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The benthic Spadella schixoptera and the planktonic Sagitta hkpidu both attacked a fine vibrating glass probe over a broad range of frequencies after being starved for l-3 days. The response curve for S. schimptem had a peak at 30 Hz and was similar in shape but far broader than that found by prior workers for Spade&z cephalopteru.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 hispidu starved for 1 day searched 2,667 mms min-' 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, Chaetognaths are important predators, feeding on a wide variety of zooplankters including crustaceans, appendicularians, fish larvae, and other chaetognaths (see Nagasawa and Marumo 11976). Yet, because the planktonic species are difficult to keep in the laboratory, few observations have been made of their feeding behavior (e.g. Parry 1944; Reeve 1970b; Cosper and Reeve 1975; Kuhlmann 1976). ming copepods and hence can select their food. Fraser (1969) tried to get a planktonic species to attack a similar probe, but without success. We have made tests with a vibrating probe with both the benthic Spaci!eZZa schfxoptera and the planktonic Sagitta hispida and have also observed the feeding behavior of S. hispida in large aquaria to determine its dependence on prey density. John ( 1933) directed a small jet of water at the body surface of the benthic SpacZeZZa to try to analyze its ability to detect prey; Bieri (1966) touched different parts of Pterosagitta with a needle and also observed that Sagitta reacted to a glass rod when the rod was moved in the water, but not when it was above the water. Horridge and Boulton (1967) found that Spadella cephaloptera would attack a fine glass probe when it was vibrated from 9 to 20 cycles per second (Hz), with a distinct peak response at 12 Hz. This led Newbury ( 1972) to speculate that chaetognaths recognize the specific vibration rates produced by swimWe are grateful to many people at the Rosenstiel School for helpful ideas and assistance, particularly C. Gordon and R. Tusting who supplied parts of the probe apparatus and L. Farmer and M. A. Walter who aided in obtaining and handling some of the chaetognaths, E. Houde suggested the use of the model which led to the attack distance and has made valuable suggestions on the manuscript. We also thank J. Sprogis for drawing the figures. In these tests a thin glass probe was mounted on a magnetic pen motor driven by a signal generator (oscillator).The probe was vibrated at various frequencies at low amplitudes and also made to perform slow lateral s...

Chaetognaths are important marine predators which rely on external sensory hairs to receive prey-produced vibrations, enabling them to recognize and locate their food. Hair-fan patterns are mapped for both larvae and adults of Sagitta hispida, Sagitta enflata, and Spadella schizoptera, for several sizes of Sagitta elegans, and for adults of Spadella cephaloptera. The fans are arranged in regular transverse and longitudinal rows with the pattern species specific, but similar, for all species studied. Larvae possess relatively longhair fans at hatching and the number of fans increases with size in the species of Sagitta, but is constant in juveniles and adults of the species of Spadella.

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Feeding rates were determined for S. elegans feeding in Vineyard Sound, Massachusetts, at a temperature of 0°C by applying a digestion time (10.2 h), obtained from laboratory observations, to an analysis of gut contents. The specific daily ration of large individuals was about l-2% of body dry wt*d-', that of small chaetognaths (3.5 mm) was about 46%. Feeding rates for all but the smallest animals are similar to those predicted from respiration data; the small& individuals consumed more than their estimated no-growth requirements.