Vertical Migration and Feeding in Sagitta Elegans Verrill

Abstract

Field studies established diel vertical migration of Sagitta elegans Verrill. Light reactions of this species were investigated in the laboratory, where positive phototaxis and light—controlled geotaxis were found. Based on these, a model for depth control in diel migration is proposed. The field studies disclosed that light inhibits feeding and suggested that the state of satiation of the animal influences its depth control mechanism. The usual method of assessing vertical migration from stratified samples is criticized, and an explanation of “midnight sinking,” involving rapid turnover of feeding animals in surface waters, is offered. The diet of Sagitta was examined in summer and winter and related to the availability of different copepods. The relationship of diel migration to feeding activity led to a preponderance of surface—living copepods in Sagitta's diet. This effect can distort calculations of food selection. Theories of adaptive value are briefly discussed and it is concluded that Sagitta left the surface waters during the day primarily in order to avoid visually directed predation.