Dragonfly larval polarization sensitivity as a contrast enhancer in turbid water
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1
The University of Bristol, Ecology of Vision Group, United Kingdom
A challenge faced by visual systems underwater is the presence of scattered light haze, or ‘veiling light’, that reduces the contrast of the scene. Light scattered underwater by sub-wavelength particles is polarized, forming a light field that is maximally polarized in a band orthogonal to the direction of the sun (Waterman, 2006). An animal able to filter out this scattered, polarized light would benefit from a greater perceived contrast, enhancing the detection of objects in an underwater scene (Rowe et al., 1995; Schechner et al., 2003). If this mechanism were present, it could potentially benefit an aquatic predator by enhancing prey detection.
We compared the responses of three larval instars of dragonfly, Anax imperator, to moving gratings with a range of spatial frequencies, seen by the insects through turbid water under different illumination conditions. Animals were placed in an optomotor drum, with a surrounding circular tank filled with diluted milk; the grating to be tested being placed on the outside of the drum. The milky solution was illuminated from above through linear Polaroid filters with transmission axes aligned either radially or tangentially with respect to the drum, thereby creating either a weakly (10%) vertically polarized or a more strongly (30%) horizontally polarized light field, respectively. The head angles of animals, during the experiment, were measured and ‘gain’, the rotational velocity (degrees per second) of the animal’s head was expressed as a fraction of that of the drum, used as a measure of their response. Data were log transformed and analysed by fitting Linear Mixed Models.
For all three larval instars, responses to the moving gratings were significantly stronger overall when the light field was polarized horizontally (Δdeviance=5.945, d.f.=1, p=0.015; Fig.1, bars represent SE) particularly at 0.09 cycles/degree. Animals responded differently to different spatial frequencies, exhibiting reduced responses at the highest and lowest frequencies tested. Responses to different spatial frequencies were dependent on instar, with response peaking at a lower frequency for earlier instars (Δdeviance=13.56, d.f.=6, p=0.035).
The increase in response, observed when animals are in a horizontally polarized light field, may be due to a contrast-enhancing effect of polarization sensitivity, a suggestion supported by preliminary data, derived from on-going behavioural experiments. This contrast enhancing effect could increase the chance of a successful strike during hunting and increase the distance at which both predators and prey can be detected.
Acknowledgements
The authors acknowledge funding from the Leverhulme Trust.
References
Rowe, M. P., Pugh, E. N., Tyo, J. S., & Engheta, N. (1995). Polarization-difference imaging: a biologically inspired technique for observation through scattering media. Optics letters, 20(6), 608–10.
Schechner, Y. Y., Narasimhan, S. G., and Nayar, S. K. (2003). Polarization-based vision through haze. Applied optics 42, 511–25.
Waterman, T. H. (2006). Reviving a neglected celestial underwater polarization compass for aquatic animals. Biological reviews of the Cambridge Philosophical Society 81, 111–5.
Keywords:
Polarization sensitivity,
Dragonfly nymph,
Contrast Sensitivity,
turbid media,
Behavior, Animal,
optomotor response
Conference:
International Conference on Invertebrate Vision, Fjälkinge, Sweden, 1 Aug - 8 Aug, 2013.
Presentation Type:
Oral presentation preferred
Topic:
Colour and polarisation vision
Citation:
Sharkey
CR,
Roberts
NW and
Partridge
JC
(2019). Dragonfly larval polarization sensitivity as a contrast enhancer in turbid water.
Front. Physiol.
Conference Abstract:
International Conference on Invertebrate Vision.
doi: 10.3389/conf.fphys.2013.25.00078
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Received:
27 Feb 2013;
Published Online:
09 Dec 2019.
*
Correspondence:
Miss. Camilla R Sharkey, The University of Bristol, Ecology of Vision Group, Bristol, BS8 1UG, United Kingdom, camilla.r.sharkey@gmail.com