Visual eyes: a quantitative analysis of the photoreceptor layer in deep-sea sharks.

The marine environment presents unique visual challenges for a range of organisms, particularly those dwelling at great depths, where sunlight may either be absent or drop to very low levels. Under these environmental conditions, the visual system must maximise light absorption in order to enhance the detection of prey, predators and potential mates. Using stereological analysis of retinal wholemounts, the distribution and number of photoreceptors (rods) was determined for 5 deep-sea shark species from a range of depths (46-1,500 m). All species possessed areas of increased photoreceptor density (with peaks between 41,000 and 82,000 rods/mm(2)) within discrete regions of the retina. The total number of rods in the photoreceptor layer also varied between 17 × 10(6) and 63 × 10(6). It is evident that increasing sensitivity of the retina is an important adaptation to life in the deep sea. The location of discrete areas of high cell density within the photoreceptor layer of the retina corresponds to discrete areas of the visual field that are sampled at a higher intensity, hence increasing sensitivity. The location of these areas of increased sensitivity differed between the species of this study. The disparity of areas of increased sensitivity seen between species is thought to reflect distinctive predator avoidance and prey capture strategies. This study reveals that the visual demands of deep-sea sharks vary interspecifically and that sampling of each species' visual field is not solely determined by its habitat.

Visual pigment in the lens eyes of the box jellyfish Chiropsella bronzie.

Box jellyfish (Cubomedusae) possess a unique visual system comprising 24 eyes of four morphological types. Moreover, box jellyfish display several visually guided behaviours, including obstacle avoidance and light-shaft attractance. It is largely unknown what kind of visual information box jellyfish use for carrying out these behaviours. Brightness contrast is almost certainly involved, but it is also possible that box jellyfish extract colour information from their surroundings. The possible presence of colour vision in box jellyfish has previously been investigated using behavioural, electrophysiological and immunohistochemical methods. However, the results from these studies are to some degree conflicting and inconclusive. Here, we present results from an investigation into the visual system of the box jellyfish Chiropsella bronzie, using microspectrophotometry and immunohistochemistry. Our results strongly indicate that only one type of visual pigment is present in the upper and lower lens eyes with a peak absorbance of approximately 510 nm. Additionally, the visual pigment appears to undergo bleaching, similar to that of vertebrate visual pigments.

The retinal topography of three species of coleoid cephalopod: significance for perception of polarized light.

The retinal topography of three species of coleoid cephalopod (one cuttlefish, one squid and one octopus) was investigated to examine and compare the structure, density and organization of the photoreceptors. The aim was to determine if there were areas of increased cell density and/or cell specialization that might be related to lifestyle or phylogeny. The orientation of photoreceptors around the curved surface of the retina was also mapped to reveal how the overall arrangement of cell microvilli might enable the perception of polarized light stimuli. It was found that all species possessed an increase in photoreceptor density in a horizontal streak approximately placed at the position of a potential horizon in the habitat. The overall arrangement of photoreceptor microvillar arrangements followed lines of latitude and longitude in a global projection that has been rotated by 90°. This arrangement seems to map polarization sensitivities on the outside world in a vertical and horizontal grid. The potential significance of this and other retinal specializations is discussed in the context of phylogenetic and habitat differences between species.

Spectral sensitivity in a sponge larva.

Cilia at the posterior pole of demosponge larvae are known to cause directional swimming, sometimes in response to light gradients, but so far neither the spectral sensitivity of, nor the molecular basis for, this response has been investigated. We exploited the fact that the larval cilia respond to sudden changes in light intensity, a shadow response, in order to determine the action spectrum of photosensitivity. Our results show that larvae of the haplosclerid sponge Reniera sp. respond most to blue light (440 nm), and have a smaller, secondary response peak to orange-red light (600 nm). These data suggest that the photoreceptive pigment in sponge larvae may be a flavin or carotenoid.