Habitat use of globally threatened juvenile Chinese horseshoe crab, Tachypleus tridentatus under the influence of simulated intertidal oyster culture structures in Hong Kong

1. Little is known about the ecological impacts of oyster culture structures on intertidal communities. In the present study, distribution and movement patterns of juvenile Chinese horseshoe crab, Tachypleus tridentatus were assessed on a mudflat at Ha Pak Nai in Deep Bay, Hong Kong.
2. As the traditional bottom-laying method of using concrete posts as cultch for collecting oyster spat is a common practice in Hong Kong, structurally similar bricks were used to simulate the potential effects of cultch on intertidal flats. 
3. Over the two-month experimental period, all the tested sediment physico-chemical characteristics, including median particle size and total organic content, remained unchanged among the treatment areas. However, juvenile densities and foraging trails at low- and high-density brick areas were significantly lower/shorter compared with the adjacent bare areas. Such effects were more evident for larger individuals since significant correlations were found between foraging distance and juvenile prosomal width in no-brick areas, but not the low- and high-density brick areas. In addition, most juveniles (> 95%) were observed feeding along the outer boundaries of brick areas. 
4. Such findings imply that the extensive artificial structures in oyster cultivation sites could induce physical disturbance and alter the habitat use of juvenile horseshoe crabs in the intertidal zone. Considering the high conservation value of Chinese horseshoe crabs, appropriate mitigation measures should be implemented to buffer the detrimental effects on horseshoe crabs and other marine organisms of conservation concern that utilize intertidal habitats as nursery and hatchery grounds.