Sensory brain areas in mesopelagic fishes

Four areas of the brain that receive primary projections from chemical sens es ([1] olfactory bulb, [2] facial and vagal lobes), the eye ([3] optic tec tum), and somato- and mechanosensory systems such as the lateral line, vest ibular and auditory systems ([4] trigeminal and octavolateral regions) have been studied and relative size differences used to deduce the sensory spec ializations of 67 species of mesopelagic fishes. One type of analysis used the average relative volumes of brain areas and identified 'specialists' wi th only one brain area above-average (36%), species 'dominated' by two sens ory brain regions (49%), and generalists (15%), with three areas above-aver age. In addition, a cluster analysis was performed that separated 49 specie s which were mostly visually oriented from 18 non-visual species, among whi ch 16 were characterized by an association of above-average trigeminal/octa volateral and gustatory areas, and a single species with a dominant olfacto ry bulb. The results support the idea that these species occupy a rich sens ory environment for which the absence of sunlight is compensated by chemica l and mechanosensory stimuli as well as by bioluminescent signals. This has lead to the development of specializations for the perception of single st imulus-modes, most notably for the visual system, as well as for combinatio ns of various receptors and central processing areas, with a preference for associating either the chemical senses, including the olfactory and facial /vagal systems, or the trigeminal/octavolateral systems. Copyright (C) 2001 S. Karger AG, Basel.