Discussion

This is one of the first studies in Singapore documenting the ecology of an unusual marine benthic community that has established itself in an inland urban drain.  The ecology of this community is unique because of the unlikelihood of its establishment given Singapore’s highly urbanised landscape as well as the study site’s far inland location.  Large resident and transient predatory organisms like the Malayan Water Monitor Lizard and Sandpiper whose diet includes a variety of small invertebrates to crustacean, worms, insects and centipedes (Briffett, 1992), were able to find survive in the drain because of the ample supply of crustacean for food. 

  Sediment analysis of the study site showed that the sediment were “gravelly sandy” which is similar to that of other natural habitat of Uca annulipes.  This finding agree with Lim et al.’s (2005) findings that Uca annulipes are generally found in sandier habitats. It was noted during tolerance testing that lethality happened rapidly at low salinities and the crabs survived very well even when exposed to high salinity levels.  This data suggests that Uca annulipes do not survive well when exposed to prolong periods of low salinity.  These two factors (sediment type and salinity), could be the reason why the Uca annulipes population is found mainly at the junction of the drain and the concretised Sungei Ketapang.

The population density of Uca annulipes at the study site (25 individuals m^-2) is found to be quite similar to that of areas with sheltered substratum like mangrove with pneumatophores and underlying cable roots or overhanging prop-roots in Pasir Ris Park (24.0 and 25.5 individuals m^-2) by Lim and Heng (2007).  This suggests that the study site may offer shelter and protection compared to exposed bare substratum lagoonal beach of Pulau Hantu Besar where population density is about 12.8 individuals m^-2 (Lim & Diong, 2003)

Euryhaline fiddler crabs inhibit intertidal regions and estuaries where they may experience fluctuations in salinity from suprasaline to almost freshwater (D’Orazio and Holliday, 1985; Zanders and Rojas, 1996).  As such, Uca species typically exhibit varying degrees of osmoregulation when exposed to different salinities.  Osmoregulatory activity may be quantified by studying the activity of the branchial enzyme Na⁺, K⁺-ATPase ESA.  When exposed to very dilute seawater, Lin (2002) showed that there seems to be an up-regulation of this enzyme but this increase is largely insignificant in Uca annulipes.  Even though the salinity of the sediment and water in which the specimens were exposed to at their respective locations were significantly different, the LC₅₀ remained largely the same, supporting evidence that Uca annulipes were able to thrive in a wide range of salinity conditions  (Thurman et al., 2010). 

Further studies could be conducted to investigate if crab zoea were transported from East Coast Park to the drain during high tide or the crab population in the drain were self-sustaining.  In addition, free swimming crabs and mud lobsters could be tagged to investigate if they were resident or transient species.