Visual Sufficiency in food detection and initiation of feeding behaviour in the Antarctic fish Trematomus newnesi Boulenger

Abstract

In order to improve the understanding of food detection mechanisms in the Antarctic nototheniid fish Trematomus newnesi BOULENGER, visual stimulation was tested. During the Antarctic summer of 1997/98,42 individuals, 21 belonging to each of two size classes, were obtained in Admiralty Bay (King George Island, South Shetlands). Fish tests were undertaken in aquaria maintained under controlled environmental conditions and a photoperiod of 22 hours light and 2 hours darkness. For the evaluation of visual stimulation, two species of amphipods, Gondogeneia antarctica and Waldeckia obesa, and the krill Euphausia superba, were offered as prey inside a transparent container in each aquarium. No contact was established between the water with prey and the water with predators, to avoid chemical stimulation. In each test 42.3% of the individuals reacted to the stimulation. Amphipods were detected from a distance of 18cm and krill from 22.5cm. The time lapse between food offer and the first reaction after visual stimulation was 17.7s and the stimulation lasted for 51.5% of the experimental time (30 min). During 30min of observation a mean of 20.9 attacks and 9.0 persecutions were performed against amphipods and 28.3 attacks and 12.6 persecutions against krill. The highest values were always obtained within the first 10 min of the test. The duration of the state of stimulation was significantly higher in the first half of the daylight period. A sequence of attitudes that result from positive visual stimulation was defined. The retina of T. newnesi is a complex matrix with neurones and four types of photoreceptors cells allowing accurate vision, which is an important tool for food detection, being sufficient to elicit feeding behaviour even in the absence of chemical and mechanical stimulation.