Computational visual ecology in the pelagic realm

Visual performance and visual interactions in pelagic animals are notoriously hard to investigate because of our restricted access to the habitat. The pelagic visual world is also dramatically different from benthic or terrestrial habitats, and our intuition is less helpful in understanding vision in unfamiliar environments. Here, we develop a computational approach to investigate visual ecology in the pelagic realm. Using information on eye size, key retinal properties, optical properties of the water and radiance, we develop expressions for calculating the visual range for detection of important types of pelagic targets. We also briefly apply the computations to a number of central questions in pelagic visual ecology, such as the relationship between eye size and visual performance, the maximum depth at which daylight is useful for vision, visual range relations between prey and predators, counter-illumination and the importance of various aspects of retinal physiology. We also argue that our present addition to computational visual ecology can be developed further, and that a computational approach offers plenty of unused potential for investigations of visual ecology in both aquatic and terrestrial habitats.     

Screening pigment,aperture and sensitivity in the dung beetle superposition eye

Summary Sensitivity to light was investigated in the refracting superposition eye of the dung beetle Onitis alexis using electrophysiological measurements and optical modelling. Intracellular recordings were made from single retinula cells over 24-h periods, with cells light and dark adapted, in order to measure the response/intensity (V-LogI) functions. The combined effects of a circadian rhythm and light adaptation allow the determination of the relative contributions of screening-pigment migration and transduction gain to changes in sensitivity in the eye. Between the extremes of dark adaptation at night and light adaptation during the day, the maximum sensitivity change possible is at least 4 log units, of which approximately 2 log units can be accounted for by changes in the transduction gain and at least 2 log units by screening-pigment migration. The role of the superposition aperture (the number of facets that contribute light to one rhabdom) in 3 species of dung beetle was investigated with an optical ray-tracing model of the eye. The facets of the superposition aperture do not contribute light equally to the target rhabdom; except in one species, the greatest contribution comes from facets located away from both the centre and periphery of the aperture. Light adaptation increases the optical density of the superposition aperture and decreases its size.     

Anatomical and physiological evidence for polarisation vision in the nocturnal bee <Emphasis Type="Italic">Megalopta genalis</Emphasis>

The presence of a specialised dorsal rim area with an ability to detect the e-vector orientation of polarised light is shown for the first time in a nocturnal hymenopteran. The dorsal rim area of the halictid bee Megalopta genalis features a number of characteristic anatomical specialisations including an increased rhabdom diameter and a lack of primary screening pigments. Optically, these specialisations result in wide spatial receptive fields (Δρ = 14°), a common adaptation found in the dorsal rim areas of insects used to filter out interfering effects (i.e. clouds) from the sky. In this specialised eye region all nine photoreceptors contribute their microvilli to the entire length of the ommatidia. These orthogonally directed microvilli are anatomically arranged in an almost linear, anterior–posterior orientation. Intracellular recordings within the dorsal rim area show very high polarisation sensitivity and a sensitivity peak within the ultraviolet part of the spectrum.     

Visual cues used by ball-rolling dung beetles for orientation

Ball rolling by dung beetles is considered to be a derived behaviour that evolved under pressure for space, and from competitors at the dung pat. Straight-line orientation away from the pat using a celestial cue should be the most successful rolling strategy to move dung to an unknown burial site. We tested this hypothesis in the field and the laboratory by presenting five species of ball-rolling beetles with different orientation tasks, involving reaction to obstacles as well as to reflected sunlight and artificial light sources. Beetles were found to consistently orientate along a chosen route, usually in the direction of the sun. Beetles rolling dung balls successfully negotiated barriers and returned to the original path as did beetles falling from ramps, or rotated about a fixed point while rolling a ball. The sun was found to be the main orientation cue, which could be substituted by reflected or artificial light. However, beetles reoriented themselves less accurately in response to lights in the laboratory, than they did to the reflected sun in the field. It is probable that phototactic orientation using the sun, which is widespread amongst arthropods, has been incorporated in the straight-line foraging behaviour that has evolved in ball-rolling dung beetles.     

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⁻¹), 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.