Reproductive biology of the three-spot swimming crab (Portunus sanguinolentus) from the west coast of Sri Lanka with a novel approach to determine the maturity stage of male gonads

Abstract

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Aspects of the reproductive biology of Portunus sanguinolentus from the west coast of Sri Lanka were studied from February 2014 to January 2015. Berried females were observed throughout the year, confirming that they were continuous spawners; however, peak spawning was in October. The gonadosomatic index of females was significantly higher than the males (GLM; p < 0.05). Sex ratio fluctuated seasonally and a significantly higher male to female ratio was observed in October (χ2 test). Size at first sexual maturity was estimated at 9.75 and 9.40 cm carapace width for males and females, respectively. Three types of external egg masses, stages I, II and III, were identified based on the colour of berried eggs. The mean (±SD) diameter of eggs in each stage was 253.8 ± 3.19, 281.8 ± 6.79 and 316.5 ± 9.78 μm, respectively. The estimated fecundity of P. sanguinolentus varied from 1.12 × 105 to 1.38 × 106. We propose a new method to identify the maturity stage of the male gonad using external characteristics. Males with mature gonads have blue colour patches on the ventral side of the chelar propodus and merus but this was not prominent in males with immature gonads and absent in females.