Vertical distribution and feeding of the copepod Chiridius armatus

Sammendrag

Adult females of the copepod Chiridius armatus inhabited the lower half of a 200-m water column during winter and spring. Their distribution became gradually shallower during summer and autumn. Female C. armatus carried out diel vertical migrations, with shallower population distribution at night compared to day, although normally with few individuals captured in the upper 50 m. Enumeration of fecal pellets produced by freshly collected individuals showed that C. armatus was foraging throughout the year. Production of fecal pellets was at a minimum in February and at a maximum in April. Individuals from all depths produced fecal pellets, but the numbers seemed to decrease by depth. Numbers of fecal pellets were greatest for individuals captured at night. The omnivorous C. armatus consistently produced many more fecal pellets than the concurrently studied, strictly carnivorous copepod Paraeuchaeta norvegica. We ascribe the relatively high number of fecal pellets to the higher content of indigestible components in plant material than in animal food. The assumption of phytoplankton as a prevailing food source got qualitative support from scanning electron microscope pictures, showing that fecal pellets mainly contained phytoplankton.