Spatial distribution of horseshoe crabs relative to different estuarine habitats

Abstract

Estuaries are complex and dynamic ecosystems that support rich biodiversity and provide a wide range of ecosystem services. As the ecosystem engineers of estuarine landscapes, structured biogenic habitats serve as nursery grounds for juvenile organisms, offer refuge from predation and provide essential trophic resources. Yet, virtually all estuaries around the world are impacted by anthropogenic pressures, and environmental degradation and modification are so severe that their ecological functions are compromised. In Hong Kong, rapid development combined with a lack of adequate protection measures has led to extensive biodiversity loss. Of the three Asian horseshoe crab species that historically occurred in Hong Kong, Tachypleus gigas disappeared completely from local waters, Tachypleus tridentatus is listed as “endangered” by the IUCN Red List, and Carcinoscorpius rotundicauda is reported to be undergoing declining populations. Increased conservation, notably the protection of remaining habitat, is advocated as an urgent priority. With the aim of establishing targeted conservation measures, previous studies on horseshoe crab habitat needs have largely focused on biological processes of different species at sites where they occur, while investigations on the larger ecosystem functionality to support these processes, are lacking. Apart from overexploitation, the causes for population declines within the remaining habitats utilized by horseshoe crabs are still mostly unresolved. A wide variety in nursing site landscapes further complicates the definition of optimal habitat and adequate restoration strategies. For example, while previous studies on habitat usage strongly associate T. tridentatus with seagrass beds, such habitats aren’t present at all its identified nursing beaches. C. rotundicauda, commonly termed “Mangrove horseshoe crab”, have been generally associated to mangroves. On the other hand, oyster beds found in diverse forms, densities and community assemblages, are present at all identified horseshoe crab nursing sites across Hong Kong, but surprisingly, there are very few studies on the ecological links between oysters and horseshoe crabs. With a recent impetus on restoring estuarine ecosystems and, notably, oyster habitats, a better understanding of such ecological links is needed, particularly if habitat manipulation can have undesired effects on endangered species. To that end, my study aimed to investigate the habitat utilization of juvenile horseshoe crabs at three known nursery and spawning grounds in the western waters of Hong Kong, with a focus on their association with oyster habitats. Each site was surveyed twice (seasonally) across a 12-month period. Horseshoe crab distribution, tidelines and oyster habitats were mapped using hand-held GPSs and ArcGIS software was used to analyze horseshoe crab abundance relative to oyster habitats at each site. I found that interactions with oyster habitats differed among sites. At one site, horseshoe crab densities were more significant away from oyster reefs, another showed a random distribution, while the third site showed significant association with oyster habitat. These findings suggest that the role oyster habitats play in supporting local horseshoe crab populations is largely dependent on their spatial and physical configurations within the site. Taking existing knowledge on horseshoe crab behavior and habitat usage into account, such differences are also likely to be species-specific: while direct associations with C. rotundicauda seem likely, potential services that support T. tridentatus are harder to detect. I conclude that more in-depth understanding of habitat function and connectivity is critical to inform sound conservation measures and restoration practices.