Larval morphology and salinity tolerance of a land crab from West Africa, Cardisoma armatum (Brachyura: Grapsoidea: Gecarcinidae)

Abstract

The complete larval development of a terrestrial crab from West Africa, Cardisoma armatum Herklots, 1851, was studied in laboratory rearing experiments carried out at various salinities ranging from 0.2‰ to 45‰. Six zoeal stages and one megalopa are described and illustrated. Our experimental results showed that zoeal stages of C. armatum are fairly euryhaline, zoea I to IV tolerating a salinity range of 15-45‰, and 15-35‰ during later development. However, salinity 15‰ tended to cause higher mortality and a significantly delayed development in most stages, while 25‰ allowed for maximum survival through metamorphosis. These observations suggest that C. armatum follows a limited export strategy, where the adults may live in brackish or even freshwater habitats, while a successful larval development is possible only in estuarine or coastal waters with higher salinities, presumbly with an optimum in the lower parts of estuaries with 25‰. Before this study, only an incomplete description of the first zoeal stage of C. armatum was available, and complete larval development had been known only for C. carnifex and C. guanhumi. In the present paper, the larval morphologies of these three congeneric species are compared. Within the Gecarcinidae, the complete larval development has been described also for Discoplax hirtipes and Gecarcinus lateralis, while only data for the morphology of the first zoeal stage are available for the other two genera of this family, Epigrapsus and Gecarcoidea. Hence, there are at present no sufficient data, in particular on megalopal morphology, to allow for conclusive intergeneric comparison and identification of familial characters. Gecarcinid zoeal morphology, as far as this is known, is briefly discussed in relation to that in other grapsoid families.