In situ feeding behavior of Sagitta elegans and Eukrohnia hamata (Chaetognatha) in relation to the vertical distribution and abundance of prey at Ocean Station “P”1

Abstract

The feeding behavior and diet in nature are described for two species of chaetognaths in an open ocean community. Sagitta elegans lives and feeds primarily in the upper water column (0–25 m) and the vertical distribution of this species may be explained primarily by its requirement of high prey densities. Eukrohnia hamata apparently needs less food and is found mainly below the mixed layer with a vertical distribution restricted by temperature. The same prey species, those with the highest densities at Station “P,” make up the diet of both chaetognaths, although Eukrohnia eats significantly more small prey. The major prey of juvenile chaetognaths (numerical frequency) are small cyclopoid copepods, a group frequently overlooked in plankton surveys. These species also comprise 41% of the diet of older chaetognaths, which prey in addition on stage V copepodites of the largest copepods. It is suggested that the development of larval chaetognaths depends on abundant, small prey but that older chaetognaths can handle prey of a very wide spectrum of sizes.