Effects of salinity on the larval development of a semiterrestrial tropical crab, Sesarma angustipes (Decapoda: Grapsidae)

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Abstract
Hatching and larval development of the semiterrestrial tropical brachyuran Sesarma angustipes were studied in relation to salinity in laboratory experiments. Adults were found in southern Brazil living in terrestrial habitats and in freshwater creeks. When a free choice was offered in the laboratory, females liberated their larvae indiscriminately in freshwater or seawater. Rearing experiments were carried out at 0, 3, 10, 20, and 32.permill. S. Unlike in other marine and brackish water species, freshly hatched zoea I larvae remained actively swimming in freshwater, where they survived for almost 2 d. Maximum survival time of zoea I exposed to 3.permill. S was ca 3.5 d. The larvae were able to pass through some or all developmental instars (4 zoeal stages and 1 megalopa) only at higher salinities. The zoea I at all salinities .gtoreq. 10.permill. S had a high survival rate to the second stage. Its minimum duration of development (with 100% survival), however, was observed at 20.permill. S. From the zoea II stage, survival was in general highest and development shortest at 32.permill. S, whilst 10.permill. S allowed only occasional development to later stages. This indicates a decreasing tolerance to low salinities and a shift in optimum salinity toward seawater in successive zoeal instars. Only 2 megalopae successfully reached metamorphosis to the juvenile crab, both at 20.permill. S. These results may reflect the following hypothetical patterns in larval development of S. angustipes under field conditions: (1) the zoea I may hatch in freshwater or brackish water; (2) if hatching takes place in freshwater, the larva has 1 to 2 d to be transported by currents to brackish water or seawater; (3) the major part of zoeal development (from the zoea II or III stage) takes place in lower estuaries and coastal oceanic waters with > 20 .permill. S; (4) the megalopa may return to less saline environments. It remains unknown whether the megalopa can successfully metamorphose in freshwater, or if recruitment in rivers and other freshwater biota takes place by upstream migration of juveniles. Our results show that S. angustipes must be considered a marine species that has only started its transition to terrestrial and freshwater habitats. Larval morphology of S. angustipes was practically identical with that presented in a previous description of Sesarma ricordi larvae, confirming the hypothesis that these 2 species are synonyms.