| Being soft bodied
(Lacking an external shell or internal skeleton),
octopuses make a perfect meal for predators, particularly larger fishes, sharks or seals.
Many octopuses take advantage of their lack of skeleton by squeezing themselves through
tiny holes into crevices or burrows. An octopus with a 30-centimetre arm span can squeeze
through a hole the size of a fivecent piece. Still octopuses have had to develop a wide
range of other defences to escape predators. The
greatest array of defence strategies occurs in shallow-water, bottom-living (benthic)
octopuses in the family Octopodidae. Diurnal octopuses such as the common Day Octopus
(Octopus cyanea), which emerge and forage during daylight hours on coral reefs, have
developed exceptional camouflage capacities. They produce elaborate colour patterns and
highly complex skin textures capable of matching a wide range of backgrounds from sand and
reef rubble, through to spiked corals and seaweeds. Their skin changes almost
instantaneously as they move over different substrates on the sea floor. Colour changes
are carried out by small, elastic, pigment-filled sacs, known as chromatophores. A square
centimetre of skin may contain hundreds of chromatophores, in up to five colours in
certain species. Each chromatophore is surrounded by a ring of muscle fibres, all of which
are under the rapid and coordinated control of the large optic lobes of the brain. As a
backup defence, most octopuses also have an ink sac that produces highly concentrated
black, red or brown pigment. Small amounts of ink are squirted out the funnel to produce
either a dummy decoy or, in some species, a full smoke screen that can mask a volume of
water up to several cubic metres, leaving predators chasing their own tails. The ink is
also thought to dull the senses of the predator. |
