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Over the years there have been many studies of cephalopods1 - among them Loligo opalescens, a species of squid common off the coast of California. These studies have turned up many things, not the least of which is the uniqueness of cephalopod adaptations in their skin and musculature. Virtually every part of a cephalopod body has specialised features that help it survive.
Musculature and Tentacle Structure
The mantles (a specialised skin) of cephalopods serve several different functions. Some of these can be found in the skins of other animals, such as the protection of vital organs. Others, however, are unique to cephalopods. Some features are in fact unique to decapods (any dibranchiate cephalopod having ten arms). Functions of the mantle that differentiate it from the skins of other animals include respiration and locomotion.
Aspects that are unique in decapods relate to the format and functionality of the musculature of the mantle. In certain genera of decapods, such as Loligo and Lolliguncula, the circular and radial muscles are bounded internally and externally by tunics of collagen fibers. These fibers are arranged helically, with alternate layers crisscrossing at an angle to the longitudinal axis. They alone are responsible for preventing longitudinal expansion of the mantle. As for the muscles among decapods, there are two distinct muscle types within the mantle tissues. The first of these contains a large amount of myofilaments and only occassional mitochondria. The second utilise a large number of mitochondria coupled with a liberal blood supply. As the act of propulsion among cephalopods interrupts the respiratory process, the muscles for locomotion are naturally anaerobic in their function.
Another area of specialisation among cephalopods deals with their tentacles. The skin of most cephalopods, specifically octopods, has been developed into suckers on each arm. Since the suckers on squid are generally not used for locomotion or anchoring (although they are certainly used in predation), they are in many instances differentiated into hooks or pads, rather than into actual suckers. In either case, during predation, if the prey attempts to escape, the suction exerted by the suckers increases relative to the force pulling away. This is a function of their design. In octopods, the suckers create a seal on the prey (or substrate, as the case may be). In decapods exhibiting hooks, the hooks will bite into the flesh of the prey. When an attempt is made to pull away, the suckers elongate (thereby increasing the suction), and the hooks dig in deeper, respectively2.
The presence of bioluminescence in cephalopods represents another area of specialisation. In fact, per capita among genera, cephalopod bioluminescence exhibits a remarkable level of occurrence. Excluding octopods (of which only two of 43 species are bioluminescent) an amazing 63 of 100 genera contain luminous species. In fact, in all but three or four of the inclusive genera, bioluminescence occurs in every species.
One of the interesting things regarding cephalopod bioluminescence (apart from the level of occurrence) is the remarkable diversity of the type of photophores (a light-emitting cell). As many as 13 recognisable types of photophores have been documented in a single species. Despite the incredible diversity, most of these photophores can be classified into two different groups, those with bacterial luminosity and those that produce their own luminescence.
Virtually all genera of cephalopods exhibit light production that is generated through an intrinsic biochemical system. These examples of photophoric cells range from very basic photogenic spots with no accessory tissue to highly complex, polymorphic photophores. Two particular families of cephalopods alone utilise bacterial photophores. These are the families Sepiolidae and Loliginidae. The bacterial photophores are paired glands seated on, and partially embedded in, the ink sac3.
One of the most widespread dermal adaptations among cephalopods is the development of chromatophores (the cells responsible for colour-changing abilities among cephalopods). The chromatophores of cephalopods are unique in form and function. Each cell consists of a pigment sac surrounded by radiating muscles. As the muscles contract, the pigment sac expands, thereby bringing out the particular colour of that chromatophore. When the muscles relax, the natural elasticity of the pigment sac causes it to contract, revealing the white of the muscle mass that lies directly under the dermis.
Chromatophores are used in a wide variety of situations by cephalopods. They are used for disguise, defense, mating, and predation. Each species has a particular series of color and patterns it utilises in any particular situation4.
Diversity tends to be the one constant throughout the oceans of the world. Cephalopod specialisation represents the best of this idea.
Herring, Peter J. 1988 - The Mollusca, Vol. 11, Academic Press, Inc. Pages 449-481.
Packard, A. 1988 - The Mollusca, Vol. 11. Academic Press, Inc. Pages 37-62.
Wells, M. J. 1988 - The Mollusca, Vol. 11. Academic Press, Inc. Pages 287-299.3Arnold, John M. and Young, Richard E. Dec. 1974 - 'Ultrastructure of a Cephalopod Photophore', Biological Bulletin, Vol. 147. Pages 507-534.
Berry, S. S. 1920 - 'Light Production in Cephalopods', Biological Bulletin, Vol. 38. Pages 141-169, 171-195.4Fox, D. L. and Crane, S. C. 1942 - 'Concerning the Pigments of the Two-spotted Octopus and the Opalescent Squid', Biological Bulletin, Vol. 82. Pages 284-291.