Feeding Ecology of Red Spotted Swimming Crab, Portunus sanguinolentus (Herbst, 1783) along Mangaluru Coast, Karnataka

Crabs are one of the major marine fisheries resources in India and they are among the most valuable seafood species due to their delicious taste, nutritional value and economic importance. The knowledge of the feeding ecology of these crabs is particularly important to understand their ecological interactions, since the abundance of preferred prey organisms strongly affects their distribution, growth, reproduction, behavior and migration.

The study was conducted along the Mangaluru coast, with fortnightly crab samples gathered from the Mangaluru fish landing centre from March 2021 - February 2022. Random sampling ensured the inclusion of all size categories and samples were analyzed for weight, morphometry measurements, sex and food and feeding habits. The level of stomach fullness was assessed visually and the contents were examined using Point’s volumetric method, identifying food items at the lowest taxonomic level and grouping them into dietary categories. The vacuity index and Gastro-somatic index (GaSI) were calculated to describe the trophic behaviour of Portunus sanguinolentus.

Gut content analysis revealed that P. sanguinolentus fed predominantly on crustaceans (31.45%), fish (22.84%), molluscs (18.70%) and larger quantities of sand and debris (20.29%) and unidentified matters (6.68%). The presence of debris and unidentified matter in the foregut suggests that P. sanguinolentus is a bottom-feeding omnivore which feeds on fresh and decaying matter. Size-wise analysis of gut contents revealed that juveniles (<80mm CW) preferred miscellaneous items followed by crustacean remains as their diet, sub-adults (80-100 mm CW) crustaceans (42.65%) and miscellaneous (50.07%) and adults (>100mm CW) had a solid preference for crustaceans and molluscs.