The effect of the alarm substance on the central nervous excitation of the black tetraGymnocorymbus ternetzi (Characidae, Ostariophysi, Pisces) indicated by dorsal light response

Abstract

The effect of the alarm substance on the central nervous excitation was measured quantitatively in the black tetra (Gymnocorymbus ternetzi) using the dorsal light response. The tetra swam freely in a tiny cage, illuminated unilaterally; its inclination towards the light was registrated by means of a videorecorder (Fig. 1). Later its angle of inclination was measured on the monitor in steps of 0.2 s with an exactness of ±1°. Various substances—one group which had previously elicited the fright reaction in experiments with schools of fish, whereas the other had not—were presented to the fish, and their effects upon equilibrium position were recorded. An enhanced optical alertness shown by an increase in the fishes' inclination was generally produced with skin-extract from the European minnow (Phoxinus phoxinus) (Figs. 2, 3a–v), whereas skin-extract from red swordtail (Xiphophorus helleri) was ineffective (Figs. 2, 4r–t). Without any additional stimulation the factorU, representing quantitatively the degree of the change of central state varied slightly within the experimental period of 1 min; however, this factor never exceededU=1.0±0.15. The increase ofU was greater with minnow skin-extract, and smaller with swordtail extract. The difference was highly significant. The three individuals tested responded uniformly. Curves from various individuals did not differ more than curves from one fish registrated at different times (Fig. 3a–v). The increase ofU usually exceeded considerably the value of 1.15 when 7–8 μg of pure substances, which in the experiments with schools had been active, were given (Figs. 3 w, x; 4a–n). However, when substances were presented that had been inactive in the experiments with schools,U generally did not exceed 1.15 (Fig. 4u–z). The differences between the highly effective substances (i.e. isoxanthopterin, 6-acetonylisoxanthopterin, and isolated pure alarm substance) and the ineffective substances (i.e. xanthopterin, 7-acetonylxanthopterin, and hypoxanthine) were highly significant (Fig. 5). A positive correlation generally existed between size and duration of the change of central state (Fig. 6). The results did not indicate the existence of any adaptation. This was also true in fishes that were stimulated repeatedly with one effective substance at intervals of a couple of minutes only.