Fecundity and reproductive output in the Brachyuran crabs Scylla serrata (forskal, 1755) and thalamita crenata (H. Milne Edwards, 1834) at the Kenya Coast

Abstract

Scylla serrata (Forskal) was sampled at Kiwambale using madema traps while Thalamita crenata (H. Milne Edwards) was sampled at Gazi using a scoop net. The objectives of the study in both crabs were: i) To determine the maturity stages of ovaries. i i ) To determine the developmental stages of the eggs. iii)To find out the mlnimum slze at sexual maturlty. iv) To estimate fecundity. v) To find the sex ratio of the crabs. vi) To determine the breeding cycle. The ovarian maturity stages were described as stage o \/irgin / Resting, stage 1- Developing, stage 2- well developed , stage 3- Ripe. In S. serrata, maturity stage 0 was only obtained once during the study - period, while stages 1-3 were obtained in low numbers in some months. The most abundant was maturity stage 2 which occurred in large numbers in each sample. The devefopmental stages of eggs were grquped into 10. Only five ovigerous S. serrata were obtained which had their eggs in stages 1-3. The smallest ovigerous crab had a carapace width (CW) of 139.75 mm and a crab of 77.1 mm (CW) had an actlvated ovary in stage 1 of development. A positive relationship exists between fecundity and carapace width (t= 3.546; d. f. = 3; p < 0.05) and egg mass weight (t = 3.251; d.f. = 3; P ~ (>.0::::,) respectively •. Jhile there lS no significant relationship fecundity and egg size (t = o . 029; d. f. = 3; P > 0 . .1). A test of homogeneity of the blnomial of the sex ratio showed homoscedacity (X2 = 14.615; d.f. = 13; P > 0.(5) and the sex ratio did not differ from a 1:1 (""X2 = 0.776; d. f. = 1; P > O. (2) . The variance test of homogeneity of the binomial distribution in relation to Slze shows a very significant difference (X2 = 32.83; d.f. 9', P < 0.(5) . There was no significant difference when the overall mean sizes for males and females were compared using the t-test (t = 0.178; d.f. = 18; P > 0.(5) . The spawning season of S. serrata in Kenya .. is from August to De2ember when ovigerous crabs were obtained. ~ Inspite. of tflis, these crabs had active , ovarles (stages 1-3) throughout the sampllng period. In Trie l ami ta c r erie t:e , ovarian maturity stage 2 "'las' the mos t abundan t throughou t _the samp ling period while maturii;y stages 0, 1 and' 3 occurred at low ~"P" " frequency during the samp 1i'f'lg period. All developmental stages 1-10 were observed in oVlgerous females but later developmental stages predominated during the two breeding peaks in September and The minimum size at sexu~l maturity 28 .9 mm (CW) . A positive relationshlp eXlsts between ..• fecundity and carapace width (t 4.95; d.f. = 205; p < 0.0(1) and egg mass weight (t 7.687; d ..f. = 205; -XlVp ~ 0.0(1) respectively while there lS no significant relatlonship between fecundity and egg size (t 0.3695; d . f . 205; p > 0.1) . The test of homogeneity of the binomial distributlon of the sex ratio showed homoscedasticy (X2 = 16.83; d.f. = 11; P > 0.(5) and the overall sex ratio did not differ from a 1:1 ( X 2 = 19.577; d.f. 1; P < 0.(5) . The varlance test of homogeneity of the binomial distribution In relation to the size shows a very significant difference (X2 = 112.2; d.t. = 12; p < 0.001). There was no signlficant difference when the overall mean sizes for males and females were compared using t-test (t = 0.83; d. f . 24; p > 0.05) . This crab species breeds throughout the year. But femal...e, crabs with carapace width 40.5 and 45.5 mm are actively breeding throughout the the sampling period. The females had activated ovaries throughout therefora they are continuous breeders.