Abstract
We investigated the spawning sites and annual variability of the number of reproductive visiting pairs of the horseshoe crab Tachypleus tridentatus along the Tsuyazaki Coast in Fukuoka, Japan, from 2004 to 2008. We found 12 spawning sites of this endangered species. The silt-clay content of the sand fraction of sediment samples ranged from 0% to 6.1%. The median diameter and size-sorting of sediments ranged from -2.32 to 2.32 phi and 0.37 to 1.98 phi, respectively. The median diameter of sediments and the number of reproductive visiting pairs at each spawning site were significantly positively related. Our results also indicated that about 70% of reproductive females laid eggs at four sites around Tsuyazaki Bridge. Moreover, a continuous monitoring survey revealed a clear decrease in the number of reproductive visiting pairs in the threatened wild population. The factor most likely associated with this decline of horseshoe crabs is the recent coastal development around Tsuyazaki Bay. Therefore, we recommend continuing the citizen-based monitoring of the horseshoe crabs to contribute to the protection of the coastal ecosystem and biodiversity.
