Lateral line stimuli can override vision to determine sunfish strike trajectory.

Although all fishes have a mechanosensory lateral line system – a system of water flow detectors (neuromasts) in canals or free on the skin's surface – particular behavioral functions are documented for only a few species. Work on lateral line use for feeding has focused on either cavefishes or night-active species (Montgomery, 1989). While surface-feeding fishes with well-developed eyes do use the lateral line to locate prey (Müller and Schwartz, 1982), it is generally assumed that diurnal/crepuscular fishes are visual predators. We show that a hydromechanical stimulus detected by the cephalic lateral line system in two sunfishes (Centrarchidae) can be the sole determinant of a strike trajectory. The response occurs without reinforcement and appears to be an unconditioned response. Green sunfish (Lepomis cyanellus Rafinesque) and largemouth bass (Micropterus salmoides Lacépède) are North American sunfishes that inhabit ponds, lakes and slow streams. Both species have diverse diets, including invertebrates and fishes (Carlander, 1977). These species and their congeners are active during the day; at night in field and laboratory they lie near or on the bottom and are lethargic (Neill and Magnuson, 1974; Helfman, 1981). Both species have cones and rods in the retina; the green sunfish visual pigments are characteristic of crepuscular mid-water fishes (Dearry and Barlow, 1987; Lythgoe and Partridge, 1989). Visual feeding of a congener of the green sunfish (L. macrochirus) has been studied previously (Li et al. 1985). The visual stimulus for this experiment was food presented through a feeding tube; the lateral line stimulus was a water jet (Fig. 1). The visual stimulus was sometimes given with the lateral line stimulus but mostly without the lateral line stimulus (see next paragraph). Green sunfish (four, 111–115mm) were housed individually in 19 l rectangular glass aquaria on one side of a vertical acrylic partition. The single largemouth bass was in a similar 38l aquarium. The white plastic feeding tube (12.3mm i.d.) penetrated the partition and to either side of it were single 10ml water-filled pipette 'jet nozzles', extending 5–6cm horizontally from the partition (Fig. 1). The visual stimulus (a piece of squid mantle) was presented on a wire pushed through the feeding tube. The lateral line stimulus, one or the other water jet, was activated via air pressure from a pump to one pipette by closing a bleed valve. Jet velocities, measured along the jet axis by hot