Morphometric Relationships in the Blue Swimming Crabs, (Portunus pelagicus) (Linnaeus, 1758) from the Palk Bay, Sri Lanka

The blue swimming crab (Portunus pelagicus) has become the focus of an important export-oriented fishery in Sri Lanka for a decade. The Palk Bay in the Northern coastal waters of Sri Lanka is one of the best-known fishing grounds for blue swimming crabs in Sri Lanka. This study was undertaken with the aim of establishing some morphometric relationships for P. pelagicus in the Palk Bay.

Biological sampling was conducted for five consecutive days per month from November 2014 - October 2015 in the Northern landing sites in the Jaffna District where the catch of Palk Bay blue swimming crab fishery was landed. P. pelagicus specimens were also randomly collected for laboratory analysis. The morphometric measurements of the specimens for the following parameters were undertaken: CW - carapace width with the spine, CWW - carapace width without spine, MW – mouth width, TL - total length, LCPL - left chelar propodus length, LMOV - left movable part Length, RMOV - right movable part length, LCPH - left chelar propodus height, RCPH - right chelar propodus height, ABW - abdominal width, ABL - abdominal length and BW - body weight. The least squared method was used to obtain the Length-Length (L-L) and Length-Weight (L-W) relationships.

Of the 65 L-L relationships obtained for P. pelagicus from the study, 34 relationships were correlated (r2>0.5). The estimated relationships between BW and CW for males and females were BW=0.0001 CW 3.01 (r2 =0.84) and BW= 0.0001 CW 2.90 (r2 =0.86), respectively.

Positive allometric growth was observed for male P. pelagicus, whereas a negative allometric growth was observed for female P. pelagicus. Most of the morphometric relationships of P. pelagicus obtained in this study were not isometric.