Abstract:

Aim: The tri-spine horseshoe crab (Tachypleus tridentatus) is an endangered marine arthropod whose juveniles (approximately instars 1–12) inhabit intertidal sand-mud flats. This study aimed to assess the current population status of juvenile horseshoe crabs and the habitat quality of their nursery grounds in Dongshan Bay, Fujian, China. 

Methods: Surveys on the distribution of juvenile populations and nursery grounds were conducted along the coastal tidal flats of Dongshan Bay. Ten seasonal surveys were carried out at Shanqian Beach, Lieyu Town, from August 2021 to June 2025. Intertidal transect surveys combined with kernel density estimation were used to analyze the spatial distribution of juveniles. Sediment grain-size composition and macrobenthic community characteristics were compared among areas with different juvenile densities. 

Results: Shanqian Beach is currently the only remaining functional nursery ground in Dongshan Bay. Juvenile density ranged from 0.0096 to 0.1536 ind./100 m², indicating an overall low population level. Juveniles ranged from instar 3 to instar 12, with instars 6–9 accounting for a high proportion in each survey year, reflecting an unstable age structure. Sediment grain-size composition differed significantly among density zones; high-density juvenile areas were characterized by sandy substrates with low gravel content and relatively higher silt content. The Shannon-Wiener diversity index, Margalef richness index, and Pielou evenness index of macrobenthos were significantly higher in high- and medium-density zones than in the low-density zone, whereas total density and biomass showed no significant differences. Macrobenthic community structure exhibited significant separation among zones, but this differentiation was primarily driven by variations in the abundance of a few dominant species, such as Simplisetia erythraeensis and Batillaria cumingi. 

Conclusion: The distribution pattern of juvenile horseshoe crabs is closely associated with sediment structure and macrobenthic community characteristics. We recommend prioritizing the protection of the core nursery area in the northern part of Shanqian Beach, controlling anthropogenic disturbances, conducting comprehensive assessments of the nursery ground and its surrounding environment, and implementing scientifically guided stock enhancement to promote long-term population recovery. The results of this study provide a theoretical foundation for the conservation and ecological restoration of nursery habitats for the Chinese horseshoe crab.

Key words: Dongshan Bay, Tachypleus tridentatus, juvenile population, nursery habitat, sediment grain size, macrobenthos