The life cycle of the semi-terrestrial mangrove crab Ucides cordatus includes pelagic larvae that are released into estuarine waters during the wet season and who may thus encounter potentially stressful low and variable salinity conditions. The effect of salinity on the survival of the zoea larvae, the number of zoeal stages and the duration of development from hatching to megalopa was experimentally studied by rearing larvae from the Caeté estuary, N-Brazil, in seven salinity treatments (0, 5, 10, 15, 20, 25, and 30). For a better interpretation of the laboratory results, estuarine salinities were measured over five consecutive years during the species´ reproductive season. The survival of the zoea larvae varied significantly with salinity, while the number of stages and the duration of their development remained 2 constant. Development to megalopa took 20.77 ± 1.57 days and comprised five zoeal stages with ZI and ZII being euryhaline and later stages stenohaline. The newly hatched larvae stayed alive for up to six days in freshwater (average 4.32 ± 0.82 days), but did not moult to the second zoeal stage. ZII larvae first occurred from salinity 5 onwards and later zoeal stages at all tested salinities ≥ 10. However, the larvae only survived to megalopa at salinities ≥ 15, with highest numbers at salinity 30 (72%) and lowest at 15 (16%). Lethal salinities ≤ 10 occurred frequently in the estuary during the reproductive season. This suggests a need for larval export to offshore and thus more saline waters to allow for significant larval survival and maintenance of viable populations of this commercially important species. A regional rather than local approach for management is suggested due to the likelihood of long distance larval dispersal by off-shore currents.

Environmental cues associated with the adult habitat induce metamorphosis in many marine invertebrate larvae and thus settlement in a place appropriate for completing their life-cycle. Ucides cordatus is a semi-terrestrial crab that exemplifies organisms living in mangrove estuaries as adults, but whose larvae develop offshore and return to the estuarine environment prior to metamorphosis. We investigated the influence of substrata (burrow mud, channel-bank mud, dead wood, sand, nylon mesh) and conspecific adult cues (crab conditioned seawater) on metamorphosis and the duration of the megalopal phase. Our results suggest that U. cordatus megalopae settle in areas populated by conspecific crabs and/or muddy habitats. High moulting rates were observed in the treatments with mud and/or adult odour (67-91 %), whereas in the other substrata treatments and in the seawater control less than 20% and 5% of the megalopae metamorphosed, respectively. Megalopae are also capable of delaying metamorphosis. Over channel bank mud they moulted 8 days later at an average, compared to the treatment with crab conditioned seawater. Such a delay could result in a reduced juvenile fitness. In seawater control almost all megalopae died prior to metamorphosis, without moulting spontaneously as observed in other species. This reflects the crucial importance of habitat cues for the settlement and recruitment of this ecologically and economically important species. We outline the necessity of future research concerning post-metamorphic carry-over effects, the possible role of biofilms in absorbing and/or producing settlement cues, as well as the question whether the megalopae react specifically to the odour of U. cordatus, or also to other species. The latter is of particular interest against the background of the spreading lethargic crab disease that results in massive mortalities of U. cordatus, but not of co-occurring-species.

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Food supply is considered critical for a successful culturing of decapod larvae. However, some species may present yolk reserve sufficient to complete their larval development without external food supply (known as lecithotrophic larval development). In the present study, two experiments were carried out in order to verify whether the callianassid Lepidophthalmus siriboia have lecithotrophic behaviour or, if they need external food for their larval development: Experiment 1, larvae submitted to an initial feeding period and Experiment 2, larvae submitted to an initial starvation period. High survival rate was observed in both experiments, in which only 2 megalopae and 1 zoea III died. These results strongly suggest that larvae of L. siriboia are lecithotrophic as they have sufficiently large yolk reserve to complete their larval development, while the megalopa stage shows facultative lecithotrophy. The larval periods of each stage of the treatments were quite similar and, despite some significant differences in some larval periods, these can be related probably to larval rearing conditions, abiotic factors or, individual variability of larval health, as well as stress caused to the ovigerous females during embryogenesis.

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RESUMOO presente trabalho estudou o efeito da salinidade na sobrevivência e na duração do desenvolvimento larval do caranguejouçá, Ucides cordatus (do estuário do Rio Caeté, Norte do Brasil), até a fase de megalopa em sete tratamentos de salinidade (0, 5, 10, 15, 20, 25 e 30). A salinidade afetou significativamente a sobrevivência das larvas zoea, no entanto não afetou a duração do desenvolvimento larval (20,77 ± 1,56 dias). Nas salinidades 0, 5 e 10 houve total mortalidade das larvas zoea. Somente a partir da salinidade 15 observou-se um desenvolvimento completo até a fase de megalopa. A taxa de sobrevivência foi maior em salinidade 30 (72%) e menor em 15 (16%). A reduzida taxa de sobrevivência das larvas zoea de U. cordatus, em salinidades baixas, indica a necessidade de dispersão larval do estuário para as águas costeiras onde as condições de salinidade para o desenvolvimento larval são mais favoráveis. Caso contrário se não houvesse a dispersão, a reduzida salinidade das águas estuarinas no período chuvoso, causaria uma elevada mortalidade, afetando desta forma o recrutamento, a manutenção e o crescimento da população de U. cordatus nos manguezais. PALAVRAS-CHAVE:Ucides cordatus, Salinidade, Sobrevivência, Dispersão larval, Zoea. The effect of salinity on the larval development of the uçá-crab, Ucides cordatus (Linnaeus, 1763) (Decapoda: Ocypodidae) in Northern Brazil ABSTRACTThe present work studied the effect of salinity on the survival and duration of larval development of the mangrove crab, Ucides cordatus (from the Caeté River estuary, North of Brazil) until the megalopal phase in seven salinity treatments (0, 5, 10, 15, 20, 25 e 30). Salinity significantly affected the survival of the zoea larvae, however it did not affect the duration of the larval development (20.77 ± 1.56 days). In salinity 0, 5 and 10 all zoea larvae died. Only from off salinity 15, complete development until the megalopal phase occurred. The survival rate was highest in salinity 30 (72%) and lowest in 15 (16%). The reduced survival rate of the U. cordatus zoea larvae, in low salinities, indicates the necessity of larval dispersion from the estuary to coastal waters, where salinity conditions for larval development are more favorable. Otherwise, if there was no dispersion, the reduced salinity of estuarine waters in the rainy season would cause a high mortality thereby affecting the recruitment, maintenance and growth of the U. cordatus population in the mangroves.