Natural Diet of Two Commercial Crab Species, Portunus segnis (Forskål, 1775) and P. sanguinolentus (Herbst, 1783), in the Coastal Waters of Karachi

Stomach contents of 558 Portunus segnis (Forskal, 1775) and 426 P. sanguinolentus (Herbst, 1783), from coastal waters of Karachi were examined. Crabs were collected from Korangi Creek, Karachi from January 2004 to December 2005. The frequency of occurrence and points methods were used for the stomach contents analysis. Out of 558 P. segnis and 426 P. sanguinolentus , 254 (45.52%) and 227 (53.3%) crabs had empty stomach, respectively. Mollusca and Crustacea dominated the diet of both the species. According to points method Mollusca and Crustacea scored 67.28% and 53.8% points in P. segnis and P. sanguinolentus , respectively. When analyzed by frequency of occurrence method Mollusca and Crustacea occurred in 88.37% and 71.4% stomachs in P. segnis and P. sanguinolentus , respectively. Majority of the Mollusca eaten by P. segnis and P. sanguinolentus were small soft-shelled bivalves while Crustacea included mostly small crabs. Small fish, shrimps, polychaete worms, sponges, starfishes, brittle stars, bryozoans, and plant (algae) materials were also present in some stomachs in small quantity. Results of the present study suggest that the two crab species are opportunistic predator competing for food in the natural environment.