Insight into intraspecific niche divergence and conservatism in American horseshoe crabs (Limulus polyphemus)

Abstract Limulus polyphemus is the sole extant representative of horseshoe crabs distributed along the Atlantic coast, and is threatened by human activities. Conservation priority to safeguard its habitat usually treats L. polyphemus as a whole, without consideration of different intraspecific ecological requirements. We seek to identify intraspecific ecological niche divergence or conservatism in L. polyphemus. In this study, ocean and coastal environmental conditions occupied by the six distinct metapopulations of L. polyphemus were collected, and compared using correlative approaches, niche overlaps among these six metapopulations were compared with geographical and genetic distances, the realized niche divergence/conservatism among these six metapopulations were tested, and ecological niche models were fitted for the six metapopulations separately and for L. polyphemus as a whole using state-of-the-art techniques. We found that mean sea surface temperature, tidal regime, chlorophyll a concentration, distance to shore and depth of seafloor were limiting factors in the geographic distribution of L. polyphemus. In addition to the former hypothesis that tidal amplitude and type account for the absence of L. polyphemus in the western Gulf of Mexico, our niche model predictions suggest that the narrow and high-depth seafloor in the western Gulf of Mexico also explain the absence. We found that realized niche overlaps among six metapopulations were not correlated with their geographic and genetic distances; rather the six metapopulations each occupied a unique combination of habitat conditions, showing different responses to environmental factors, although these divergences might be due a background effect. Results here showed that the distribution of one metapopulation of L. polyphemus cannot be inferred from changes in distribution of another, highlighting the inadequacy of one-size-fits-all conservation actions. We recommend that conservation priority assignment by IUCN and Natureserve for L. polyphemus would better consider the six metapopulations separately rather than L. polyphemus as a whole, which will deliver more useful information on guiding regional conservation efforts.