Colour and spatial vision in a reef fish, Rhinecanthus aculeatus

Champ, Connor Michael (2012). Colour and spatial vision in a reef fish, Rhinecanthus aculeatus PhD Thesis, Queensland Brain Institute, The University of Queensland.

       
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Author Champ, Connor Michael
Thesis Title Colour and spatial vision in a reef fish, Rhinecanthus aculeatus
Formatted title Colour and spatial vision in a reef fish, Rhinecanthus aculeatus
School, Centre or Institute Queensland Brain Institute
Institution The University of Queensland
Publication date 2012-02
Thesis type PhD Thesis
Supervisor Justin Marshall
Misha Vorobyev
Ulrike Siebeck
Guy Wallis
Total pages 247
Total colour pages 247
Language eng
Subjects 060604 Comparative Physiology
060809 Vertebrate Biology
060603 Animal Physiology - Systems
Formatted abstract To the human observer the reef environment is perhaps the most colourful of all natural environments and coral reef fishes present us with a stunning assemblage of colours and patterns. The diversity of colours and patterns in these fish has long fascinated researchers and several explanations have been put forward. Factors that may influence such colouration include display, metabolic by-product, phylogeny, external factors and camouflage. Given the diversity of lifestyles and habitats on the reefs it is likely that many of the colours and patterns we see are compromises dictated by a number of these elements, however, our inquiry into such factors is limited by a current lack of knowledge about the vision of these animals. As such, assumptions about reef fish colouration are often based on our own perception of colours and patterns.

This thesis has used a test species, the Rhinecanthus aculeatus to examine the perception of reef fishes. This species has been examined because it was found to be highly amenable to behavioural testing and because it displays both the diverse colouration and intricate patterning associated with reef fishes. To begin with the ability of the animal to perceive colours was examined. Using modelling and behavioural testing it was found that the animal was capable of using a particular type of photoreceptor known as a double cone for colour vision. This photoreceptor type is found in abundance in reef fish but it function had previously been unknown. The use of this photoreceptor meant that this fish and potentially many more reef fish possess better colour vision than previously thought. This is the first time double cone involvement in colour discrimination has been shown directly in any animal.

Having examined the colour vision system of the animal and finding the receptors responsible for colour vision the thesis then examined the thresholds of this colour vision system i.e. how different colours have to be for the fish to discriminate them. To do this the animal was behaviourally tested once more with colours that were designed to test these thresholds. The retina of the animal was then mapped and the photoreceptors in the retina measured in detail using a variety of techniques that included confocal microscopy, wholemounting of tissues and intracellular dying. Results obtained indicated that the behavioural results could be predicted well and that the anatomy of the photoreceptors set a limit on the behavioural thresholds of the animal. This indicated that this fish species, and perhaps other fish, could have their behavioural thresholds predicted if the anatomy of the photoreceptors is known. This is also the first time that behavioural thresholds had been obtained in a reef fish. Using these results we can now reasonably predict what colours this species can or cannot see.

The ability of the fish to perceive patterns (visual acuity) was then examined. Fish were behaviourally tested with a number of paradigms to ascertain the visual acuity of the species. This was then compared to predictions made by examining the spacing and numbers of cells in the retina of the animal. It was found that the behaviour and the anatomical estimates of acuity did not match well. Behavioural acuity was found to be approximately 1.75 cycles per degree while estimates from the photoreceptors and ganglion cells in the retina were 7.75 cycles per degree and 3.4 cycles per degree respectively. This indicated that the anatomy of the animal does not predict the visual acuity of this species well and that the triggerfish had acuity levels similar to that of a goldfish and far below that of humans (30-60 cycles per degree). This also meant that acuity of a reef species with intricate patterning is not necessarily 'high'.

The final part of the thesis has integrated the colour and acuity results and examined them in relation to the colour and patterns of the Rhinecanthus aculeatus itself. It can be seen that the colour displayed by the fish are readily perceivable to itself. Additionally it can be seen that the colours displayed on the animal are perceivable against the backgrounds on the reef it is commonly viewed upon. This was established by taking light measurements of the animal's environment via underwater spectrophotometry and scuba. Interestingly however it was found that the patterns on the animal become indistinguishable to other members of the species at relatively short distances (1-2m). Intricate patterning may therefore be a very short range communication device and the blurring of colours and patterns at distance may act as camouflage. This fits in well with theories of reef fish colouration put forward by Marshall and Vorobyev previously (Marshall 2000, Marshall and Vorobyev 2003).
Keyword Triggerfish
Vision
Acuity
Reefs
Photoreceptors
Double cones
Visual system
Teleost fishes
Behaviour
Colour vision

 
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Created: Mon, 24 Sep 2012, 18:43:08 EST by Mr Connor Champ on behalf of Scholarly Publishing and Digitisation Service