Description
Size differs between species, but
they range from four to six centimetres long, with arms reaching lengths
of seven to 10 centimetres. The group is named for the iridescent blue
markings that dot their bodies; however these are usually only seen when
the octopus feels threatened and is about to attack. This change in colour
is due to pigment cells known as chromatophores. They, along with all
other octopuses, have eight arms which are attached around their mouth.
These arms have rows of broad, muscular suckers.
The brain of an octopus is shaped
like a donut, and is centred around their oesophagus. They have two very
well-developed eyes that are similar to those possessed by vertebrates.
Octopuses have three hearts, with a central heart and one over each gill.
These gills in turn are suspended in a cavity under the body. Seawater
enters the octopus through this cavity, due to the pumping action of the
mantle, a muscular bag-like structure within which is stored the organs of
the octopus. The mantle is not responsible for disposing of the seawater
from the body however, rather the water is ejected through a funnel, which
can be aimed in different directions. The propulsion of water from this
funnel allows the octopus to move rapidly in escape. The funnel can also
shoot out ink in some blue-ringed octopuses, which comes from a gland
located in the liver.
Although molluscs in general are
known for their shells, in the octopuses this shell has been greatly
reduced through evolution, and now exists only as two small rods. Another
distinctive feature of the octopuses is the colour of their blood:
transparent blue. This is due to the respiratory pigment of the octopuses
being based on a copper atom; the respiratory pigment of a human is based
on an iron atom, which makes our blood red.
Family
Blue-ringed octopuses belong to the
Phylum Mollusca (the molluscs) which includes snails, slugs and bivalves.
They are part of the Class Cephalopodea, a distinctive group of animals so
named because their limbs are attached to their head. The name cephalopod
comes from the Greek words kephalos for head and podos for
foot. Within the cephalopods they belong to Subclass Coleoidea, which
includes cuttlefish, squid and octopuses. All octopuses belong to the
Order Octopoda, while the blue-ringed octopuses can be found within the
Family Octopodidae. They make up the genus Hapalochlaena.
Habitat
The blue-ringed octopuses range from
the Sea of Japan down to the waters of southern Australia; across from the
Philippines to Vanuatu. They inhabit depths from intertidal flats down to
50m. They tend to hide in crevices or under rocks during the day, and
emerge at night.
Mating
One of the limbs of the male
octopus, usually the third right arm, is modified for the purpose of
mating. This arm has a groove embedded into it and the end is shaped like
a spoon. During mating, males insert the spoon-like structure into the
oviduct of the female. They then place spermatophores (little "packets" of
sperm) into the groove on this arm. These then slide down the arm and into
the oviduct of the female. She does not conceive immediately, but rather
keeps the sperm until she is ready to lay her eggs. The female takes care
of the eggs until they hatch; females usually die once this job is
complete.
Toxins
The blue-ringed octopus has a nasty
surprise for any potential prey or predators. Within its salivary glands
live bacteria, which produce the chemical tetrodotoxin. This is a strong,
fast-acting toxin that paralyses the target by blocking the nerves from
transmitting messages. This toxin can be fatal; it has known to have
caused the deaths of at least three people: two in Australia and one in
Singapore. Many more people have come close to death as a result of the
bite of the blue-ringed octopus. The paralysis that overcomes the victim
is only to their voluntary muscles; they remain fully conscious. Death
usually occurs as a result of lack of oxygen. Thus, if mouth to mouth
resuscitation is given to a victim of a blue-ringed octopus, they should
fully recover. The good news for swimmers in the waters where blue-ringed
octopuses are found, is that they are retiring creatures and will only
bite if they are being harassed and poked.
Reference:
- Norman, M (2000) Cephalopods:
A World Guide Hackenheim, ConchBooks
