Feeding ecology of Spinocalanus antarcticus, a mesopelagic copepod with a looped gut

Abstract.

Spinocalanusantarcticus, an abundant mesopelagic copepod in polar seas, has a greatly elongated and looped midgut, contrary to most other copepod species. The total gut length is 1.77, 1.86 and 1.90 times the total body length in adult females, CV and CIV, respectively. Gross morphology of the midgut is similar in all copepodite stages and adults. It is described here from specimens collected in the Arctic Ocean. In stratified samples from the deep Amundsen and Makarov Basins S. antarcticus showed a clear preference for the depth layer between 100 and 500 m. Generally, the guts were packed with material, but most of it was impossible to identify. In most specimens the digestive tract was filled with undefined detritus particles ("detritus balls"). They were almost spherical, heterogeneous organic aggregates of 40–100 µm diameter, with small clay-sized mineral flakes imbedded. Mineral particles in the size range of 1–10 µm were found in large quantities in the guts of many specimens. Cysts of Chrysophycea and dinoflagellates and fragments of dinoflagellates, diatoms, tintinnids and radiolarians, as well as skeletons of silicoflagellates, were rather rare; some animal remnants were also found. A high carbon/nitrogen ratio (8.9) and very high lipid content (54% of dry weight) indicated a very good nutritional state. The adaptive significance and possible feeding strategy of this deep-water copepod is discussed.