Spawning site selection of the endangered horseshoe crab tachyplenus tridentatus at tsuyazaki cove in fukuoka, japan

Abstract

Spawning site selection of Tachyplenus tridentatus in relation to elevation, gradient and grain size were examined using a topological model combined with field investigation results of spawning frequency and particle size analysis in Tsuyazaki, Fukuoka, Japan. The most abundant spawning activities occurred in the range from 0.61 meters and 5.97 degrees to 0.89 meters and 8.26 degrees, suggesting that T. tridentatus might prefer moderate shaped mounds regarding elevation and gradient for their spawning sites. However, some spawning activities were found at a slope with lower elevation, and a lesser gradual area where the original shape of spawning ground was destroyed due to construction works suggesting that the species lay their eggs reluctantly at that site. The medium diameter and size-sorting of sediments ranged from 0.19 to 0.38mm and 0.59 to 1.06 phi respectively. More spawning activities were found at the sites in which the medium diameter and size-sorting of sediments from 0.19 to 0.22 mm and 0.59 to 0.73 phi respectively. Our results indicated that T. Tridentatus prefers the areas where the original mound shape was relatively well-preserved offering suitable elevation, gradient and grain size for their spawning. Possible reasons for these results are discussed, and management implications are suggested.