Comparison of functional and anatomical estimations of visual acuity in two species of coral reef fish

The high-contrast, complex patterns typical of many reef fish serve several purposes, including providing disruptive camouflage and a basis for vision-based communication. In trying to understand the role of a specific pattern, it is important to first assess the extent to which an observer can resolve the pattern, itself determined, at least in part, by the observer's visual acuity. Here, we studied the visual acuity of two species of reef fish -and- using both anatomical and behavioural estimates. The two species share a common habitat but are members of different trophic levels (predator versus herbivore/omnivore) and perform different visual tasks. On the basis of the anatomical study, we estimated visual acuity to lie between 4.1 and 4.6\ua0cycles\ua0degforand 3.2 and 3.6 cycles degforBehavioural acuity estimates were considerably lower, ranging between 1.29 and 1.36 cycles degforand 1.61 and 1.71 cycles degforOur results show that two species from the same habitat have only moderately divergent visual capabilities, despite differences in their general life histories. The difference between anatomical and behavioural estimates is an important finding as the majority of our current knowledge on the resolution capabilities of reef fish comes from anatomical measurements. Our findings suggest that anatomical estimates may represent the highest potential acuity of fish but are not indicative of actual performance, and that there is unlikely to be a simple scaling factor to link the two measures across all fish species

Eye Movement Strategies and Vision in Teleost Fish

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

Australian Loggerhead sea turtle hatchlings do not avoid yellow

When emerging from the nest, sea turtle hatchlings primarily orient using visual stimuli, with light pollution known to disrupt effective sea localization behavior. Previous studies have shown that sea turtle hatchlings respond differently to different wavelengths of light but Loggerhead hatchlings, exclusively among species tested, have a strong aversion to yellow light (at 600 nm). This study repeats these experiments with an Australian population of Loggerhead hatchlings (Caretta caretta) and Flatback hatchlings (Natator depressus). The orientation preference was measured using a modified y-maze set-up with the animals response observed using an infrared camera. This study showed that both Loggerhead and Flatback hatchlings can see and are attracted to light in the ultraviolet waveband (365 nm) and, to a lesser extent to longer wavelengths of 600 nm and above. The surprising finding was that the Loggerhead hatchlings tested here, unlike their conspecifics in Florida, do not show any avoidance to yellow but are attracted to bright lights of wavelength between 365 nm (UV) and 600 nm. This suggests potential differences in the visual behavior among different populations of sea turtles of the same species. No difference was detected in the response of Loggerhead hatchlings to flickering or steady light stimuli

Sound, chemical, and light detection in sea turtles and pelagic fishes: sensory-based approaches to bycatch redution in longline fisheries

Mortality due to capture in longline fisheries has been implicated as a significant factor contributing to population declines for several species of threatened or endangered sea turtles. Identification of methods to reduce or prevent sea turtle bycatch is a high priority for fisheries managers and a necessary component of conservation efforts. One approach to reducing sea turtle interactions with longline fisheries is to take into account the behavior of sea turtles and the factors that lead them to interact with fishing gear. An understanding of the sensory cues that attract sea turtles to longline gear could help guide efforts to develop gear and bait that is less attractive, non-detectable, or even repellent to sea turtles. This paper presents a review of morphological, physiological, and behavioral studies conducted to assess the auditory, chemosensory, and visual capabilities of sea turtles, as well as the large pelagic fishes that are targeted by longline fisheries. We discuss the potential for exploiting differences in the sensory biology of these evolutionarily distinct groups to refine longline fishing techniques and reduce incidental bycatch of sea turtles without impacting the catch rates of targeted fish species. Based on the current evidence, differences in visual capabilities of sea turtles and pelagic fishes provide a promising avenue for development of a sensory-based deterrent

Lens optical properties in the eyes of large marine predatory teleosts

The optical properties of the crystalline lenses were studied in a variety of large predatory teleosts (bony fishes) that forage in the open ocean, some of them at considerable depths. We found the first fish lenses that are free of measurable longitudinal spherical aberration, i.e., are perfectly monofocal, in contrast to the multifocal lenses that are typical for smaller fishes living close to the surface. In fact, none of the lenses investigated in this study were clearly multifocal. Most, but not all, of the lenses had long normalized focal lengths (focal length/lens radius) of up to 3.3 lens radii. A monofocal lens of long focal length, combined with spectrally suitably placed cone pigments, may be the optimal solution for vision of high spatial and spectral resolutions in a habitat where the available spectrum of light is limited

The visual ecology of a deep-sea fish, the escolar Lepidocybium flavobrunneum (Smith, 1843)

Escolar (Lepidocybium flavobrunneum, family Gempylidae) are large and darkly coloured deep-sea predatory fish found in the cold depths (more than 200 m) during the day and in warm surface waters at night. They have large eyes and an overall low density of retinal ganglion cells that endow them with a very high optical sensitivity. Escolar have banked retinae comprising six to eight layers of rods to increase the optical path length for maximal absorption of the incoming light. Their retinae possess two main areae of higher ganglion cell density, one in the ventral retina viewing the dorsal world above (with a moderate acuity of 4.6 cycles deg), and the second in the temporal retina viewing the frontal world ahead. Electrophysiological recordings of the flicker fusion frequency (FFF) in isolated retinas indicate that escolar have slow vision, with maximal FFF at the highest light levels and temperatures (around 9 Hz at 23°C) which fall to 1-2 Hz in dim light or cooler temperatures. Our results suggest that escolar are slowly moving sit-and-wait predators. In dim, warm surface waters at night, their slow vision, moderate dorsal resolution and highly sensitive eyes may allow them to surprise prey from below that are silhouetted in the downwelling light