The final spawning ground of Tachypleus gigas (Müller, 1785) on the east Peninsular Malaysia is at risk: a call for action

Abstract

Tanjung Selongor and Pantai Balok (State Pahang) are the only two places known for spawning activity of the Malaysian horseshoe crab - Tachypleus gigas (Müller, 1785) on the east coast of Peninsular Malaysia. While the former beach has been disturbed by several anthropogenic activities that ultimately brought an end to the spawning activity of T. gigas, the status of the latter remains uncertain. In the present study, the spawning behavior of T. gigas at Pantai Balok (Sites I-III) was observed over a period of thirty six months, in three phases, between 2009 and 2013. Every year, the crab's nesting activity was found to be high during Southwest monsoon (May-September) followed by Northeast (November-March) and Inter monsoon (April and October) periods. In the meantime, the number of female T. gigas in 2009-2010 (Phase-1) was higher (38 crabs) than in 2010-2011 (Phase-2: 7 crabs) and 2012-2013 (Phase-3: 9 crabs) for which both increased overexploitation (for edible and fishmeal preparations) as well as anthropogenic disturbances in the vicinity (sand mining since 2009, land reclamation for wave breaker/parking lot constructions in 2011 and fishing jetty construction in 2013) are responsible. In this context, the physical infrastructure developments have altered the sediment close to nesting sites to be dominated by fine sand (2.5Xφ ) with moderately-well sorted (0.6-0.7σφ), very-coarse skewed (-2.4SKφ), and extremely leptokurtic (12.6Kφ) properties. Also, increased concentrations of Cadmium (from 4.2 to 13.6 mg kg(-1)) and Selenium (from 11.5 to 23.3 mg kg(-1)) in the sediment, and Sulphide (from 21 to 28 µg l(-1)) in the water were observed. In relation to the monsoonal changes affecting sheltered beach topography and sediment flux, the spawning crabs have shown a seasonal nest shifting behaviour in-between Sites I-III during 2009-2011. However, in 2012-2013, the crabs were mostly restricted to the areas (i.e., Sites I and II) with high oxygen (5.5-8.0 mg l(-1)) and moisture depth (6.2-10.2 cm). In view of the sustained anthropogenic pressure on the coastal habitats on one hand and decreasing horseshoe crabs population on the other, it is crucial to implement both conservation and management measures for T. gigas at Pantai Balok. Failing that may lead to the loss of this final spawning ground on the east coast of P. Malaysia.

Keywords: Anthropogenic disturbance; Conservation and management; Living fossil; Monsoonal impact; Nest shifting behaviour; Seasonal nesting.

Figure 1. Study area.

(A) Geographic location of State Pahang and Pantai Balok (square box) on the east coast of Peninsular Malaysia; (B) The sampling sites (I–III) in River Balok estuary along with the locations of sand mining (▴), wave breaker/parking lot construction and, fishing jetty construction (*) in the vicinity (Google Map ^© 2013). Photographs shows the physical infrastructural developments at Site-I - (C) beach condition in 2009–2010, (D) wave breaker/parking lot construction in 2011 and, (E) fishing jetty construction in 2013.

Figure 2

Egg/nest yield of Tachypleus gigas at Pantai Balok in relation to - (A) season, (B) lunar period, (C) sampling sites and, (D) the number of male and female spawning crabs arriving at Balok beach.

Figure 3. Principal Component Analysis (PCA) showing the % variance in sedimentological parameters in relation to –(A) Tachypleus gigas egg count, (B) season, (C) lunar period and, (D) sampling sites.

The numbers 1–3 indicate study phases in the present investigation (1, Phase-1: 2009-2010; 2, Phase-2: 2010-2011 and 3, Phase-3: 2012-2013). The circle in each panel represents correlation circle and the orientation of the environmental (sediment) lines approximate their correlation to the ordination axes. Abbreviated environmental parameters: Temp, Temperature; Sort, Sorting; Skew, Skewness; Kurt, Kurtosis; Grav, Gravel; S&C, Silt and clay; MD, Moisture depth; and TOC, Total organic carbon.

Figure 4. Principal Component Analysis (PCA) showing the % variance in water quality parameters in relation to –(A) Tachypleus gigas egg count, (B) season, (C) lunar period and, (D) sampling sites.

The numbers 1–3 indicate study phases in the present investigation (1, Phase-1: 2009–2010; 2, Phase-2: 2010–2011 and 3, Phase-3: 2012–2013). The circle in each panel represents correlation circle and the orientation of the environmental (water) lines approximate their correlation to the ordination axes. Abbreviated environmental parameters: Temp, Temperature; Sal, Salinity; DO, Dissolved oxygen; Chl-a, Chlorophyll-a; NO₂⁻, Nitrite; NO₃⁻, Nitrate; ${\mathrm{PO}}_4^{3-}$, Phosphate; S²⁻, Sulphide.

Figure 5. Principal Component Analysis (PCA) showing the % variance in heavy metals in relation to –(A) Tachypleus gigas egg count, (B) season, (C) lunar period and, (D) sampling sites.

The numbers 2–3 indicate study phases in the present investigation (2, Phase-2: 2010–2011 and 3, Phase-3: 2012–2013). The circle in each panel represents correlation circle and the orientation of the environmental (heavy metals) lines approximate their correlation to the ordination axes. Abbreviated environmental parameters: Cd, Cadmium; Cr, Chromium; Cu, Copper; Pb, Lead; Se, Selenium; Zn, Zinc.

Figure 6. Schematic diagram showing the nest shifting behaviour of Tachypleus gigas between the three sampling sites (I–III) at Pantai Balok (Google Maps©2013).

The impact of environmental processes (indicated in boxes) in relation to monsoon (SW/IM/NE) seasons are found to be important for spawning site(s) selection by the adult crabs.