The distribution and biology of the swimming crab Charybdis smithii McLeay,1838 (Crustacea; Brachyura; Portunidae) in the NW Indian Ocean

Abstract

On seven cruises with the R.V. Tyro, R.R.S. Discovery and R.V. Malcolm Baldrige in the NW Indian Ocean between 1992 and 1995, the pelagic swimming crab Charybdis smithii was found in great densities during July–August (SW monsoon) and January (NE monsoon). Discrete depth sampling, using RMT1 +8 and Mocness-10 gear, collected the species predominantly in the upper 200 m during night and day. Most of the crabs were concentrated above the thermo- and oxycline, both situated at around 150–200 m. In July–August the stock of crabs showed considerable variation, with maximum values of 1.9–2.4 crabs m −2 (0–500 m, night) foundin the Southern Somali Basin the Omani Basin and in the Central Arabian Sea. The maximum biomass (wet weight) of crabs was 12.7 g m −2 (0–500 m, day) off Somalia and in the Central Arabian Sea during the SW monsoon. In January stocks and biomasses were lower, with maximum values of 0.1 and 2.0, off Somalia at 7°N. The size-frequency distribution of all crabs found during the SW monsoon in the Somali Basin demonstrated the occurrence of smaller instars at the near-coast stations compared with the open ocean stations. During the NE monsoon there was a tendency for nearshore crabs to be larger than offshore. High concentrations of zoea and megalopa larvae of portunid crabs together with juvenile C. smithii were found in neuston samples off Somalia in May. Dense swarms of crabs occurred in the pelagic NW Indian Ocean in June–September (SW Monsoon). In October, crabs began to migrate onshore, where they characteristically formed surface swarms at night. Analysis of combined ship and literature data showed a 1 year life cycle for C. smithii. Aggregation of the crabs on the continental shelf precedes their breeding from about October to January (NE Monsoon). The larvae hatching in shelf waters disperse offshore and, after metamorphosis, form dense pelagic swarms from about July to August (SW monsoon).