Living fossils in highly urbanized areas: Interventions aiming to mitigate local extinction risk and species range shifts in Asian Horseshoe crabs

Abstract

Horseshoe crabs are integral to the coastal estuarine ecosystem of Hong Kong and Asia, providing important ecosystem services (e.g., bioengineers of benthic environments, components of trophic webs). The strong economic demand of HSC, together with the rapid development of coastal areas and other anthropogenic pressures (e.g., invasive species, climate change), are driving drastic declines of HSC populations in this region. From the three Asian HSC species, only the Chinese HSC T. tridentatus and the mangrove HSC C. rotundicauda are currently found in Hong Kong. Although previously present (before 1950s), the southern HSC T. gigas is now locally extinct in this area. T. tridentatus and C. rotundicauda are following the same fate (threatened by local extinction) as evidenced by the drastic declines (over 90%) in the density of these species since 2002, likely the result of habitat destruction, fragmentation and overfishing. Local ex-situ conservation efforts have been developed in Hong Kong in order to decelerate the decline of these species. Some of these programs have included laboratory culture of eggs and juveniles, and posterior release of individuals to the field. However, these programs were designed to increase population size without previous assessment of genetic diversity, effective population size, connectivity among natural populations, levels of inbreeding. Even more, these programs did not monitor the effects of interventions on the long-term persistence and genetic background of HSC populations. Genetic variation and structure among local populations are paramount factors to take into consideration when planning active interventions.