New funded PhD project: Crustacean vision

THIS POSITION IS NO LONGER AVAILABLE: Crustacean vision: adaptable eyes for extreme changes in light – Martin J How

Photographers are familiar with the idea of controlling exposure by adjusting shutter speed and aperture, thus altering the amount of light that hits the camera chip. Most complex animal eyes have evolved to do something similar. For example, in humans the size of the iris and the sensitivity of the underlying light receptors and neurons can be quickly tuned, thereby allowing us to see in light levels varying by over 100 million times, from dark starlit nights to bright sunny days.

Crustaceans are no exception. Their compound eyes are composed of hundreds, or sometimes thousands, of repeated eye units (ommatidia) each viewing a different area in the world around. Each one of these ommatidia needs to adapt to local light conditions and does this using a system of moveable screening pigments within the light-sensitive cells. The intricacies of how crustaceans deal with rapidly changing light environments is not well understood. For example, many species (such as fiddler crabs) have eyes with very wide fields of view. This means that, while part of the eye could be experiencing very high light levels (e.g. in the direction of the sun), another part could be in the dark. How eyes simultaneously adapt different parts of their eyes to different light levels is relatively unknown. There are also suggestions that light adaptation in the eye of fiddler crabs could alter their sensitivity to the polarization of light.

This project will address key questions around how crustacean eyes deal with changing light levels. The student will take a multi-scale approach using a wide range of techniques, ranging from micro-anatomical studies using electron microscopy, through to behavioural experiments in the animal’s natural environment in Spain, Panama, and Australia.  

The student will join the Ecology of Vision Group (www.ecologyofvision.com) at the School of Biological Sciences, University of Bristol, under the supervision of Dr Martin How and Dr Nicholas Roberts. The group currently hosts one principal investigator, two research fellows, two postdocs, and four PhD students, all working on diverse aspects of animal visual ecology.

Eligibility criteria: At least an upper second-class honours degree (e.g. MSci) or equivalent. Applicants with a good BSc degree may be considered if they can demonstrate very good potential for research. A keen interest in animal sensory ecology is essential, as well as demonstrable lab and fieldwork skills.

Scholarship details: This studentship is fully funded by the Royal Society, covering full UK/EU PhD tuition fees, research costs, and an annual stipend of £14,057. This award is available to UK/EU applicants only, unless suitable top-up funding can be identified for international candidates. If English is not your first language, you must have IELTS 6.5, or equivalent. 

Start date: March-Sept 2016

Informal enquiries: Dr Martin J How (m.how@bristol.ac.uk)

Application Details: To apply for this studentship submit a PhD application using our online application system [http://www.bristol.ac.uk/study/postgraduate/apply

ANOTHER new paper from the lab!

Optics of cone photoreceptors in the chicken (Gallus gallus domesticus)

 

Wilby D., Toomey M.B., Olsson P., Frederiksen R., Cornwall M.C., Oulton R., Kelber A., Corbo J.C. & Roberts N. W. 

Journal of the Royal Society Interface 12 20150591. doi: 10.1098/rsif.2015.0591

Vision is the primary sensory modality of birds, and its importance is evident in the sophistication of their visual systems. Coloured oil droplets in the cone photoreceptors represent an adaptation in the avian retina, acting as long-pass colour filters. However, we currently lack understanding of how the optical properties and morphology of component structures (e.g. oil droplet, mitochondrial ellipsoid and outer segment) of the cone photoreceptor influence the transmission of light into the outer segment and the ultimate effect they have on receptor sensitivity. In this study, we use data from microspectrophotometry, digital holographic microscopy and electron microscopy to inform electromagnetic models of avian cone photoreceptors to quantitatively investigate the integrated optical function of the cell. We find that pigmented oil droplets primarily function as spectral filters, not light collection devices, although the mitochondrial ellipsoid improves optical coupling between the inner segment and oil droplet. In contrast, unpigmented droplets found in violet-sensitive cones double sensitivity at its peak relative to other cone types. Oil droplets and ellipsoids both narrow the angular sensitivity of single cone photoreceptors, but not as strongly as those in human cones.

 

NEW paper from the lab!

Polarization sensitivity as a visual contrast enhancer in the Emperor dragonfly larva, Anax imperator (Leach, 1815).

 

Sharkey, C. R., Partridge, J. C., Roberts N. W. 

Journal of Experimental Biology. Advanced online publication. doi: 10.1242/eb.122507

Polarization sensitivity (PS) is a common feature of invertebrate visual systems. In insects, PS is well known for its use in several different visually guided behaviours, particularly navigation and habitat search. Adult dragonflies use the polarization of light to find water but a role for PS in aquatic dragonfly larvae, a stage that inhabits a very different photic environment to the adults, has not been investigated. The optomotor response of the larvae of the Emperor dragonfly, Anax imperator, was used to determine whether these larvae use PS to enhance visual contrast underwater. Two different light scattering conditions were used to surround the larval animals: a naturalistic horizontally polarized light field and non naturalistic weakly polarized light field. In both cases these scattering light fields obscured moving intensity stimuli that provoke an optokinetic response in the larvae. Animals were shown to track the movement of a square-wave grating more closely when it was viewed through the horizontally polarized light field, equivalent to a similar increase in tracking ability observed in response to an 8% increase in the intensity contrast of the stimuli. Our results suggest that larval PS enhances the intensity contrast of a visual scene under partially polarized lighting conditions that occur naturally in freshwater environments.