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MALAYSIAN FRESHWATER CRABS: CONSERVATION PROSPECTS AND CHALLENGES
The conservation of freshwater crabs hinges almost entirely on preserving patches of natural
forest large enough to maintain the good water quality of the original streams. Potamids are
extremely sensitive to polluted or silted waters, and will not survive when exposed to these
factors. In Singapore for example, the small patch of primary forest of Bukit Timah Hill (ca.
70 hectares) is quite sufficient to maintain a small but thriving population of the potamid
Johora singaporensis. This species is known from only one other area in Singapore, which is
threatened with development, and Bukit Timah is probably its last refuge (see Ng 1988, 1989,
1990b). The same is true for Parathelphusa reticulata, which is known to occur only in a
small remnant patch of peat swamp forest patch of less than 50 hectares in the Central Catchment
Area of Singapore (Ng 1989, 1990a, b). Similar patterns have been recorded for the freshwater
crabs of Sri Lanka (Bahir et al. 2005).
Development, agriculture and exploitation of forest products probably cannot be halted, but
compromises will have to be made if many freshwater crab species are not to be extirpated. It
is likely that some species have already become extinct through extensive developments in
some areas before their taxonomy can be better understood. Judicious and careful exploitation
(e.g., controlled logging) is unlikely to cause extinctions as long as the water drainages are not
polluted or redirected and the forest cover not completely stripped away. The recolonisation
of many lowland plantations and estates by more adaptable species like Parathelphusa maculata
is encouraging. How more montane taxa like potamids will cope is not known, but considering
their fastidious habitat requirements, most species will not be able to adapt as readily as
parathelphusids.
The subjectivity of threat levels assigned here must be emphasised, as some of the limitations
of this study echo the challenges faced in conservation. Conservation challenges are often
associated with the amount of knowledge available on the species. The freshwater crabs of
Singapore and southern Peninsular Malaysia are better known, and their biology and distribution
better understood, as are the potential threats. This, of course, stems from an inherent bias for
conservation efforts to target the better studied species, which are better known simply because
they are more easily caught by workers in more accessible areas, e.g., Johora tiomanensis, a
large, locally common aquatic species found in the lower stretches of the forest streams of the
southern half of Pulau Tioman, which are mostly in close proximity to villages. Conversely,
hard-to-find species tend to be neglected as we simply do not know enough to initiate directed
conservation efforts, e.g., Geosesarma tiomanicum, a tiny terrestrial species that dwells among
the leaf litter of the forest floor in the rugged, hilly parts of Pulau Tioman, often some distance
away from water sources – encountering this species in the middle of the forest is purely a
matter of chance, subject to weather, seasons, and their own fluctuating populations (Ng 1988;
Yeo et al. 1999).
Another aspect of our limited knowledge of some freshwater crabs that proves challenging
for conservation, is the evolving taxonomy of some taxa. Some wide-ranging “species” that
we might try to conserve (or worse, not see the need to conserve, presuming that they are
widespread and common enough) may actually prove to be complexes of several distinct
cryptic taxa, which could differ in various ways such as diets, habits, microhabitat preferences,
ecological niches, local distribution, etc. One such possibility is Johora intermedia, which is
here assigned the status of “Least Concern” primarily because it has been recorded from more
than 20 sites throughout the lower half of the Main Range of Peninsular Malaysia (Selangor,
Pahang, Negri Sembilan and northwestern Johor) in an estimated Extent of Occurrence of
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