ABSTRACT: The diel feeding cycle of Sagitta enflata, the dominant chaetognath in the Indian Ocean, was investigated, using gut content analyses, between the monsoon periods on 2 occasions, October 1995 and May 1996. Zooplankton samples were collected every second hour during a 24 h period close to a drifting buoy in the middle of the Zanzibar Channel. S. enflata made up 64% (October) and 77% (May) of all chaetognaths in this study. S. enflata and all chaetognaths contributed as much as 30 and 33%, respectively, of the total 90 µm net zooplankton wet weight in October (May samples could not be weighed due to occurrence of phytoplankton). S. enflata fed most actively at night. Small copepods with a mandible width <0.07 mm (prosome length <1 mm) were its predominant prey. Feeding rate and mean number of small copepods found in S. enflata were 3 times higher in October than in May. In contrast, the estimated daily predation impact on the standing stock of small copepods (by number) was around 1% for both periods. This may be a negligible predation effect considering the short life-span of tropical copepods. It is suggested that predation effects are related to temperature, since a similar predation impact may be important in cold polar waters where copepod life-spans are much longer. Daily cannibalistic predation impact (by number) was estimated to be between 1 and 4% (October) and between 2 and 9% (May) using assumed maximum and minimum digestion times of 8 and 2 h. Cannibalism, therefore, may be an important source of mortality in S. enflata populations.

KEY WORDS: Zooplankton · Chaetognatha · Sagitta enflata · Flacisagitta enflata · Copepods · Feeding · Predation · Cannibalism · Indian Ocean

Published in MEPS Vol. 193 (2000) on February 28
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 2000