Tuesday, December 11, 2012

Hiding Out With The Cuttlefish

Common Cuttlefish (Sepia officinalis)
The ability of cephalopods to vary their colour has been known since antiquity and while most species can achieve impressive colour changes, few can match the common cuttlefish (Sepia officinalis) for sheer dramatic quality. This is in part due to the size (1.5 mm in diameter) and density (50 per square mm) of the chromatophores (1), the neurally controlled colour bearing organs that can change the pigmentation and hence appearance of the animal with incredible detail. However in part it is, as the chromatophores are just one movement in the symphony that makes each animals body pattern.

Body pattern change is used in feeding feeding, avoiding predators and communication, and is therefore an integral part of S. officinalis life history. Its most striking aspect are the chromatophores, organs that are unique in the animal kingdom to cephalopods. Body pattern is controlled in a hierarchical fashion in S. officinalis: behaviour will dictate body pattern and hence organ response. Body pattern is constructed using four components, such as coloration of which chromatophores play a part. However, they are aided by organs such as leucophores which scatter light of all waveslengths and iridiphores, which produce interference colours when viewed from certain angles, often giving pink and iridescent greens and blues (1). The other three components are textural (the smoothness or papillation of the skin), postural (the orientation of the body parts) and locomotor (the action of the animal, e.g. resting, burying, scuttling, etc.). These components are themselves divided into units which are in turn divided into elements, such as the previously mentioned chromatophores. This complex hierarchy of organisation allows for the wide variety of body shapes observed in S. officinalis.
 
Such an intricate response mechanism is under tight control of the central nervous system and is driven by visual stimuli. Environmental cues taken in by the eye and transferred to the optic lobe where information is processed and transferred to the lateral basal lobe which will control motor response (1). Amazingly, these neural areas are already well developed upon hatching in S. officinalis and newly emerged cuttlefish are immediately able to conceal themselves from predators (2). They use strategies such as colour resemblence, disruptive coloration, obliterative shading, shadow elimination, disguise and adaptive behaviour to avoid becoming a meal from fish such as the Comber (Serranus cabrilla).
 
Human ability to distinguish symmetrical objects easily and quickly lead to the assumption that the use of these behaviours would be greatly enhanced by the use of asymmetrical patterns. However, it has been demonstrated that in cryptic behaviour, S. officinalis will exhibit a high degree of bilateral symmetry (3). This seems counter-intuitive: symmetrical objects would stick out much more obviously in a random, asymmetrical environment. Yet S. officinalis is notoriously difficult to spot in its native environment. This may be due to a number of factors (3). Predators of S. officinalis may not use symmetry as a visual clue. Also the orientation of the axis of symmetry is important, as unless the axis is horizontal or vertical, symmetry becomes less apparent to the viewer. Alternatively, symmetry may play a vital role in concealment. By highlighting a symmetrical pattern on its body, S. officinalis may be taking the emphasis off its own body shape, making it seem just an interesting but inedible artifact to its predators.
References:
  1. Hanlon and Messenger, 1998. Cephalod Behaviour pp. 31-46
  2. Langridge, 2006. Proceedings of the Royal Society Series B 273 pp. 959-967
  3. Hanlon and J. B. Messenger, 1988. Philosophical Transactions of the Royal Society of London, Series B, Biological Sciences 320 pp. 437-487
Cuttlefish picture taken at Galway Atlantiquaria, Salthill, Co. Galway.

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