Phil. Trans. R. Soc. B doi:10.1098/rstb.2008.0270 Published online Review Cephalopod dynamic camouflage: bridging the continuum between background matching and disruptive coloration R. T. Hanlon1,*, C.-C. Chiao1,2, L. M. Ma¨thger1, A. Barbosa1,3, K. C. Buresch1 and C. Chubb4 1Marine Biological Laboratory, Woods Hole, MA 02543, USA 2Department of Life Science, National Tsing Hua University, Hsinchu 30013, Taiwan, Republic of China 3ICBAS—Institute of Biomedical Sciences Abel Salazar, University of Porto, 4099-003 Porto, Portugal 4Department of Cognitive Sciences and Institute for Mathematical Behavioral Sciences, University of California at Irvine, Irvine, CA 92617-5100, USA Individual cuttlefish, octopus and squid have the versatile capability to use body patterns for background matching and disruptive coloration. We define—qualitatively and quantitatively—the chief characteristics of the three major body pattern types used for camouflage by cephalopods: uniform and mottle patterns for background matching, and disruptive patterns that primarily enhance disruptiveness but aid background matching as well. There is great variation within each of the three body pattern types, but by defining their chief characteristics we lay the groundwork to test camouflage concepts by correlating background statistics with those of the body pattern. We describe at least three ways in which background matching can be achieved in cephalopods. Disruptive patterns in cuttlefish possess all four of the basic components of ‘disruptiveness’, supporting Cott’s hypotheses, and we provide field examples of disruptive coloration in which the body pattern contrast exceeds that of the immediate surrounds. Based upon laboratory testing as well as thousands of images of camouflaged cephalopods in the field (a sample is provided on a web archive), we note that size, contrast and edges of background objects are key visual cues that guide cephalopod camouflage patterning. Mottle and disruptive patterns are frequently mixed, suggesting that background matching and disruptive mechanisms are often used in the same pattern. Keywords: body pattern; vision; colour pattern; contrast; defence 1. INTRODUCTION they encounter, and they can implement the change in Camouflage versatility is probably no better developed milliseconds (Hill & Solandt 1935; Hanlon 2007). anywhere in the animal kingdom than in the cephalo- This unique ability, combined with (i) the enormous pods (octopus, squid and cuttlefish), which are marine visual diversity of their habitats (e.g. coral reefs, kelp molluscs possessing soft bodies, diverse behaviour, forests, sand plains, rock reefs, seagrass beds, etc.) and elaborate skin patterning capabilities and a sophis- (ii) the vast array and capabilities of their visual predators ticated visual system that controls body patterning for (e.g. marine mammals, diving birds, teleost and elasmo- communication and camouflage (cf. Packard 1972; branch fishes, etc.; Clarke 1996), provide an integrated Hanlon & Messenger 1996; Messenger 2001). Detailed biological system in which many aspects of camouflage ethograms of body patterning have been developed for can be described and tested experimentally, and it is more than 20 species of cephalopods (Hanlon & hoped that this group may inform us of general principles Messenger 1996), and one surprising finding is that of how visual camouflage works (Hanlon 2007). the seemingly vast numbers of camouflage patterns can A good deal of fieldwork on cephalopod camouflage be synthesized into three pattern types: uniform; has been accomplished (e.g. Forsythe & Hanlon 1988; mottle; and disruptive (Hanlon & Messenger 1988, Hanlon & Messenger 1988, 1996; Mather & Mather 1996; Hanlon 2007). Cephalopods can effectively 1994; Hanlon et al. 1999, 2007) and very recently a camouflage themselves on almost any natural habitat significant literature on laboratory experimentation with cuttlefish has begun to sort out the visual stimuli * Author for correspondence ([email protected]). that evoke different camouflage patterns (Marshall & Electronic supplementary material is available at http://dx.doi.org/10. Messenger 1996; Chiao & Hanlon 2001a,b; Barbosa 1098/rstb.2008.0270 or via http://journals.royalsociety.org. et al. 2004, 2007, 2008a,b; Chiao et al. 2005, 2007; One contribution of 15 to a Theme Issue ‘Animal camouflage: Ma¨thger et al. 2006, 2007; Shohet et al. 2006, 2007; current issues and new perspectives’. Kelman et al. 2007, 2008). 1 This journal is q 2008 The Royal Society 2 R. T. Hanlon et al. Review. Cephalopod camouflage (a) (b) (c) (a) (b) (c) (d) (e) ( f) (d) 1.0 disruptive mottle uniform 0.5 Figure 2. Three forms of background matching in cephalo- pods. Specific background match: (a) Octopus vulgaris in mottle showing ‘high-fidelity’ match to calcareous algae at Grand Cayman, BWI, 3 m depth. (b) Sepia officinalis normalized energy in the laboratory showing high-fidelity match to a coarse 0 1 2 3 4 5 6 sand of moderate contrast. General background match: (c) O. vulgaris in mottle showing a generalist (but not exact) match to a complex background of soft corals, sponges and sand at Saba Island, W. Indies, 2 m depth. Octopus is exactly in the middle of the image. (d ) S. officinalis showing mottle granularity bands with weak striping amidst silt covered rocks and sand near Izmir, Turkey. Deceptive resemblance or masquerade: Figure 1. (a–c) Representative uniform, mottle and disruptive (e) S. officinalis showing large- and small-scale mottles that patterns, respectively, in the cuttlefish Sepia officinalis. match patches of algae on a sand plane near Vigo, Spain at (d ) The granularity analysis; see text §4 for details. The 20 m depth under murky water conditions. ( f ) Sepia apama three spectra shown are typical of uniform, mottle and showing mottle to resemble or masquerade as clumps of algae disruptive patterns (modified from Barbosa et al. 2008b). on a sand plain at Whyalla, S. Australia, 5 m depth. 2. BACKGROUND MATCHING IN CEPHALOPODS: Camouflage is a key evolutionary development in UNIFORM AND MOTTLE PATTERN visual predator–prey interactions. Like other research- DESCRIPTIONS ers who study animal camouflage, we have been A chief characteristic of uniform body patterns is little positively influenced by the masterful accounts of or no contrast, i.e. there are no light/dark demarcations Poulton (1890), Thayer (1896, 1909) and Cott that produce spots, lines, stripes or other configu- (1940). A key question—yet unanswered—is whether rations within the body pattern (figure 1a). Uniform (and how) disruptive coloration is a distinctive patterns can vary in colour and brightness yet both mechanism of camouflage, and how it can be attributes are held constant, or uniform, within any distinguished from background matching. Part of the answer lies in the definitions of disruptiveness, a subject single uniform body pattern. Stipple patterns are that was formally presented by Cott (1940) but not considered a subset of uniform; they usually have narrowly defined in his superb comparative treatise small clumps of expanded dark chromatophores that (with due credit here to some of his predecessors, create a uniform distribution of small roundish spots. including Bates, Poulton and Thayer). In this paper, we Stipples represent an early transition phase from (i) provide testable definitions of background matching uniform to mottle patterns (figure 2b). The individual and disruptiveness in cephalopods, (ii) describe those skin components in uniform and stipple patterns major pattern types quantitatively, (iii) summarize generally match the size of surrounding background laboratory psychophysical experiments suggesting objects (e.g. sand, mud, small pebbles) to achieve that cephalopod camouflage responses are controlled background matching on a spatial scale (Hanlon & by a modest set of visual cues, (iv) provide field and Messenger 1988). Uniform backgrounds are most laboratory evidence that some disruptive patterns have often observed on open uniform sand, uniformly higher contrast than adjacent backgrounds, and coloured rocks, in shadows and by squids in the (v) show that cephalopods often mix their mottle and water column. disruptive patterns, and speculate on the function of Mottle body patterns are characterized by small- such hybrid responses. to-moderate-scale light and dark patches (or mottles) Phil. Trans. R. Soc. B Review. Cephalopod camouflage R. T. Hanlon et al. 3 distributed somewhat evenly and repeatedly across the (a) (b) body surface. There is low-to-moderate contrast between the light and dark patches of the pattern. (i) The light or dark patches can vary in shape and size, yet each corresponds to some adjacent background objects to achieve general matching. Many visual backgrounds consist of small-to-moderate objects of moderate contrast, thus mottle camouflage is extremely common (ii) in cephalopods as well as many animals (Cott 1940; Hanlon & Messenger 1996). Figures 1 and 2 and the electronic supplementary material, figure 1, illustrate various examples of mottle patterns in octopus, cuttlefish and squid. (iii) We note several forms of background matching achieved with uniform and mottle patterns in cepha- lopods. The first is a specific background match to the pattern, contrast, physical surface texture, overall intensity and colour of the immediate background (figure 2a,b). From our extensive field experience and (iv) digital library, this sort of ‘high-fidelity’ match to the background occurs infrequently; this makes sense when one considers that cephalopods could