Fine Analysis of Predatory and Defensive Motor Events in Torpedo Marmorata (Pisces)

Abstract

ABSTRACT Predation and defence of Torpedo marmorata, elicited with biological., and inanimate stimuli, were studied in experimental conditions. Patterns of responses of neonates and adults are described, and chains of separate, coordinated motor acts are defined. Jumping predation, creeping predation, defence of the disc and defence of the tail are distinguished. Each predatory behaviour consists of stimulus reaction chains, whereas defensive types of behaviour involve single stimulus reactions. During jumping behaviour, tail strokes provoked by the initial response in the chain are displayed during the whole period of predation; these ‘delayed’ tail strokes provoke displacements of the prey, thus probing the substrate and furnishing biological cues. Large behavioural variations are reported, including incomplete chains of responses, repetition of responses, oriented responses, stimulus intensity/response magnitude relationships and missing of certain motor acts. Ontogenetic maturation of the behaviour patterns is observed in embryos and in immature neonates. Sets of electric organ discharges (EODs), which accompany the defensive responses and certain predatory responses, show a stereotyped pattern, characteristic for each EOD motor act. Variations found in the EOD pattern are often related to the effects of fatigue on the EOD rate. Electric shock provokes the immobilization of the prey, or the flight of the aggressor. Effective use of the EOD by neonates as a weapon against prey is reported. The possible neuronal basis of the sensorimotor organization which may account for the results is discussed.