The breeding cycle of Scylla serrata (Forskal, 1755) at Ramisi River estuary, Kenya

Abstract

The mud crab Scylla serrata was collected from Ramisi river estuaryfor a period of 14 months (January 1990–February 1991) using madema traps. For each crab, the carapace width (mm), sex and weightwere noted, and the number of ovigerous female crabs was recorded. Both ovigerous and non-ovigerous females were dissected in order to observeof the maturity stages of ovaries. The ovarian maturity stages weredescribed as stage zero – virgin/resting, stage one – developing, stage two– well developed and stage thre e– ripe. The most abundant was stage twothroughout the study period. The smallest ovigerous crab had a carapacewidth of 139 mm. A test of homogeneity of the binomial distributionof the sex ratio showed homoscedasticity (χ² = 14.615; d.f. = 13; p> 0.05) and the overall sex ratio did not differ from 1:1 (χ² = 0.776; d.f. = 1; p> 0.05). The variance test ofhomogeneity of the binomial distribution of sex in relation to size showeda very significant heterogeneity (χ² = 32.83; d.f. = 9; p< 0.05). There was no significant difference when the overall mean sizesfor males and females were compared using t-test (t = 4.26; d.f. = 18;p< 0.001). The relatively high numbers of females with stage two ovariesindicated that spawning took place throughout the year with a possible peakin the second half of the year.