Population biology of the sponge crab Cryptodromia hilgendorfi (Dromiacea) in Moreton Bay, Queensland, Australia

Abstract

Cryptodromia hilgendorfi de Mann 1888 is an intertidal sponge crab that uses sponges and ascidians as places to live and for concealment. Most crabs occur alone on sponges, but when the tide is in, there are frequent movements. The crab is a biennial species with a maximum life span of approximately 2–5 yr. Generations overlap by 1.5 yr and during early summer as many as 3 generations may occur together in the same area. Both males and females live to approximately the same age and females are iteroparous, producing an average of 3–4 broods in a lifetime (maximum 6–7). Females moult after each brood and must remate in order to produce fertile eggs. Eggs (mean diameter 0.73 mm) are carried from September to February and recruitment occurs during December to March. The young are not brooded by the female and have a planktonic phase lasting 2–3 wk. Brood size increases with age from 150 eggs for the first brood to over 600 eggs for the largest females. Average lifetime production of offspring is approximately 1 500 (maximum 3 600). The largest contribution to recruitment comes from first brood offspring. Compared to other Brachyura of similar size C. hilgendorfi produces smaller broods composed of larger eggs. Life table analysis suggests that reproduction is more than sufficient to achieve replacement. The young crabs have a specific settling response to the sponge Suberites carnosus. Sex ratio at settling does not differ from 50/50, but as a result of differential growth and perhaps differential mortality, females predominate in the larger size classes. Both males and females have indeterminate growth but their parabolic growth curves differ. Females grow more rapidly so that males of the same age are smaller. Moult increments and intermoult intervals differ between the sexes. Male increments are smaller and independent of initial size whereas female increments are larger but decrease with increasing size. Intermoult intervals for males tend to be longer than for females although female intervals increase more rapidly as the crab grows. Sexual size dimorphism differs from the typical brachyuran pattern of males being larger than females.