The most abundant life on this planet is found deep beneath the waves at depths that sunlight hardly penetrates. Species that live at these depths are impossible to capture for behavioural studies where questions can then be asked about how fish at these depths 'see' the world. Thus, deep sea scientists are restricted to undertaking morphological analysis on these organisms and interpreting the results. One of the long held beliefs is that vision becomes less important for fish the deeper you go, and this is obvious when you look at eye size. Generally, after a certain point, when sunlight can no longer penetrate, the size of fish eyes becomes increasingly smaller and the non-visual senses become elaborate and highly specialised. In this post I discuss two papers that investigate what senses are utilised by mesopelagic and abyssal demersal fish species. For those of you who do not know mesopelagic fish are those that swim in the water column at depths of 500 and 1500m while abyssal demersal fish are those found near the bottom of the sea floor at depths of 2000-6000m.
What the author did was capture fish during deep water trawls in the Atlantic and Pacific oceans, and investigate their brain morphology. He described the brains with special reference to the differentiation of the sensory centres - olfactory bulb (smell), optic tectum (vision), octavolateral region (water motion via lateral line), and gustatory lobes (taste). For those who do not know what the lateral line is it is a hair cell based sensory system that detects local water movements surrounding the fish. Such as the wake of a passing prey. By comparing the size of each brain region for a specific species, with the overall average across all species, the author was able to determine whether the fish was a specialist for that particular sensory system.
Some species were 'specialised' in one particular sensory system (mesopelagic fishes 36%; abyssal demersal fishes 40%). Other species 'dominated' in two sensory systems (mesopelagic fishes 49%; abyssal demersal fishes 46%), while the remaining species were generalists and specialised in three senses (mesopelagic fishes 15%; abyssal demersal fishes 14%). No fish therefore were not specialised in any sense.
For mesopelagic fishes that were specialists 92% were masters of vision. This pattern, although not as strong, continues to hold true when you take into account 'dominated' and 'generalist' species. Sixty one percent of fish had above average volumes of the optic tectum. This would suggest that vision is the most important sense in the mesopelagic environment. This pattern is not as strong for abyssal demersal fish with fish specialising in vision in only 50% of cases both for specialists and when dominated and generalist species are accounted for. Vision therefore seems to play a lesser role in the deeper abyss.
The use of the lateral line shows the reverse trend becoming more important the deeper you go. Mesopelagic fish that specialised in the lateral line (25%) were less than those found in the abyss (49%). However, in both environments the lateral line was the second most important sensory system. This pattern of increasing importance of non-visual senses in deeper waters continues to hold for the other senses also. When shifting from the mesopelagic to the abyss, species with above average gustation areas (taste) increased from 10% to 34% , and from 3% to 37% for olfaction.
This pattern shows that the sensory environment of the abyss is markedly different than the open waters of the deep sea. Vision was clearly dominant in the mesopelagic waters, and this may be due to the abundant sources of bioluminescence found in this environment. This is quite evident when comparing the species between the two depth categories. Bioluminescent species were common in the mesopelagic, but not a single bioluminescent species was found in the abyss. With a lack of any visual cues for feeding or mating it is not surprising that non-visual senses begin to become more dominant at greater depth.
Wagner, H. (2001). Sensory Brain Areas in Mesopelagic Fishes Brain, Behavior and Evolution, 57 (3), 117-133 DOI: 10.1159/000047231
Wagner, H. (2001). Brain Areas in Abyssal Demersal Fishes Brain, Behavior and Evolution, 57 (6), 301-316 DOI: 10.1159/000047249