Conserving the understudied invertebrates: a call for a systematic monitoring protocol for Asian horseshoe crabs in nursery habitats

Abstract

Baseline information on population size, trends and spatial distribution are important for population status assessments of threatened taxa. The 3 species of Asian horseshoe crabs (Tachypleus tridentatus, T. gigas, Carcinoscorpius rotundicauda) are declining throughout their geographic ranges, and identifying their essential nursery habitats and monitoring juvenile population changes is necessary for conservation management. We reviewed juvenile population studies published during 2001-2019 to evaluate which sampling methods were commonly adopted and how baseline data were presented. The applied methods included random or systematic quadrat (35%), random visual search (41%) and belt transect (47%). Juvenile population sampling and data reporting were generally insufficient for assessing population status and clarifying nursery habitat configuration. We propose a conceptual framework and minimum standards for collecting and reporting juvenile population and nursery habitat baseline information. We also outline the importance of showing population density, rather than abundance, and spatial distribution in relation to habitat seascape or landscape satellite images. The framework and recommendations will improve the feasibility of robust, long-term monitoring of juvenile Asian horseshoe crab population status and nursery habitat, thus serving regional conservation management purposes.