Reproductive Biology of the Three Spot Swimming Crab (Portunus sanguinolentus) Occurring in the Coastal Waters off Negombo, Sri Lanka with Novel Approach to Determine the Maturity Stage of Male Gonads

Abstract

The reproductive biology of Portunus sanguinolentus (Three Spot Swimming Crab) was
investigated in the coastal waters off Negombo, Sri Lanka from February 2014 to January
2015. Reproductive activity was assessed using macroscopic and microscopic observations of
gonad characteristics, trends of gonad indices, size at first sexual maturity and differences in
the monthly sex ratio. Though P. sanguinolentus females have higher body weight than
males, carapace length, carapace width or body weight of males and females were not
significantly different (Mann-Whitney test, p>0.05). Ovigerous females were observed
throughout the year, confirming that they are continuous spawners however, peak spawning
was in October. Percentage Gonadosomatic Index (GSI) of females is significantly higher
than the males (t-test; p<0.05; df=193) and GSI values ranged from 0.02 to 3.69 in males and
0.12 to 9.44 in females. Sex ratio of P. sanguinolentus population fluctuated seasonally and
significantly high male to female sex ratio was observed in October 2014 (χ2 test). Size at
first sexual maturity (Lm50) was estimated at 9.75 and 8.90 cm (carapace width, CW) for
males and females, respectively. Three types of external egg masses; stage I, II and III were
identified based on colour and size of the berried eggs. The mean diameter (μm) of eggs in
each stage was reported as 253.8±3.19, 281.8±6.79, 316.5±9.78, respectively. The average
fecundity of P. sanguinolentus varied from 2.04×104 to 1.24×106 and it increased with
increasing CW. Present study proposes a new method to identify mature and immature males
by using their external characteristics. Accordingly, in mature males, blue color patches can
be observed on the ventral side of chelar propodus and merus but this colour pattern is not
prominent in immature males and absent in mature females.

Keywords: Portunus sanguinolentus, Sri Lanka, Chelar propodus, Merus, Size at first sexual
maturity