The role of the visual information in the fright reaction of Dawkinsia filamentosa (Valenciennes, 1844) (Teleostei: Cypriniformes: Cyprinidae)

The Blackspot Barb, Dawkinsia filamentosa (Valenciennes, 1844), is a shoaling fish of India. As in many ostariophysians, alarm cues released after skin injuring elicit striking anti-predator behaviour in conspecifics. The alarm substance necessary for tests was obtained from a single donor specimen (female). Each fish was tested individually (in visible light or with IR illuminators) in an aquarium with size: 105×8×20cm; water height: 8cm. The tank was subdivided in 9 parts and the stimulation (water or alarm substance) was released just under the water surface at the higher posterior right edge: the elongated shape of aquarium allowed evidencing a possible avoidance response. Fish were tested in light or dark conditions both in absence or presence of a chemical alarm cue. In both test conditions, fish behavior was monitored with a camera, which was sensitive to both infrared and visible light. In each of two experimental conditions, 12 tests were conducted. In each test, 3 consecutive registrations were made: 15 minutes in absence of stimulus, 15 minutes after administration of 50 ml of pure water, and 15 minutes after administration of a solution of 48 ml of water and 2 ml of alarm substance. Each sector of tank was correlated with a number in crescent order from 1, where administration was being carried out, to 9, in the opposite side. In each 15 minutes part of a test, fish position and associated number were relived every 30 seconds and the medium score of each fish obtained in absence of stimulus, in presence of water, and in presence of alarm substance were calculated. The medium scores obtained in presence of water and in presence of alarm substance were subtracted from the medium score obtained in absence of stimulus, obtaining a Changing Spatial Use Index. Indexes obtained for each fish (in presence of alarm substance and in presence of water) were compared with Wilcoxon test. As in others cyprinids, Blackspot Barb fright reaction differs in light or darkness. In particular, its alarm reaction in darkness seems to consist simply in moving to the bottom without any swimming rapid movements and without any avoidance component of the alarm substance.