Since the summer of 1991, a field project has been conducted by biologists from Albion College and the Apex Predators Program to investigate the reproductive biology of nurse sharks (Ginglymostoma cirratum) in the south western Florida Keys. This project differs from many other studies of shark reproduction by using direct underwater observation of mating activities. The clear waters of the Keys and the apparent preference of ovulating females for shallow water permit detailed observation and videography of sharks by divine biologists.
In addition to disclosing these largely, undescribed activities, the study promises to provide information on population genetics through the use of DNA comparisons, to more completely reveal details of social organization in sharks, and also to provide access to an ongoing wild population of nurse sharks for studies of reproductive physiology. The study has demonstrated the importance of habitat to actively reproducing sharks, and measures have been taken to close the mating grounds for this population during the mating season.
The most recent aspect of the long-term study has taken parts of the investigation's field component to the captive laboratory (Sea World of Florida) for research on gestation. For the last several summers, shark mating activities were observed and recorded in the wild, and two females that were noted as participants in mating events were subsequently captured. Each year, selected females were examined in the field using ultrasonography to determine the extent of the presence of eggs and transferred to holding facilities at Sea World. In collaboration with veterinarians and laboratory staff, serial blood samples were taken from both animals during the one year captive period, and one animal was examined monthly using ultrasound and intrauterine endoscopy to assess the progress of the presumed pregnancy.
While in captivity, the female's blood was sampled periodically and changes in steroid hormones were monitored. Endoscopic procedures verified the presence of numerous eggs and revealed the appearance of young that had emerged from egg cases. Ultrasonic imagery was used to further confirm the presence of uterine hatchlings. One of the animals eventually carried young to term. The first births occurred after five months. These earliest births occurred with large yolk sac remnants in place and most of the animals did not survive. A total of 15 offspring were born, 5 from one female and 10 from the second. One pup has survived, and both of the females and the surviving offspring were tagged and returned to the site of capture. Though unique, this study can serve as a valuable "template" with which to understand mating, pupping and reproduction in other species of large sharks.