Distribution, abundance, and feeding ecology of decapods in the Arabian Sea, with implications for vertical flux

Macrozooplankton and micronekton samples were collected on two cruises in the Arabian Sea conducted during the Spring Intermonsoon period (May) and the SW Monsoon period (August) of 1995. Discrete depth samples were collected down to depths of 1000–1500 m. Quantitative gut content analyses were performed on four species of decapod shrimps, Gennadas sordidus, Sergia filictum, Sergia creber, and Eupasiphae gilesii, as well as on the pelagic crab Charybdis smithii. Of the shrimps, only S. filictum and S. creber increased significantly in abundance between the Spring Intermonsoon and SW Monsoon seasons. These four species were found at all depths sampled, and most did not appear to be strong vertical migrators. G. sordidus and S. filictum did appear to spread upward at night, especially during the SW Monsoon, but this movement did not include the entire population. S. creber showed signs of diel vertical migration only in some areas. All four shrimp species except, to some degree, S. creber lived almost exclusively within the oxygen minimum zone (150–1000 m), and are likely to have respiratory adaptations that allow them to persist under such conditions. Feeding occurred at all depths throughout these species’ ranges, but only modest feeding occurred in the surface layer (0–150 m). G. sordidus appeared to feed continuously throughout the day and night. Estimated contribution of fecal material to vertical flux ranged from <0.01–2.1% of particulate flux at 1000 m for the shrimps and 1.8–3.0% for C. smithii.