Deep-sea barreleye fish...

Read about the very unusual visual system of the deep-sea barreleye fish in this paper coauthored by PhD student Tom Jordan.

Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis)

Partridge, JC, Douglas, RH, Marshall, NJ, Chung, WS, Jordan, TM, and Wagner, HJ

Abstract: We describe the bi-directed eyes of a mesopelagic teleost fish, Rhynchohyalus natalensis, that possesses an extensive lateral diverticulum to each tubular eye. Each diverticulum contains a mirror that focuses light from the ventro-lateral visual field. This species can thereby visualize both downwelling sunlight and bioluminescence over a wide field of view. Modelling shows that the mirror is very likely to be capable of producing a bright, well focused image. After Dolichopteryx longipes, this is only the second description of an eye in a vertebrate having both reflective and refractive optics. Although superficially similar, the optics of the diverticular eyes of these two species of fish differ in some important respects. Firstly, the reflective crystals in the D. longipes mirror are derived from a tapetum within the retinal pigment epithelium, whereas in R. natalensis they develop from the choroidal argentea. Secondly, in D. longipes the angle of the reflective crystals varies depending on their position within the mirror, forming a Fresnel-type reflector, but in R. natalensis the crystals are orientated almost parallel to the mirror's surface and image formation is dependent on the gross morphology of the diverticular mirror. Two remarkably different developmental solutions have thus evolved in these two closely related species of opisthoproctid teleosts to extend the restricted visual field of a tubular eye and provide a well-focused image with reflective optics.

Sensory ecology course in Lund, Sweden...

This September two PhD students from the Ecology of Vision group, Ilse Daly and David Wilby, will be heading out to Sweden for the foremost workshop in the field of Visual Ecology. Organised by the prestigious Vision Group at Lund University, the Sensory Ecology course is renowned as an international forum for early career researchers in sensory biology to meet, present posters showcase their work and attend lectures from 28 internationally leading scientists over two weeks.

In addition to lectures on other sensory modalities, speakers on Visual Ecology include Lund's Dan-Eric Nilsson, Eric Warrant, Almut Kelber and Marie Dacke as well as Sönke Johnsen from Duke University. Ilse and David will also present posters on their PhD research.
To support this trip, David has been awarded a travel grant from the University of Bristol Alumni Foundation.

For more on the Lund Sensory Ecology course see here. (http://www4.lu.se/vision-group/courses/sensory-ecology)

Sea slug conspicuousness...

A new paper with Martin How as a coauthor: Conspicuous visual signals do not coevolve with increased body size in marine sea slugs

KL Cheney, F Cortesi, MJ How, NG Wilson, SP Blomberg, AE Winters, S Umanzör and NJ Marshall

Abstract: Many taxa use conspicuous colouration to attract mates, signal chemical defences (aposematism) or for thermoregulation. Conspicuousness is a key feature of aposematic signals, and experimental evidence suggests that predators avoid conspicuous prey more readily when they exhibit larger body size and/or pattern elements. Aposematic prey species may therefore evolve a larger body size due to predatory selection pressures, or alternatively, larger prey species may be more likely to evolve aposematic colouration. Therefore, a positive correlation between conspicuousness and body size should exist. Here, we investigated whether there was a phylogenetic correlation between the conspicuousness of animal patterns and body size using an intriguing, understudied model system to examine questions on the evolution of animal signals, namely nudibranchs (opisthobranch molluscs). We also used new ways to compare animal patterns quantitatively with their background habitat in terms of intensity variance and spatial frequency power spectra. In studies of aposematism, conspicuousness is usually quantified using the spectral contrast of animal colour patches against its background; however, other components of visual signals, such as pattern, luminance and spectral sensitivities of potential observers, are largely ignored. Contrary to our prediction, we found that the conspicuousness of body patterns in over 70 nudibranch species decreased as body size increased, indicating that crypsis was not limited to a smaller body size. Therefore, alternative selective pressures on body size and development of colour patterns, other than those inflicted by visual hunting predators, may act more strongly on the evolution of aposematism in nudibranch molluscs.

A new way of seeing colour

Martin How has a new paper in Science this week - A Different Form of Color Vision in Mantis Shrimp

Abstract - One of the most complex eyes in the animal kingdom can be found in species of stomatopod crustaceans (mantis shrimp), some of which have 12 different photoreceptor types, each sampling a narrow set of wavelengths ranging from deep ultraviolet to far red (300 to 720 nanometers) (1–3). Functionally, this chromatic complexity has presented a mystery (3–5). Why use 12 color channels when three or four are sufficient for fine color discrimination? Behavioral wavelength discrimination tests (Δλ functions) in stomatopods revealed a surprisingly poor performance, ruling out color vision that makes use of the conventional color-opponent coding system (6–8). Instead, our experiments suggest that stomatopods use a previously unknown color vision system based on temporal signaling combined with scanning eye movements, enabling a type of color recognition rather than discrimination.

A nice write up on the paper by Ed Yong on NotExactlyRocketScience