Anchialine ponds are isolated habitats frequently associated with unique biota that differ dramatically from nearby coastal systems. Opportunities to study speciation in the wild are rare, but often begin by locating a population that is geographically isolated. In this study, we investigated the distribution of lined seahorses (Hippocampus erectus) by season, size and sex across an estuarine, tidal pond (1.6 km by 0.8 km) on the island of Eleuthera (The Bahamas). Thirty-five 30-60 m2 benthic transects were established around the pond margin and assessed four times over two years (2014-2016). Seahorse population size of the lake and individual movement patterns, using both traditional mark-recapture techniques and facial recognition techniques using the Interactive Individual Identification System (I3S), have been analyzed for 1,861 seahorses. Seahorses were mapped along each transect and photographed for morphological analysis.
Results/Conclusions
Across the lake, seahorse density differed both seasonally and spatially, with 40% higher densities observed in the wet than dry season, and 48.8% lower densities observed in the south vs. the north end of the lake. Males were observed more often than females, with a significantly male-biased sex-ratio in both the north and during the wet months. Male mating effort varied seasonally and spatially, with a significantly higher frequency of gravid males in the dry season, and in the north compared to the south. Finally, male seahorses were significantly larger than females with size differences that were larger both spatially and seasonally, with additional morphological traits that indicate broader sexual dimorphism in this species. Compared to other seahorse species, Sweetings Pond animals have a higher population density, exhibit sexual size dimorphism, and are shaped differently than other H. erectus, indicating a population that is potentially a discrete management unit. Given the CITES Appendix II listing of all seahorses and their decreasing numbers worldwide due, this population represents a valuable and unique biological resource that should be protected.