Population Genetic Structure of Juvenile Tri-Spine Horseshoe Crabs in Hong Kong

Populations of tri-spine horseshoe crab Tachypleus tridentatus are declining in Hong Kong due to the destruction of natural shore habitat and overfishing. Understanding their phylogeographic pattern is thus important to the conservation of this important living fossil. Owing to the very low numbers of horseshoe crabs found locally, a preliminary study of the genetic diversity of T. tridentatus was conducted by sequencing a mitochondrial gene region from a total of 33 juvenile samples collected from six locations in Hong Kong. Eight haplotypes were identified, and the relationship among haplotypes was inferred using Network Analysis. Results indicated that although the sample size was relatively small, T. tridentatus tended to express a high level of endemism, and the population in Deep Bay (sample size = 12) was differentiated from that on the northwestern coast of Lantau Island (sample size = 11). Horseshoe crabs at Shui Hau on the southern coast of Lantau Island (sample size = 10) also appeared to be geographically isolated. These genetic patterns may indicate the limited dispersal ability of horseshoe crabs in Hong Kong. Very low haplotype diversity observed on the northwestern coast of Lantau Island also implies the problem of a population bottleneck, which may eventually result in local extinction. Such information is important for the evaluation of current conservation plans and the future management of this endangered species.