Dr. Samuel Gruber
Bimini Biological Field Station
Dr. Kevin Feldheim
Pritzker Lab, Chicago Field Museum
Doc Gruber began what is now known as the PIT Project over 25 years ago. The project is one of the longest studies of sharks in the region, and has contributed to dozens of publications and dissertations over the many years. This long term research study led to the development of the "lemon shark family tree", which has enabled us to expand on our previous research on mating and foraging behaviour. With over 25 years of DNA samples collected during the PIT project, we collaborate with Dr. Kevin Feldheim to examine how common natal philopatry is in Bimini's mangrove nursery habitats, and determine genetic connections between sharks in each nursery. Expanding sampling in other nurseries will help to determine whether or not adult females occasionally give birth at nearby nurseries.
THE PIT PROJECT: Every year from April - June, mature female lemon sharks return to Bimini to drop their pups in the mangroves that surround the island. Beginning in 1995, the Shark Lab began monitoring the population of juvenile lemon sharks through this annual survey. 90% of all juvenile lemon sharks are captured and tagged with a microchip called a PIT (Passive Integrated Transponder) tag. DNA and stable isotope samples are taken from each animal, as well as measurements to monitor growth through future recapture. Over 25 years we have caught and tagged over 4,000 individual lemon sharks.
With this data, we’ve been able to understand how the lemon sharks parentage works, reconstruct their genetic mating systems, characterize their long-term use of Bimini and understand life history traits, mortality, and natural selection.
Genotype reconstruction, using highly variable microsatellites (short tandem repeats of DNA), was used by Dr. Feldheim as the DNA marker of choice. Over 3200 fin clips have been genotyped to date! This reconstruction can show the siblings, pups, mother, father etc. We were able to prove females are philopatric down to the mangrove nursery (South Bimini, "Sharkland", and North Sound). Males rarely return to the mangroves, and don’t need to use the habitat in the same way the females do.
The DNA was also able to show that each female has about 10 pups per litter, and they pup every other year. Reproduction is very taxing on lemons due to placental connection to each pup. We found there were up to 4 males per litter, and polyandry was evident in some cases! Approx. 10% of the litters per year were sired by single males, and females are capable of sperm storage for long period of time. There is no evidence of greater survival or genetic diversity for sharks born from polyandrous litters, and is likely that females mate with multiple males to avoid harassment and to convenience mating.
SUMMARY: We sampled 246 litters from philopatric females from the North island and 22 from the South island. There is no example of a female returning to Bimini and switching islands. So, ALL females from the North nurseries came back and gave birth at the North island, and ALL females from the South nurseries gave birth at the south island.
The nursery areas of most coastal sharks are typically in state waters where state jurisdiction applies, but feeding grounds, mating areas, and the migratory paths in between can often be in federal waters. Data on the frequency of natal philopatry in the lemon shark is therefore needed for the effective protection of such species and to better preserve these associated nursery grounds as essential fish habitat.