Lateral Phototaxis Behaviour and Foraging Strategies of Juvenile Horseshoe Crabs Tachypleus tridentatus (Leach, 1819) (Merostomata: Xiphosurida: Limulidae) in Taiwan: an in-situ Study

The foraging activity of the juvenile horseshoe crabs Tachypleus tridentatus (Leach, 1819) was examined in natural conditions on a sandy substrate during ebb-tide periods in the intertidal zone at Chingluo Wetland, Taiwan, during July and August 2019. The movement patterns, length and width of the crawling paths and the azimuths of direction to sunlight were examined. Juvenile T. tridentatus at the seventh- to tenth-instars (95.5%, 21of 22) could emerge from sand and start to feed on beach surface. Their crawling trails did not follow a straight-line directed pattern; D : Wall was 0.52±1.99; 0.1–0.83 (mean ± s.d.; range). The juveniles rarely faced the sunlight when departing from the starting point or stayed at the end-point of crawling paths. We first detected a lateral phototaxis tendency of juvenile T. tridentatus that was more often with a body lateral to the sunlight than a direct orientation of head towards (81.8% and 18.2%, Binomial test, N = 22, P < 0.01). Their body left-side was more frequently exposed in direction to sunlight the right-side (75% and 25%, Binomial test, N = 24, P < 0.01). We suppose that such a lateral phototaxis and related movement patterns are adaptive foraging strategies of juveniles of T. tridentatus.