(Reinsel and Rittschof, 1995). Fidd

(Reinsel and Rittschof, 1995). Fiddler crabs exhibit morphological, physiological and
behavioural adaptations to thrive under desiccation and thermal stress. Takeda et al.
(1996) demonstrated that the well-developed inner linings of the branchiostegites form
‘lungs’ in six species of Uca living in semi-arid environments. They also showed that
the branchiae are reduced in species occupying higher ground. Thurman (1998)
revealed that U. subcylindrica, which lives in semi-terrestrial habitats, has a large
extravascular water source and good cellular osmoregulatory capabilities. This
enables the crab to lose large amounts of water from its gill chamber to cool the
body during short-term desiccation without compromising cellular functions. The
deep, well-hydrated burrow is also an ideal refuge for Uca and prevents excessive
water loss (Wilkens and Fingerman, 1965; Smith and Miller, 1973; Wolcott, 1992).
To avoid desiccation, the crab is active only during the coolest parts of the day
(Thurman, 1984). Under elevated temperatures and bright sunlight, Uca can also
change its body colour or its orientation toward the sun (Smith and Miller, 1973).
The distribution of Uca species has been largely related to sediment types (e.g., Ono,
1962; Icely and Jones, 1978; Bertness and Miller, 1984; Zimmer-Faust, 1987), environmental
factors (e.g., Miller and Maurer, 1973) or behaviour (e.g., Crane, 1943, 1966;
Altevogt, 1955; Christy, 1979). A more recent study by Reinsel and Rittschof (1995)
suggested that the sediment organic and water content are the most important factors
determining the foraging behaviour of fiddler crabs. Fewer studies, however, have
been directed towards the semi-tropical uciids. In sub-tropical Hong Kong (22
200 N;
114
100 E), the fiddler crabs U. vocans borealis (Crane 1975) and U. lactea lactea (de
Haan 1935) are widely distributed on sandy shores (Bones, 1983). On many shores,
both species coexist in the upper intertidal zone and share similar home ranges. It is
therefore of interest to explore the factors controlling their distribution in the same
habitat. In the present study, we investigated the distribution of U. vocans borealis
and U. lactea lactea on an intertidal shore and examined whether behavioural adaptations
by these two sympatric species were important for their coexistence on the shore