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Sexual Maturity and Reproductive Strategy of the Rock Crab Grapsus Adscensionis (Osbeck, 1765) (Brachyura, Grapsidae) on Ascension Island

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The rock crab, Grapsus adscensionis (Osbeck, 1765), was studied on Ascension Island, South Atlantic Ocean. By day crabs were inactive above water level at high tide, but during low tide fed on algal films exposed on the low shore. By night they were inactive at all tidal states. Both sexes reached a maximum carapace width of 74 mm, but were potentially capable of further moulting. Sexual maturity was reached at ∼38 mm carapace width in males, and ∼43 mm carapace width in females. In females one batch of eggs was produced in most intermoults. The ovaries had matured by the time that the eggs had completed incubation. The females moulted shortly after the hatching of the eggs, and re-laid early in the next intermoult. Consequently 78% of mature females were ovigerous. It is estimated that an egg batch is laid every 24 days. Reproductive investment (dry weight) per egg batch averaged 5.5%. Le crabe des rochers Grapsus adscensionis (Osbeck, 1765), a été étudié sur l'île d'Ascencion, océan Atlantique Sud. De jour, les crabes sont inactifs au dessus du niveau de l'eau à marée haute, mais durant la marée basse ils se nourrissent sur le film algal de la côte émergée. La nuit ils sont inactifs quelle que soit la marée. Les deux sexes ont atteint une taille maximum de 74 mm de largeur de carapace, mais sont susceptibles de continuer à muer. La maturité sexuelle est atteinte à ∼38 mm de largeur de carapace pour les mâles, et ∼43 mm pour les femelles. Chez les femelles, une ponte est émise dans la plus part des intermues. Les ovaires sont matures au moment où les œufs terminent leur incubation. Les femelles muent rapidement après l'éclosion des œufs et pondent à nouveau au début de l'intermue suivante. En conséquence 78% des femelles mâtures sont ovigères. Il est estimé qu'une ponte est émise tous les 24 jours. L'investissement dans la reproduction (en poids sec) par portée est en moyenne de 5,5%.

Affiliations: 1: School of Biological Sciences, Biosciences Building, Liverpool L69 7BZ, United Kingdom;, Email: hartnoll@advsys.co.uk

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