Null point of discrimination in crustacean polarisation vision

J Exp Biol Advance Online Articles. First posted online on 15 April 2014 as doi:10.1242/jeb.103457 Access the most recent version at http://jeb.biologists.org/lookup/doi/10.1242/jeb.103457

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Null point of discrimination in crustacean polarisation vision

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Martin J. How1,2,3, John Christy3, Nicholas W. Roberts1 and N. Justin Marshall2

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1 – School of Biological Sciences, Woodland Road, University of Bristol, Bristol, BS8 1UG, UK

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2 - Queensland Brain Institute, University of Queensland, St Lucia, QLD 4072, Australia

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3 - Smithsonian Tropical Research Institute, Panama City, Republic of Panama

The Journal of Experimental Biology – ACCEPTED AUTHOR MANUSCRIPT

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Abstract

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The polarisation of light is used by many species of cephalopods and crustaceans to discriminate

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objects or to communicate. Most visual systems with this ability, such as that of the fiddler crab,

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include receptors with photopigments that are oriented horizontally and vertically relative to the

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outside world. Photoreceptors in such an orthogonal array are maximally sensitive to polarised light

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with the same fixed e-vector orientation. Using opponent neural connections, this two-channel

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system may produce a single value of polarisation contrast and, consequently, it may suffer from

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null points of discrimination. Stomatopod crustaceans use a different system for polarisation vision,

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comprising at least four types of polarisation-sensitive photoreceptor arranged at 0°, 45°, 90° and

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135° relative to each other, in conjunction with extensive rotational eye movements. This

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anatomical arrangement should not suffer from equivalent null points of discrimination. To test

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whether these two systems were vulnerable to null points, we presented the fiddler crab

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heteropleura and the stomatopod Haptosquilla trispinosa with polarised looming stimuli on a

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modified LCD monitor. The fiddler crab was less sensitive to differences in the degree of polarised

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light when the e-vector was at -45°, than when the e-vector was horizontal. In comparison,

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stomatopods showed no difference in sensitivity between the two stimulus types. The results

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suggest that fiddler crabs suffer from a null point of sensitivity, while stomatopods do not.

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Keywords: polarisation distance, fiddler crab, mantis shrimp, discrimination threshold

© 2013. Published by The Company of Biologists Ltd

Uca

The Journal of Experimental Biology – ACCEPTED AUTHOR MANUSCRIPT

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Introduction

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The polarisation of light provides an independent channel of visual information that some animals

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use to discriminate objects or for communication (Shashar et al., 1998; Wehner and Labhart, 2006;

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Glantz, 2007; Chiou et al., 2011; How et al., 2012; Temple et al., 2012; Marshall et al., 2014b). Visual

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systems that detect and identify objects using polarised light rely on comparative processing of the

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signals generated by polarisation-sensitive photoreceptors distributed across parts of the visual field.

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This is a process somewhat analogous to colour vision and adds to the well-known uses of the

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polarisation of light for navigation, orientation and habitat localisation (Wehner, 1976; Schwind,

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1983; Labhart and Meyer, 1999). Animals may use this channel to improve the detection of objects

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through veiling light or glare (Schechner and Karpel, 2005; Alkaladi et al., 2013), to enhance the

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contrast of transparent prey (Shashar et al., 1998; Tuthill and Johnsen, 2006; but see Johnsen et al.,

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2011), to break camouflage (Shashar et al., 2000; Jordan et al., 2012; Temple et al., 2012) or to allow

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the detection of polarised body patterns for communication (Shashar et al., 1996; Marshall et al.,

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1999).

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Object-based polarisation vision systems that have so far been studied are restricted to

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cephalopods and crustaceans. In these examples, the photoreceptors that act as polarised light

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detectors in the eye are aligned horizontally and vertically (Tasaki and Karita, 1966; Shaw, 1969;

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Waterman, 1981) forming two channels of polarisation sensitivity. Furthermore, these animals

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maintain precise alignment of their eyes relative to the visual world around them (Zeil et al., 1986;

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Talbot and Marshall, 2011), thus ensuring that the polarisation-sensitive receptors remain horizontal

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and vertical relative to the visual scene. Stomatopods are an exception. These crustaceans possess

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an anatomically complex array of polarisation receptors with e-vector sensitivities arranged at 0°,

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45°, 90° and 135° relative to each other (Marshall et al., 1991; Kleinlogel et al., 2003). They also have

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receptors in rows 5 and 6 of the mid-band of the eye that are sensitive to left and right-handed

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circularly polarised light (Chiou et al., 2008; Roberts et al., 2009). In addition to this anatomical

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complexity, these animals regularly rotate their eyes around the axis of view (Cronin et al., 1988;

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Land et al., 1990), thereby altering the orientation of the polarisation detectors relative to the

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outside world. Such a system is likely, therefore, to have very different properties from the more

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common two-channel polarisation vision.

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An understanding of the underlying anatomy of polarisation vision opens the possibility for

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modelling an animal’s sensitivity to polarised light cues. One approach, inspired by analogous

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systems in colour vision (Vorobyev and Osorio, 1998), is to simulate the proposed neural

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architecture of early visual processing. For simple systems, two orthogonally oriented polarisation

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channels are assumed to have opponent connections (similar to dichromatic colour vision models)

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and produce a single level of neural activity (Bernard and Wehner, 1977). Subsequent comparison of

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this activity between two different parts of the visual field (one viewing an object and one viewing

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the background) results in a measure of polarisation contrast, termed ‘polarisation distance’(How

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and Marshall, 2014). Presumably this output is modulated at some stage by intensity variations, so

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that intensity signals and polarisation signals are fused into a single measure of contrast; however

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this falls beyond the scope of the current paper. Such models allow us to predict how well animals

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will perform in a discrimination task, assuming that they are using a given neurosensory apparatus.

The Journal of Experimental Biology – ACCEPTED AUTHOR MANUSCRIPT

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Previous studies using this modelling approach identified the problem of potential ambiguities

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and null points in two-channel polarisation vision systems (Bernard and Wehner, 1977; How and

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Marshall, 2014). For example, polarised light cues with e-vector axes oriented at +45° or -45°

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stimulate horizontal and vertical polarisation receptors equally, and so are indistinguishable from

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each other and from unpolarised light of the same intensity. This is widely accepted as a problem for

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two-channel polarisation vision, yet, with the exception of some associative learning experiments

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with cephalopods (Moody and Parriss, 1961; Shashar and Cronin, 1996), this has not been

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investigated in detail at the behavioural level. To this end, we used a modified liquid crystal display

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(LCD) monitor (Glantz and Schroeter, 2006; Pignatelli et al., 2011; How et al., 2012; Temple et al.,

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2012) to present polarised looming stimuli to the two-channel polarisation vision system of the

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fiddler crab Uca heteropleura (Smith, 1870) and to the dynamic, multi-channelled polarisation vision

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system of the stomatopod Haptosquilla trispinosa (Dana, 1852). These looming stimuli differed in

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the degree of polarisation, while the overall light intensity remained constant. We compared the

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response probability of these two species to stimuli that fell, either within, or outside a predicted

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null point of discrimination for a two-channel polarisation vision system.

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Results

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Fiddler crabs U. heteropleura responded to polarised looming stimuli in a number of ways similar to

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those reported for U. vomeris (How et al., 2012), including cessation of all movement (freeze),

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drawing of the legs or claw closer to the body (tuck), a jerk or twitch, and a sprint. A small number

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of crabs responded to the zero contrast stimulus (Fig. 1a, dotted line) as a result of coincidental

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changes in behaviour during stimulus presentation. These therefore estimate the probability of false

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positive responses during the experiment. A significant response was recorded for crabs viewing the

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horizontally polarised stimuli down to differences in degree as low as 8% (linear mixed effects model

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(lmer) for 8% stimulus vs. background: family binomial, number of crabs (n) = 22, df = 1, χ 2 = 18.7, p