Breeding Biology

Photo Credit: Matthew D Potenski
Despite the fundamental importance of sharks as predators in marine ecosystems, there are very few data regarding mating systems or population structure of any elasmobranch species. Further, there is a complete lack of knowledge regarding the evolutionary dynamics of large marine vertebrates in the wild. Therefore, via a collaborative research effort (McGill University, Canada; Natural History Field Museum, Chicago; Bimini Biological Field Station, Bahamas), we combined long-term field (mark-recapture) and laboratory studies (microsatellite DNA markers) to examine the mating system and evolutionary dynamics of the lemon shark, Negaprion brevirostris, at two tropical nursery lagoons in the western Atlantic. At our primary study site, Bimini, Bahamas, we examined genetic data over 13 breeding seasons based on studies beginning in 1995. We also considered nine years of data at an additional site in the Marquesas Keys, Florida. With microsatellite markers we developed for the lemon shark we were able to infer parent/offspring and sibling relationships between sampled individuals as well as reconstruct genetic information from most unsampled parents. These pedigree data were used to estimate quantitative genetic parameters in each population and explore mating patterns in general.

1. How do lemon shark mating systems vary between populations (i.e. mating strategies, birthing cycle, number of reproducing females, philopatry to breeding grounds)?

2. Can early life history trait differences (e.g., body size at age and growth rate) between populations be explained by differential selection at each nursery site? This issue was explored by estimating the strength of selection and heritability for a number of traits using an "animal model" approach.

3. Are neutral and quantitative genetic impacts of on-going coastal development at the North Sound nursery evident at the molecular level?

The results provided by this project are important in the management of shark fisheries and ultimately in the survival of this heavily impacted group of vertebrates. The project is highly relevant to the NOAA strategic goals of building sustainable fisheries by increasing our understanding of the role of nurseries for successful recruitment of coastal shark species and by providing data on life history parameters (fecundity, reproductive cycles, growth rates, evolutionary potential) critical for successful management plans. The project is also relevant to the Foundation's strategic goal to conserve protected species. The 1993 NOAA/NMFS management plan gives protected status to 39 shark species, including the lemon shark, and the data provided by this study will be critical for developing stock rebuilding plans for this and other shark species.
Lemon Shark Mating Systems:
We found that adult female sharks consistently returned to Bimini on a biennial cycle to give birth, and over 86% of the litters had multiple fathers. Similar patterns were observed at the Marquesas Keys with polyandrous, philopatric females returning every two years to pup. In contrast, adult male sharks rarely sired more than one litter at Bimini (or Marquesas) and may mate over a broader geographic area, thus introducing gene flow between nurseries. No single adult female was ever detected as having switched between nurseries for parturition, although the mating habits of adult males we know with less certainty.
One argument for the adaptive significance of female sharks mating with multiple males despite the risk of injury associated with mating events is that it provides indirect genetic benefits to the offspring like greater genetic diversity and survival. In Bimini we found that this was not the case and it may instead be the result of convenience polyandry, whereby females make the best of a bad situation and mate multiply to avoid excessive harassment from males.
Natural Selection:
The juvenile lemon sharks of North Bimini are unique in that they are smaller at age and grow up to five times slower than individuals sampled at all other lemon shark nursery areas we have surveyed to date. By estimating the strength of natural selection acting on these traits in the North Sound we found that small size and slow growth were indeed consistently favored and heritable, which contradicts the conventional “bigger is better” theory for life history traits in fishes. Our results instead support those of some other recent studies in suggesting that bigger / fatter / faster is not always better, and may often be worse. In this case, we suggest that bigger, faster growing juvenile lemon sharks may take more risks when foraging and thus expose themselves more often to predators in the North Sound (i.e., larger, subadult sharks). This hypothesis is currently under investigation using stable isotope data and personality studies (see current research).
Anthropogenic Effects:
Coincident with coastal development on North Bimini, we found significant changes in the level of neutral genetic variation in the lemon shark, additive genetic variance (i.e., a key component of heritability) for body length and mass, and strength of selection on these same traits. This finding suggests that anthropogenic forces can induce dramatic shifts in the selective forces that a population must adapt to over a short time period.
Principal Investigators
Dr. Joseph DiBattista - King Abdullah University of Science and Technology, SA
Dr. Kevin Feldheim - Field Museum, Chicago Pritzker Laboratory , US
Dr. Samuel H Gruber - University of Miami, US
Dr. Demian D Chapman - Stony Brook University, US
Dr. Nigel Hussey - Windsor University, US
Dr. Tristan Guttridge - Cardiff University, UK
Dr. Steve Kessel - Windsor University, US
Dr. Andrew Hendry - McGill University, CA
Dr. Bryan Franks - Rollins College, US
Dr. Dany Garant - University of Sherbrooke, CA
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