Modeling potential habitats of threatened horseshoe crabs in Indo-Pacific Asia: Suggestions for conservation management

Abstract

Identifying critical habitats of threatened marine species is key to area-based management measures, while it can be of great challenge for those species that distribute continentally. Asian horseshoe crabs, including tri-spine horseshoe crab Tachypleus tridentatus, coastal horseshoe crab T. gigas, and mangrove horseshoe crab Carcinoscorpius rotundicauda, are globally threatened marine chelicerates, which are largely distributed in developing countries in Indo-Pacific Asia with limited baseline information. Here, we performed the maximum entropy model (MaxEnt), a widely used correlative species distribution model, to explore potential habitats of the Asian horseshoe crabs. Presence-only data of the adult Asian horseshoe crabs and the seven sea benthic environmental predictors, which included temperature, salinity, chlorophyll a concentration, dissolved oxygen, current velocity, bathymetry, and slope, were input into the MaxEnt. The Cloglog output format was adopted to estimate the presence probability of these three species under species-specific regularization multipliers. Our results showed that all three species have potential habitats in Southeast Asia. The range of potential habitats is the most widespread for T. tridentatus, but the most restricted for T. gigas. Bathymetry, temperature, and chlorophyll a concentration are the main contributing environmental predictors for the projected distribution of T. tridentatus, while for T. gigas and C. rotundicauda, temperature and bathymetry have contributed the most. Temperature of the potential habitats is the lowest for T. tridentatus (25.6 ± 4.4 °C), compared to the more tropically distributed T. gigas and C. rotundicauda (around 29 °C). Core habitats of the estuary-bound C. rotundicauda have shallower bathymetry of 5.8 ± 3.5 m in depth. Overall, Asian horseshoe crab species share similar niches, with the common characteristics of shallow bathymetry (<15 m in depth), high temperature (>25 °C), and relatively higher chlorophyll a concentration in coastal waters. We conclude that the projected habitats serve as a critical reference for the conservation management of threatened Asian horseshoe crabs under extensive baseline gaps. Generalized conservation strategies are suggested, depending on the sufficiency of baseline data, to facilitate the formation of specific decision-making.