This is a comparative study of eye movement behaviour of teleost fish from 5 families with diverse visual specialisations and oculomotor function. In chapter 3 I compared basic oculomotor parameters in three species of fish from the families Creediidae, Syngnathidae and Pinguipedidae, that show asynchronous eye movements and a fovea. All three species showed a close correlation between their specific retinal specialisation, oculomotor range and the lifestyles and feeding habits. Direction of gaze was correlated in the two independently moving eyes in both sandperch (Pinguipedidae) and pipefish (Syngnathidae) but not in the sandlance (Creediidae). Properties of spontaneous and fixational fast eye movements (saccades) in the species studied show many similarities to those found in other vertebrates. The apparent independence of the two eyes in the teleosts studied seem to set them apart from many other vertebrates, where eye movements are largely correlated with respect to each other. The results presented in chapter 4, however, reveal a regular switching of saccadic activity between the left and the right eye in sandlance, pipefish and sandperch, suggesting that the two eyes are in some way correlated. Since saccades are often a motor correlate of attention this finding suggests that these teleosts with asynchronous eye movements may show periodic shifts of attention while observing their environment. In chapter 5 the correlation between the two eyes was also tested during optokinetic nystagmus. This basic response shown by all animals stabilises the gaze against rotational head movements and translation. In most vertebrates the optokinetic response is tightly yoked in both eyes. This is also the case for the butterflyfish (Chaetodontidae) which shows strong yoking of the eyes during spontaneous eye movements. However some capacity for independent optokinesis in the two eyes was observed. Both sandlance and pipefish are capable of following two conflicting stimuli independently. However monocular occlusion in the pipefish unmasks a link between the two eyes, which is overridden when both eyes receive visual input. The sandlance never showed any correlation between eyes during optokinesis, even during monocular stimulation. This suggests that there are different levels of linkage between the two eyes in the oculomotor system of teleosts, depending on the visual input. One of the main functions of the oculomotor system in vertebrates and most invertebrates is to keep the image of the world relatively still on the retina. As shown in chapter 6 the sandlance breaks this universal rule of image stabilisation by showing large postsaccadic drifting eye movements as part of its normal oculomotor behaviour. In these animals, up to 40% of spontaneous saccades are followed by a drifting movement, either binocularly or in one eye only. The drifts are large and are always directed towards the most relaxed position of the eye, indicating that this form of eye movement is not visually driven. However the eye is visually responsive and saccades and an optokinetic response can be elicited during a drift. The drifting speed and the known acuity of the sandlance eye suggest that, during the drift, the image quality is not degraded. Several advantages of this unusual oculomotor behaviour can be related to the unusual optics and lifestyle of the sandlance. A unique modification of the eye muscles of billfish (Xiphiidae) maintains the eye and brain above ambient temperature; however the function of this adaptation and its effect on the oculomotor system is unknown. Chapter 7 aims to provide an insight into the visual abilities of billfish derived from anatomical observations of their retinal structure. The observations help explain the effect the increased retinal temperature might have for vision and eye movements. The blue marlin (Makaira nigricans) shows a well developed temporal area centralis and no visual streak, suggesting that a functional oculomotor system is required in this fish. A convergence of cones to ganglion cells at a ratio of at least 5:1 is present even in the area of highest acuity. The finding of two cone types suggest that the animal is capable of wavelength discrimination. Regional differences in size and composition of photoreceptors between dorsal and ventral retina potentially affect colour vision and sensitivity. The anatomical results suggest that sensitivity and spatial summation are of high priority to billfish. The possible function of the warm retina for increasing temporal resolution is discussed. These findings show the adaptability of the oculomotor system to suit the needs of different teleost lifestyles. However most of the parameters established for the oculomotor system of higher vertebrates also hold for teleosts.
| Identifer | oai:union.ndltd.org:ADTP/253732 |
| Creators | Fritsches, Kerstin Anna |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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