Digestion time and feeding of the pelagic chaetognath Zonosagitta nagae in the laboratory

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Digestion time and feeding of the pelagic chaetognath Zonosagitta nagae, which occurs predominantly in warm waters around Japan, were investigated in the laboratory. Young individuals of Z. nagae ranging from 4.1–7.6 mm in body length were used for the experiments with adult females of the copepod Acartia steueri as prey. The mean digestion time of Z. nagae was 115±3 minutes at 20°C. Temporal change in individual’s feeding intensity was observed every 1.5 h for 24 h to determine the diel feeding pattern, feeding interval, feeding rates and daily ration. Almost all individuals showed no clear diel rhythm within 24-h at 20°C under 12L:12D cycle; time interval between feeding was 4.5–6 h (mode) for individuals that ingested multiple copepods. The number of copepods ingested by a chaetognath ranged from 0 to 5 and on average 2.6 copepods were consumed a day per individual. Specific daily ration in most individuals corresponded to ca. 50% of its dry weight while some individuals showed the extremes of non-feeding or voracious feeding (>200% d⁻¹), indicating that some of the young Z. nagae had an intermittent feeding mode, which would be adaptive to save energy expenditure at low prey densities and to reduce mortality by lowering the risk of predation.