Abstract
The horseshoe crab Tachypleus tridentatus is known to lay its eggs on beaches in the upper intertidal zone in summer, and the embryos develop and hatch in the sand. The hatched larvae emerge from the sand surface and move into the sea on the night of the high tide in spring tide. Up to now, however, larval emergence, seasonal change, and their relationship to the physical environment at spawning sites have not been fully studied. The duration and frequency pattern of emergence and peculiarities of the physical environment were investigated at two spawning sites (A: Kubihineri; B: Eitaibashi) in Moriye Bay, Oita Prefecture. The number of emerged hatched larvae was investigated using a fish-luring light around high tide during the spring tide. Furthermore, the spawning period was identified to analyze its effect on the emergence period. The temperature and salinity of the interstitial water were measured at the spawning sites, and pH, turbidity and dissolved oxygen in the surface water were measured around the high tide in the spring tide. The relationships with lunar phase and tidal height were investigated. More larvae emerged in autumn (from mid-September to mid-November) than in spring-summer (from late April to late June). At site B, the beginnings of both larval emergence and spawning were delayed by about half a month in comparison with site A. The beginning of emergence in autumn thus appeared to be influenced by the timing of the spawning season. The majority of hatched larvae emerged during the period between one hour before and two hours after the high tide. Most of the larvae emerged after seawatel had flooded the spawning site. Therefore, hatched larvae appeared to be stimulated to move by seawater flooding into the sand, and began moving within the sand. The larvae emerged when the temperature was higher than 16°C, salinity was higher than 19 psu, and the dissolved oxygen was higher than 6 mg/l. Hatched larvae did not emerge at neap tide when the tidal height was less than 190 cm. There was no significant difference between the number of larvae that emerged during spring tides at full moon and at new moon.
