Mass sedimentation of the swimming crab Charybdis smithii (Crustacea: Decapoda) in the deep Arabian Sea

During cruise Meteor 33/1 in the northern Arabian Sea in September/October 1995, large numbers of the portunid crab Charybdis smithii were observed swimming in the open ocean. In a photographic survey at three abyssal stations in the northern Arabian Sea (NAST, WAST, CAST), even higher densities of Charybdis smithii – up to 1 crab m−2 – were found dead on the sea floor. Average sizes of the crabs were around 34–44 mm carapace width, indicating that the animals died prematurely, before returning to the breeding grounds presumable on the shelves of India or Oman. The average weight of the crabs was 10–14 g wet weight. From the photographic quantification it can be deduced that these large food falls represent a significant carbon input of at least 10–30% of the annual flux of POC as measured in sediment traps in this region. The exceptionally high microbial chitinase activity in the surface sediment layers detected at the same stations indicates that this energy is utilized and channelled into the deep-sea benthic food web of the deep Arabian Sea. There are frequent observations of dense Charybdis smithii swarms in the Arabian Sea from different years; however, it is not certain whether such large food falls as observed during M 33/1 are regular seasonal events that repeat each year.