Studies on the pebble crab, Eriphia smithi MacLeay 1838 (Xanthoidea Menippidae): patterns of relative growth and population structure

Morphometry and population structure have been investigated in a large sample (about 800 specimens) of the tropical Xanthoidea Eriphia smithi MacLeay 1838 (Decapoda Brachyura), inhabiting a rocky cliff in Somalia. The analysis of relative growth in two sexual dimorphic characters (i.e., chelipeds and abdomen) showed that both sexes undergo the puberty molt at approximately 19 mm CL. However, functional maturity sensu HARTNOLL (1969) is attained only by larger instars. In the females, the ovaries begin to grow allometrically in weight with respect to animal weight at the size of 25 mm CL, when the ovigerous-ratio reached a constant value (about 42\%) within the female population. Fecundity is size-dependent, the number of eggs per brood ranging from 14,000 in small ovigerous specimens to 60,000 in the largest ones. In the males, mating capability is limited not only by the developmental status of the gonads (gonad weight is inconsistent in individuals smaller than 25 mm CL), but also by the ownership of a copulatory burrow. The suitability of crevices for copulation depends on the presence of a narrow entrance (to prevent predation by fishes during high tides), and of an enlarged «nuptial» chamber. Due to their shape, these burrows are scarce within the cliff inhabited by this species, thus being a strong limiting factor for mating, and also requiring for their defence or conquest powerful weapons, i.e. long and wide chelipeds. Large males (35 mm CL), which are both the best armoured (chelipeds exhibit in this sex a strong positive allometry), and the most fertile (gonopods grow allometrically in length, and the gonads in weight) are almost completely absent in the samples. It has been hypothesized that these individuals may increase their mating opportunity as owners of copulatory burrows where the largest females can be accomodated. Thus, a male has to fight with conspecifics of the same sex to defend or conquer a burrow, and/or, if resident, he never leaves his burrow undefended (i.e., he does not make long foraging excursions around the plateau). It is likely that either a high mortality (due to the frequent agonistic interactions) or a sedentary life-style could reduce the number of large males in our sample.