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First published online November 17, 2006
Journal of Experimental Biology 209, 4717-4723 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02580

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Cuttlefish responses to visual orientation of substrates, water flow and a model of motion camouflage

A. J. Shohet¹, R. J. Baddeley², J. C. Anderson, E. J. Kelman and D. Osorio^*

School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QG, UK

View larger version (57K): [in a new window]   Fig. 1. Images of cuttlefish resting on backgrounds: (A) fine, (B) medium and (C) coarse.

View larger version (126K): [in a new window]   Fig. 2. Evidence that there is no systematic effect of orientation relative to background on the coloration patterns expressed by cuttlefish. Upper row: Average images of all cuttlefish camouflaging (A) parallel and (B) perpendicular to the small-scale background and, (C) the mean difference between these two sets of patterns. Lower row: average images of cuttlefish camouflaging (D) parallel and (E) perpendicular to the medium background and (F) the mean difference between these two sets of patterns. Differences were even smaller for the coarse stripes.

View larger version (62K): [in a new window]   Fig. 3. The effect of applying horizontal and vertical spatial filters to representative images of a cuttlefish camouflaged perpendicular and parallel to the sand and gravel backgrounds (Fig. 1). Fine background (sand) - the horizontal filter (A) found no horizontal structure in the cuttlefish camouflage pattern (B) and the vertical filter (C) found no vertical structure in the cuttlefish camouflage pattern (D). For the medium background (gravel) - the horizontal filter (E) found no horizontal structure in cuttlefish camouflage pattern (F) and the vertical filter (G) found no vertical structure in cuttlefish camouflage pattern (H). This analysis was run for a range of filter widths; the images shown are those where the filtered scale most closely matched the width of background stripe, and so one might expect an effect of orientation on the pattern.

View larger version (11K): [in a new window]   Fig. 4. The effect of applying horizontal and vertical spatial filters to representative images of a cuttlefish camouflaged perpendicular and parallel to the fine and medium backgrounds (see Fig. 1). The number of times in 60 tests that the five subjects settled at each of the three orientations relative to backgrounds: (A) fine, (B) medium and (C) coarse stripes.

View larger version (8K): [in a new window]   Fig. 5. Where camouflage relies on matching stripes on a body pattern to stripes in the background, mismatches due to small errors may severely compromise camouflage. (A) A cartoon of the camouflage situation faced by our cuttlefish, together with the optimal camouflage pattern (stripes that exactly match the substrate). If this were possible, it would be the best strategy. To achieve this perfect match, the animal needs to both accurately estimate the frequency, orientation and phase of the substrate, and also generate to a matching pattern. (B) Shows the results of a small error (10%) in estimating the frequency of the substrate. As can be seen, this small error results in highly visible structure around the cuttlefish. (C) Shows the result of a 10% error in frequency, phase and orientation. These small errors can result in a pattern that is more visible than one that either simply matched the average luminance, or employs a disruptive pattern.

View larger version (26K): [in a new window]   Fig. 6. The effect of motion on visibility. (A) An optimal static camouflage pattern where the pattern matches the background, and the orientation minimises the number of lines obscured. Unfortunately, if the animal moves, this pattern is far from optimal: the relative motion between the stripes on the animals back and the background is highly visible, and there is a strong relative motion signal together with occlusions along the side (marked by the arrows). (B) A pattern without stripes minimises motion signals. Further, orientating the body orthogonally to the substrate, the area of high relative motion and occlusion is minimised. (C) Sand ripples in the natural environment are oriented at 90° to the water flow (Ayrton, 1910). Thus an orientated substrate provides important information about the current. Similar principles apply to shadows cast by waves. Orientating the body in the direction shown by the arrow, as was observed, minimises drag, and maximises the efficiency with which the animals can compensate for involuntary movements.

View larger version (4K): [in a new window]   Fig. 7. Body orientation of cuttlefish in flowing water. Data are for ten animals each measured four times. There is a clear preference for a posture parallel to the direction of flow (2 = 26.6, d.f.=2, P<0.001).