Journal of Sustainability Science and Management Volume 11 Number 1, June 2016: 1-13 ISSN: 1823-8556 © Penerbit UMT WIDTH-WEIGHT AND LENGTH-WEIGHT RELATIONSHIPS OF THE TRI-SPINE HORSESHOE CRAB, Tachypleus tridentatus (LEACH 1819) FROM TWO POPULATIONS IN SABAH, MALAYSIA: IMPLICATIONS FOR POPULATION MANAGEMENT FARIDAH MOHAMAD 1 *, AZWARFARID MANCA 1 , AMIRRUDIN AHMAD 1 , MUHD FAWWAZ AFHAM MOHD SOFA 1 , AMIRUL ASYRAF ALIA’M 2 AND NORAZNAWATI ISMAIL 2 1 School of Marine and Environmental Sciences, 2 Institute of Marine Biotechnology, Horseshoe Crab Research Group, Universiti Malaysia Terengganu, Terengganu. *Corresponding author: azwarfarid@ymail.com Introduction The Tri-spine horseshoe crab, Tachypleus tridentatus (Leach, 1819) is among the three Asian horseshoe crab species that belongs to sub-family Tachypleinae. Tachypleus tridentatus distributes within 12° N to 31° N and 90° E to 125° E geographical areas (Sekiguchi & Shuster, 2009). Recent fossil discovered in Manitoba, Canada, resembles the extant horseshoe crabs proved that the horseshoe crab has lived at least 445 million years ago with very little morphological changes from their ancestor (Mattei & Bekeey, 2008; Rudkin et al., 2008). All the three Asian species (tri-spine horseshoe crab Tachypleus tridentatus, coastal horseshoe crab Tachypleus gigas and mangrove horseshoe crab Carcinoscorpius rotundicauda) were reported to exist in the waters along the coast of Malaysia (Zaleha et al., 2008; Srijaya et al., 2010; Ismail et al., 2011; Ismail et al., 2012; Chatterji & Pati, 2014; Robert et al., 2014). Even though the Asian horseshoe crab does not have multiple utilization like Limulus polyphemus that commercially harvested in the production of LAL endotoxins detection kit and as commercial bait to catch eel and conch in United States (US), but still the public is unconscious of the biological and ecological importance of these horseshoe crabs (Tan et al., 2012). Abstract: A morphometry study of 259 individuals of Tri-spine horseshoe crab Tachypleus tridentatus was carried out from April to October 2014 in two populations in Sabah, Malaysia, one in Tanjung Limau and another in Inderasabah. Two methods were used, i.e. visual search and gill netting. Both sites showed a higher number of males with 1.01:1 OSR (χ²=0.005, df=1, P>0.05) in Tanjung Limau, and 2.42:1 OSR (χ²=19.55, df=1, P<0.05) in Inderasabah. T. tridentatus collected from Tanjung Limau are larger than the population from Inderasabah. The prosomal width (PW) frequency for males ranged from 30-31 cm and 26-27 cm whereas for females ranged from 38-39 cm and 30-31 cm in Tanjung Limau and Inderasabah respectively. The prosomal width-weight relationship were W=0.0028 PW 3.9273 and W=0.0125 PW 3.5245 , while the total length- weight relationship are W=0.0152 TL 2.8465 and W=0.0202 TL 2.7536 for Tanjung Limau and Inderasabah respectively. Two-way ANCOVA revealed a signiicant difference among PW-weight relationship in males and females in these two locations (F=4.74, df=1, P<0.05). It conirms that both populations demonstrate allometric growth (b≠3) with a higher increment of PW-weight as compared to TL-weight, which agrees with previous studies that reported similar traits. The differences in these morphometric parameters in both populations of T. tridentatus in this study suggest that they may derive from two discrete populations. The indings of this study are useful to estimate the size of adults and spawning individuals of T. tridentatus for better management of the population of this unique animal. Keywords: Horseshoe crab, Tachypleus tridentatus, width-weight relationship, length- weight relationship, Sabah, Malaysia.