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 sw imming in the open ocean. In a photographic survey at three abyssal station s 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 crab s was 10-14 g wet weight. From the photographic quantification it can be de duced that these large food falls represent a significant carbon input of a t least 10-30 parts per thousand of the annual flux of POC as measured in s ediment traps in this region. The exceptionally high microbial chitinase ac tivity in the surface sediment layers detected at the same stations indicat es that this energy is utilized and channelled into the deep-sea benthic fo od web of the deep Arabian Sea. There are frequent observations of dense Ch arybdis 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. (C) 2000 Elsevier Science L td. All rights reserved.