The topographic analysis of retinal ganglion and photoreceptor cell distributions yields valuable information for assessing the visual capabilities and behavioral ecology of vertebrates. This study examines whole-mounted retinas of four elasmobranch species, the ornate wobbegong, Orectolobus ornatus; the whitetip reef shark, Triaenodon obesus; the epaulette shark, Hemiscyllium ocellatum; and the east Australia shovelnose ray, Aptychotrema rostrata, for regional specializations mediating zones of improved visual ability. These species represent a range of lifestyles: benthic, mid-water, diurnal, and nocturnal. Both photoreceptors (visualized using differential interference contrast optics) and ganglion cells (stained with cresyl violet) in the retina are extensively sampled, and their spatial distribution is found to be nonuniform, exhibiting areae or In general, the topographic distributions of both cell populations are in register and match well with respect to the location of regions of high density. However, the location of peaks in rod and cone densities can vary within a retina, indicating that preferential sampling of different regions of the visual field may occur in photopic and scotopic vision. Anatomical measures of the optical limits of resolving power, indicated by intercone spacing, range from 3.8 to 13.1 cycles/deg. Spatial limits of resolving power, calculated from ganglion cell spacing, range from 2.6 to 4.3 cycles/deg. Summation ratios, assessed by direct comparison of cell densities of photoreceptors (input cells) and ganglion cells (output cells), at more than 150 different loci across the retina, show topographic differences in signal convergence (ranging from 25:1 to over 70:1). Species-specific retinal specializations strongly correlate to the habitat and feeding behavior of each species.

SUMMARYCoral reef fish live in a complex world of colour and patterns. If they are to survive they need to be able to correctly identify the things they see (e.g. predators, prey) and act accordingly (e.g. flee, feed). This paper investigates whether discrimination is limited to ecologically relevant stimuli or is in fact more adaptable. Our work focuses on the reef damselfish Pomacentrus amboinensis. Within a day or two of capture the fish demonstrated an ability to associate an arbitrary stimulus with a food reward and to discriminate the reward stimulus from a distractor matched along various physical dimensions. In our initial experiments the reward was directly associated with the target. In the final experiment, however, the reward was separated from the target in both space and time, thereby eliminating a weakness applicable to the majority of food reward experiments involving fish; namely, the presence of olfactory cues emanating from the feeding tubes. All fish were not only able to solve this task but also showed anticipatory behaviour (also referred to as goal tracking). We conclude that freshly caught reef fish not only are able to quickly learn and discriminate between novel stimuli on the basis of shape but are also able to interpret stimuli as a predictor for the availability of food at a different time and place (anticipatory behaviour).

SUMMARY Over many millions of years, sea creatures have developed a range of light reflectance properties. One example is the large variation in the patterns and colours of fish inhabiting the world's coral reefs. Attempts to understand the significance of the colouration have been made, but all too often from the perspective of a human observer. A more ecological approach requires us to consider the visual system of those for whom the colours were intended, namely other sea life. A first step is to understand the sensitivity of reef fish themselves to colour. Physiological data has revealed wavelength-tuned photoreceptors in reef fish, and this study provides behavioural evidence for their application in colour discrimination. Using classical conditioning,freshly caught damselfish were trained to discriminate coloured patterns for a food reward. Within 3–4 days of capture the fish selected a target colour on over 75% of trials. Brightness of the distracter and target were systematically varied to confirm that the fish could discriminate stimuli on the basis of chromaticity alone. The study demonstrates that reef fish can learn to perform two-alternative discrimination tasks, and provides the first behavioural evidence that reef fish have colour vision.

SUMMARYElasmobranch fishes utilise their vision as an important source of sensory information, and a range of visual adaptations have been shown to reflect the ecological diversity of this vertebrate group. This study investigates the hypotheses that visual optics can predict differences in habitat and behaviour and that visual optics change with ontogenetic growth of the eye to maintain optical performance. The study examines eye structure, pupillary movement, transmission properties of the ocular media, focal properties of the lens, tapetum structure and variations in optical performance with ontogenetic growth in two elasmobranch species: the carcharhinid sandbar shark, Carcharhinus plumbeus, inhabiting nearshore coastal waters, and the squalid shortspine spurdog, Squalus mitsukurii, inhabiting deeper waters of the continental shelf and slope. The optical properties appear to be well tuned for the visual needs of each species. Eyes continue to grow throughout life, resulting in an ontogenetic shift in the focal ratio of the eye. The eyes of C. plumbeus are optimised for vision under variable light conditions, which change during development as the animal probes new light environments in its search for food and mates. By contrast, the eyes of S. mitsukurii are specifically adapted to enhance retinal illumination within a dim light environment, and the detection of bioluminescent prey may be optimised with the use of lenticular short-wavelength-absorbing filters. Our findings suggest that the light environment strongly influences optical features in this class of vertebrates and that optical properties of the eye may be useful predictors of habitat and behaviour for lesser-known species of this vertebrate group.Key words: shark, eye growth, ocular media, focal ratio, tapetum, visual ecology. THE JOURNAL OF EXPERIMENTAL BIOLOGY 3584ocular parameters adapt to different light environments and influence the optical image reaching the retina. We reveal that the visual system of C. plumbeus has adapted to facilitate vision in the fluctuating light environment of coastal neritic waters, while S. mitsukurii has adapted to maximise light capture in the dim, semiextended light environment of the mesopelagic zone. MATERIALS AND METHODS Source and maintenance of animalsA population of S. mitsukurii was sampled (N26) from the insular shelf of the Hawaiian Island chain in the Central Pacific. Three populations of C. plumbeus were sampled to represent the extremes in visual environment encountered by this species; a tropical population inhabiting the insular shelf of the Hawaiian Island chain (C. plumbeus H1 ; transparent oceanic waters, depth >70m, N88, , a temperate population inhabiting the continental shelf in the north-western Atlantic Ocean (C. plumbeus H2 ; turbid estuarine waters, depth <10m, N40, TL55-110cm) and a subtropical population inhabiting the continental shelf of eastern Australia in the Western Pacific (C. plumbeus H3 ; green coastal waters, depth >50m, N57, TL50-187cm) (Table1). Tissue samples ...

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