Prey detection in the Chaetognatha: Response to a vibrating probe and experimental determination of attack distance in large aquaria1

The benthic Spadella schizoptera and the planktonic Sagitta hispida both attacked a fine vibrating glass probe over a broad range of frequencies after being starved for 1–3 days. The response curve for S. schizoptera had a peak at 30 Hz and was similar in shape but far broader than that found by prior workers for Spadella cephaloptera. Sagitta hispida responded differently, with a peak response at 150 Hz and no apparent preference among lower frequencies. It also had a strong (37%) response to nonvibrating lateral sweeps. Neither species would attack the probe at a distance >3 mm. This extremely short attack distance was confirmed, for S. hispida, in a 50‐liter aquarium by a series of feeding experiments, the results from which were applied to a simple model. Sagitta hispida starved for 1 day searched 2,667 mm3 min−1 and had an attack distance of 0.88 mm. These experimental data were used to determine the ability of S. hispida to sustain itself nutritionally in its natural environment.