Visually guided behaviour of juvenile tri-spine horseshoe crab Tachypleus tridentatus : Implications for designing racks for oyster cultivation compatible with horseshoe crab conservation

Visually guided behaviour of juvenile horseshoe crabs Tachypleus tridentatus was studied in both the field and laboratory. Foraging juveniles avoided high contrast black or grey objects placed in front of their foraging routes, but a high percentage collided with obstacles that were transparent. The obstacles, however, did not affect the total distance travelled or displacement during foraging. In the laboratory, each juvenile horseshoe crab was placed in a tray either shaded on top by boards with different amount of shade available or without a cover. No preference to any type of board was observed but the juveniles preferred to stay under boards, particularly the black ones rather than in the open area. This indicates that the presence of a cover and the associated shady environment did not affect the foraging behaviour of the juvenile horseshoe crabs. The results have raised the potential of introducing off-bottom rack-and-bag methods for intertidal oyster cultivation in Hong Kong to minimize the conflict with the conservation of this endangered species.