| One of the best known features of Octopuses
is the fact that they have eight arms. These arms can be considered 'super lips' as they
are modified appendages surrounding the mouth. The inner surface of each arm is lined with
one to two rows of adhesive suckers, which are used to seize prey and, for bottom-dwelling
species, to 'walk' along the substrate. The individual suckers and the muscles of the arms
are very strong, capable of prizing apart bivalve shells or pushing weighted lids off
aquariums. In some species, an individual sucker can hold weights in excess of two
kilograms. All the body organs and gills are stored in a muscular bag (the mantle).
Expansion and contraction of the mantle pumps water in and out of a cavity in the mantle,
allowing oxygen to be absorbed through the gills. The exhaled water can be harnessed to
provide jet propulsion by forcing water through the funnel. The mouth is made up of a parrot-like beak and a toothed tongue,
the radula. The radula is used to drill holes into the shell of crabs, shrimp, even clams
and snails, through which paralysing saliva is injected. For bivalve molluscs, the toxins
in the octopus' saliva relax the muscles that hold the shells together, and the octopus
can then prize them apart. Octopuses may control more active prey like crabs by nicking
one of the crab's eye tips with the radula and injecting toxins straight down the optic
nerve into the animal's brain. It is a quirk of evolution that the brain of octopuses and
other cephalopods developed surrounding the oesophagus. This means that, to avoid serious
migraines, octopuses must chew their food into a puree before swallowing it.
The well-developed eyes of octopuses are similar to the
eyes of humans with an image-forming lens and large retina, which enable excellent vision,
although not in colour. The brain is very large for an invertebrate. Many studies have
examined the intelligence of octopuses and demonstrated learning and problem-solving
skills. For example, octopuses soon learn to screw the lids off glass jars in order to
capture prawns or crabs placed inside. There are even reports from the last century of
tool use by an octopus, which used small stones to wedge open clams before squeezing
inside to devour the shellfish. |
