HABITAT SELECTION AND ENERGETICS OF THE FIDDLER-CRAB (UCA-TANGERI)

We tried to unravel the possible links between the skewed predation ri sk in Uca tangeri (where large individuals are more at risk from avian predators) and size-dependent changes in the physiology and habitat c hoice of this fiddler crab species. Over a transect running from low t o high in the tidal zone of a beach in Mauritania, the temperature pro file at various depths in the substrate, the water-table level of seep water, salt concentration of seep water, depth of the aerobic level, operative temperatures on the surface, and size distribution of crabs were assessed. In addition, resting metabolic rates, Q10 and thermal a nd starvation tolerances were estimated. Going from low to high in the tidal zone, crab size and burrow depth increased. At the preferred bu rrowing depth, microclimatological conditions appeared to be equally f avourable at all sites. At the surface, conditions were more favourabl e low in the tidal zone, where also food availability is sufficient to enable small crabs to forage in the vicinity of their burrows. Large crabs have higher energy requirements and are thereby forced to forage in flocks low in the tidal zone where food is probably more abundant. Low in the tidal zone, digging deeply is impossible as the aerobic la yer is rather thin. Large crabs prefer living high in the tidal zone a s (1) deep burrows ensure better protection against predators, (2) mor e time is available for digging holes and (3) the substrate is better suited for reproduction. Energy reserves in late summer ensured an ave rage of 34 days of survival. It is argued that the allotment of energy to growth must be considerable even in reproducing animals; the rewar ds of growth being the disproportional increase in reproductive output with size.