The mirror-based eyes of scallops demonstrate a light-evoked pupillary response.

Light levels in terrestrial and shallow-water environments can vary by ten orders of magnitude between clear days and overcast nights. Light-evoked pupillary responses help the eyes of animals perform optimally under these variable light conditions by balancing trade-offs between sensitivity and resolution [1]. Here, we document that the mirror-based eyes of the bay scallop Argopecten irradians and the sea scallop Placopecten magellanicus have pupils that constrict to ∼60% of their fully dilated areas within several minutes of light exposure. The eyes of scallops contain two separate retinas and our ray-tracing model indicates that, compared to eyes with fully constricted pupils, eyes from A. irradians with fully dilated pupils provide approximately three times the sensitivity and half the spatial resolution at the distal retina and five times the sensitivity and one third the spatial resolution at the proximal retina. We also identify radial and circular actin fibers associated with the corneas of A. irradians that may represent muscles whose contractions dilate and constrict the pupil, respectively.

Polarization vision seldom increases the sighting distance of silvery fish.

Although the function of polarization vision, the ability to discern the polarization characteristics of light, is well established in many terrestrial and benthic species, its purpose in pelagic species (squid and certain fish and crustaceans) is poorly understood [1]. A long-held hypothesis is that polarization vision in open water is used to break the mirror camouflage of silvery fish, as biological mirrors can change the polarization of reflected light [2,3]. Although, the addition of polarization information may increase the conspicuousness of silvery fish at close range, direct evidence that silvery fish - or indeed any pelagic animal - are visible at longer distances using polarization vision rather than using radiance (i.e. brightness) vision is lacking. Here we show, using in situ polarization imagery and a new visual detection model, that polarization vision does not in fact appear to allow viewers to see silvery fish at greater distances.

Visual acuity in pelagic fishes and mollusks.

In the sea, visual scenes change dramatically with depth. At shallow and moderate depths (<1,000 m), there is enough light for animals to see the surfaces and shapes of prey, predators, and conspecifics. This changes below 1,000 m, where no downwelling daylight remains and the only source of light is bioluminescence. These different visual scenes require different visual adaptations and eye morphologies. In this study we investigate how the optical characteristics of animal lenses correlate with depth and ecology. We measured the radius, focal length, and optical quality of the lenses of pelagic fishes, cephalopods, and a gastropod using a custom-built apparatus. The hatchetfishes (Argyropelecus aculeatus and Sternoptyx diaphana) and the barrel-eye (Opisthoproctus soleatus) were found to have the best lenses, which may allow them to break the counterillumination camouflage of their prey. The heteropod lens had unidirectional aberrations that matched its ribbon-shaped retina. We also found that lens angular resolution increased with depth. Due to a similar trend in the angular separation between adjacent ganglion cells in the retinas of fishes, the perceived visual contrast at the retinal cutoff frequency was constant with depth. The increase in acuity with depth allows the predators to focus all the available light bioluminescent prey animals emit and detect their next meal.

Nocturnal homing: learning walks in a wandering spider?

Homing by the nocturnal Namib Desert spider Leucorchestris arenicola (Araneae: Sparassidae) is comparable to homing in diurnal bees, wasps and ants in terms of path length and layout. The spiders' homing is based on vision but their basic navigational strategy is unclear. Diurnal homing insects use memorised views of their home in snapshot matching strategies. The insects learn the visual scenery identifying their nest location during learning flights (e.g. bees and wasps) or walks (ants). These learning flights and walks are stereotyped movement patterns clearly different from other movement behaviours. If the visual homing of L. arenicola is also based on an image matching strategy they are likely to exhibit learning walks similar to diurnal insects. To explore this possibility we recorded departures of spiders from a new burrow in an unfamiliar area with infrared cameras and analysed their paths using computer tracking techniques. We found that L. arenicola performs distinct stereotyped movement patterns during the first part of their departures in an unfamiliar area and that they seem to learn the appearance of their home during these movement patterns. We conclude that the spiders perform learning walks and this strongly suggests that L. arenicola uses a visual memory of the burrow location when homing.

Adaptation in the optical properties of the crystalline lens in the eyes of the Lessepsian migrant Siganus rivulatus.

Vision is an important source of information for many animals. The crystalline lens plays a central role in the visual pathway and hence the ecology of fishes. In this study, we tested whether the different light regimes in the Mediterranean and Red Seas have an effect on the optical properties of the lenses in the rivulated rabbitfish, Siganus rivulatus. This species has migrated through the Suez Canal from the Red Sea and established a vital population in the Mediterranean Sea. Longitudinal spherical aberration curves and focal lengths of the fish lenses were measured by laser scans and compared between the two populations. In addition, rivulated rabbitfish from the Mediterranean Sea were exposed to colored light (yellow, green and blue) and unfiltered light for periods of 1 or 13 days to test for short-term adjustments. Lens focal length was significantly longer (3%) in the Rea Sea population. The shorter focal length of the Mediterranean population can be explained as an adaptation to the dimmer light environment, as this difference makes the Mediterranean eyes 5% more sensitive than the eyes of the Red Sea population. The difference may be due to genetic differences or, more likely, adaptive developmental plasticity. Short-term regulatory mechanisms do not seem to be involved.

Adjusting a light dispersion model to fit measurements from vertebrate ocular media as well as ray-tracing in fish lenses.

Color dispersion, i.e., the dependency of refractive index of any transparent material on the wavelength of light, has important consequences for the function of optical instruments and animal eyes. Using a multi-objective goal attainment optimization algorithm, a dispersion model was successfully fitted to measured refractive indices of various ocular media and the longitudinal chromatic aberration determined by laser-scanning in the crystalline lens of the African cichlid fish, Astatotilapia burtoni. The model describes the effects of color dispersion in fish lenses and may be applicable to the eyes of other vertebrates as well.

Optical plasticity in the crystalline lenses of the cichlid fish Aequidens pulcher.

One of the reasons that the crystalline lenses of vertebrate eyes are highly transparent is that most of the cells have broken down all of their organelles, including the nuclei. These cells can neither synthesize new proteins nor generate energy by electron transport in the mitochondria. Only in the peripheral layers--in the cichlid fish Aequidens pulcher, beyond 92% of the lens radius--are there cells with full complements of organelles. We report here that the optical properties of the lens change between the light-adapted and dark-adapted states in A. pulcher. Changes occur even in cell layers free of organelles, and they occur in parallel with changes in retinal function between the light-adapted (all-cone, color vision) and dark-adapted (all-rod, grayscale vision) states. Depleting the eye of dopamine also caused changes in the optical properties similar to those of dark adaptation. Our results indicate that the refractive index of the organelle-free lens fiber cells can be adjusted quickly and accurately.

Effects of the peripheral layers on the optical properties of spherical fish lenses.

We created a computational optical model of spherical fish lenses that takes into account the effects of the peripheral layers, which differ in cellular composition from the bulk of the lens. A constant refractive index, except for the lens capsule, in the outer about 6% of lens radius made it possible to uniquely infer the refractive index gradient in more central layers from a known or desired longitudinal spherical aberration curve using the inverse Abel transform. Since the zone of constant refractive index is wider than necessary to make the solution unique and for optimal optical performance of the lens, we propose that its width be set by the metabolic needs of the lens.