductive cycle. The exact date of conception was not known, but it was estimated to have occurred within a month of implementing changes in the ray's daily ration that involved a large increase in the amount of food available. The gestational period was estimated to be 4 months.

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.  Henningsen A. 2003. Notes on reproduction in the Southern Stingray, Dasyatis americana (Chondrichthyes: Dasyatidae), in a captive environment. Copeia 2003: 826-828.

2.  Henningsen A., M.J. Smale, and I. Gordon. 2004. Elasmobranch Husbandry Manual; Pp. 244.