Abalone

O
ctopuses or octopi? Whatever we choose to
call these cryptic cephalopods, the eight-
armed octopus is among the most sophisticated of
all invertebrates and a key player in the food webs
of most marine ecosystems.
Octopuses, together with squids, cuttlefish and nautiluses,
are cephalopods – members of a class which forms part of
the phylum Mollusca. The cephalopods represent the peak of
molluscan evolution and, with finely-tuned sensory organs and
complex nervous systems, they are thought to be the most
highly developed of all invertebrates. Unlike other cephalopods,
octopuses have no shell at all and have abandoned a swim-
ming life. They are bottom-dwellers that spend most of their
time hidden in holes which they defend against other octopuses.
Octopuses move in two different ways. They may pull them-
selves along by attaching their suckers to rock faces or, like
other cephalopods, they swim by means of jet propulsion. Their
mantles are strong and muscular and, by exhaling water from
the mantle cavity through the funnel, they are able to effect a
form of jet propulsion. When escaping from predators, octo-
puses will shoot ink out of their funnel, rapidly change their
colour and escape to a hole where they can hide. The ink blocks
the predator’s ability to see or smell the escaping octopus.
C O A S TA L A N D M A R I N E L I F E – A N I M A L S : I N V E RT E B R A T E S – M O L L U S C S
Octopus 3B
Octopuses are able to change their colour very rapidly by
expanding or contracting their chromatophores – cells in their
skin that contain pigment. Each chromatophore is operated
by a ring of radiating muscles so that the animals can change
their colour, and blend in with their environment, almost
instantaneously.
The common octopus – Octopus vulgaris
The octopus’ characteristic eight arms are equipped with
suckers which are arranged in two rows along each arm. The
arms are joined by a web which extends about a quarter of
their length. At the centre of the web is the octopus’s parrot-
like mouth. This tough beak comprises an upper and lower
jaw and is used to tear at the flesh of prey items. Vital
organs, such as the gills, gonads and digestive system are
contained in the octopus’s mantle or hood which is situated
in the pouch behind the animal’s eyes.
Thirteen species of octopus occur in South African waters.
The most common, Octopus vulgaris, grows to a length of
1,5 m and is often caught by bait collectors who use lures
resembling crabs to bring the animals out of their holes. It is
distinguished from a rarer species, Aphrodoctopus schultzei,
by having two rows of suckers on each arm, rather than
alternate rows. Aphrodoctopus schultzei has brush-like tips to
the arms. The giant octopus, Octopus magnificus, is regularly
caught by bottom trawlers on the south coast. It can grow to
a length of 3 m and reach a maximum weight of 25 kg.