Growth and development of the monitoring method for juvenile horseshoe crab Tachypleus tridentatus at Tsuyazaki mudflat in Fukuoka, Japan

We investigated the growth of juvenile horseshoe crab Tachypleus tridentatus at Tsuyazaki mudflat in Fukuoka Japan from September 2003 to October 2006, thereby approaching the development of the monitoring method for the endangered species. The growth of juvenile horseshoe crabs was estimated using carapace width (CW) frequency analysis. The CW frequency was separated into four instars with 17.00-62.00 mm of CW range, resulting that each area of unimodal distribution showed the CW ranges from 5 to 8 instars. The mean CW of juvenile horseshoe crabs increased 1.35, 1.33 and 1.31 times with the molting process, and these values were higher than the growth rate under the rearing condition for the previous study. Dividing the captured individuals into age groups according to the CW ranges obtained from this study, it would become possible to monitor the increase and decrease of the threatened wild population. The endangered horseshoe crab can be potentially an important flagship species for the conservation of the coastal ecosystem, because diverse coastal environments are indispensable for its life cycle. Therefore, in order to protect not only the endangered species but also its natural habitat or biodiversity, the citizen-based monitoring of horseshoe crabs should be continued in this field.