Master of Science

The time spent and suite of behaviours associated with water uptake and circulation of three sympatric species of fiddler crabs (Austruca annulipes, A. perplexa and Gelasimus vocans) on Pulau Hantu Besar were determined. Since G. vocans individuals live nearer the water edge, they allocated significantly less time to water uptake and circulation. There were also slight, non-significant differences in the suite of behaviours displayed by each species of crabs, revealing the relative efficacies of each behaviour in relation to moisture content of the substrate. Results suggest that ‘dip’, the predominant behaviour, was associated with drier substrates and ‘sit’ was observed more commonly on wet substrates. Increase in feeding duration consistently increased water uptake activity across species and significantly for A. perplexa individuals, validating ‘flotation process’ as a possible feeding mechanism. Elevation in substrate temperature did not significantly increase water uptake activity or time spent underground, suggesting that other factors could also influence water uptake and circulation from the surface of substrate in the field. The presence of major chela appeared to hinder water uptake in A. perplexa individuals, but not significantly so. This is likely a small cost relative to the importance of this enlarged structure in increasing reproductive fitness. The removal of setal tufts in laboratory experiments showed that these setae could be more useful in water uptake only for G. vocans individuals and smaller individuals of A. annulipes. These findings suggest that species dwelling on moist substrates (e.g., G. vocans) depend more on capillary water uptake through the setae while those living on drier substrates (e.g., A. annulipes and A. perplexa) could favour the use of pressure difference between the inside of the branchial chambers and external environment, warranting further work in this aspect Examination of setal morphology revealed substantial differences in the area of setal coverage and types of setae in individuals of G. vocans compared to those of the other two species, indicating that some morphological adaptation to their respective microhabitats. New types of setae were also discovered, which could possibly be unique to the populations at the study site. The behavioural and morphological aspects of this study corroborated to suggest that capillary water uptake could be a derived trait. This is the first study to characterise the water-uptake and circulatory behaviours and uncover the functional morphology and importance of setal tufts in water uptake among fiddler crabs. These results point to the possibility of behavioural and morphological adaptations of each species of fiddler crabs for water uptake in relation to the moisture content in their microhabitats and shed light on their success in occupying the semi-terrestrial intertidal zone, where large fluctuations in microhabitat conditions often pose as a challenge for many other organisms. The scope of further work was also described to expand the study in greater relevance to current environmental issues.