Taxonomic Key:
Kingdom Animalia | Phylum Mollusca | Class Cephalopoda | Sub-class
Coleoida | Order Dibranchia/ Octopoda | Family Octipididae | Genus
Octopus | Species maorum
Introduction:
To look at an octopus it is hard to believe that it is a mollusc,
it has lost all remnants of a shell and appears so far removed from
the familiar snails and shellfish we associate with this phylum!
The octopus is the most highly organised and advanced of all molluscs
, along with its relatives the squids.
At the 'class' level there are 600-700 living species of cephalopods,
(including the largest invertebrate ever to have lived- the 'giant
squid'), and with the exception of the Nautilus, which has an external
shell, they all belong to the sub class Coleoida.
There are between 150and 200 species of octopus in the world, 39
different species in New Zealand waters alone!
The octopus is one of the few cephalopods to assume a benthic (bottom
living) habitat, most cephalopods (ie squid), have a free swimming
existence, and therefore a more hydrodynamic form.
The term Cephalopoda comes from the Greek, and means 'head-footed',
referring to the way this mollusc has evolved into a 'head' surrounded
by arms, (which are equivalent to the foot of a mollusc such as
a Paua).
Physiology
and Ecology:
Eyes:
The octopus has highly developed eyes and excellent
vision, a necessary adaptation to its mode of life.
It can visually distinguish between different shaped objects, forms
3-dimensional images, and is believed to have colour vision.
Although the octopus eye is a 'direct' eye, (photoreceptors in the
retina directed towards the light source), it is very like the human
eye in construction and complexity.
Theoretically the octopus eye is more efficient than our own, with
a wider field of clear vision and very good resolution and light
sensitivity.
Like a camera, the lens of the octopus eye has a fixed focal length,
and the octopus deforms its whole eyeball to focus instead of just
the lens as in a vertebrate.
Brain:
The cephalopods have the most highly developed nervous
system among the invertebrates. In the octopus the ganglia have
concentrated and almost fused, to form a brain enclosed in a cartilaginous
'skull'.
As well as areas to control motor responses, arm and mantle muscles,
circulatory system etc, there are lobes of the brain to control
behaviour, learning and memory,(not what you would expect to find
in a mollusc!). the largest brain lobes are those associated with
the eye, as would be expected of a species so dependant on vision.
Circulatory
System:
The octopus has one of the most developed circulatory
systems outside of the vertebrates; unlike other molluscs where
dissolved oxygen never forms more than 3% of blood volume, the octopus
achieves a level closer to that of mammals, reaching 11% by volume.
This is because the muscularity of the mantle allows strong gill
ventilation.
Their circulatory system includes two branchial hearts to force
blood through the gills, in addition to the systematic heart. (see
fig.1, appendix)
Interestingly, mollusc blood has no red corpuscles, instead it has
blue corpuscles, copper bearing haemocyanin, which performs the
same functions as haemoglobin does in mammals, although with less
oxygen carrying capacity.
Movement and Respiration:
Like its relatives the squid, the octopus swims by
jet propulsion. This form of locomotion is produced by muscular
contraction of 'ring muscles' sealing the mantle cavity, increasing
the pressure of the water trapped inside before expelling it through
the funnel.
The water is drawn in between the anterior of the mantle and the
posterior of the head (see fig.1, appendix).
The funnel is mobile to allow for forward or backward movement;
when fleeing a predator the octopus swims backwards, and when an
octopus is observed in the water, it becomes obvious why. This gives
it the most streamlined (hydrodynamic) shape. (NB, The author has
seen octopus bump into objects when swimming backwards, so obviously
there are some disadvantages to this method!).
A medium sized octopus is capable of jetting along at up to 13kmh!
The circulation of water through the mantle has the additional,
(and more important function) of bringing in fresh oxygenated water
for the gills and respiration.
The dilation of the mantle and movement of the funnel 'lips' is
a lot less noticeable when the animal is at rest.
In general the octopus seems to prefer 'walking' around on its arms
unless really disturbed.
Arms
and Suckers:
An octopus has approximately 240 suckers per arm,
1928 in total). It has been calculated that a 2cm sucker requires
a pull of 6oz to break its grip; the average adult maorum has suckers
up to 3cm, which even allowing for the variation in sizes from base
to tip means a considerable force is required to remove an unwilling
octopus!
Arms
are well supplied with tactile (touch) cells, and chemoreceptors,
enabling the octopus to distinguish textures and chemicals, but
interestingly, they cannot discern by touch between objects varying
in weight, size or shape.
This is because they have no tactile proprioceptive ability, (ability
to receive stimuli from within the organism and measure own position
and movements); possibly because of the enormity of accurately computing
the whereabouts of 2000 suckers on eight independently mobile arms.
Feeding Habits:
The octopus is a carnivorous predator that preys
upon crustaceans such as crabs and crayfish, and also molluscs,
particularly clams, cockles and other bivalves.
(The author has observed octopus feeding on fish scraps and taking
baited hooks, so they will obviously take dead matter as well).
The octopus will remain in its 'lair' until a potential 'meal' comes
into its range of vision, then either lasso the prey item with an
arm, or swoop over it and enfold it with its interbrachial web and
arms.
Alternatively the octopus hunts its prey by crawling over the substrate
and exploring among the rocks with the tips of its arms, these are
very flexible enabling the animal to penetrate almost any nook and
cranny.
At times the octopus will catch several food items before returning
to its lair to feed.
This habit of 'dining in' means their holes are often easily recognisable
to a diver due to the large amount of discarded shells and carapaces
surrounding the entrance.
The
mouth of an octopus is very much like a parrot's beak and is located
in the centre of the arms 'underneath' the animal. Inside the mouth
is a radula, or tongue covered, in small teeth which is used to
draw food into the mouth.
Once the octopus has bitten its prey it injects poisonous saliva
through the wound which quickly paralyses then kills its victim.
The octopus then waits for twenty minutes before eating, possibly
because it is not immune to its own poison.
A proteolytic (digestive) enzyme is then injected, and the partially
digested food is passed into the gut, the indigestible material
being discarded, much like a spider!
Size:
Some species of octopus can grow quite large, Octopus
hongkongensis from the Pacific has arms 5m long and a body of 0.46m,
and Octopus dofleini has been recorded up to a total length of 5.6m…
a large intimidating creature for a diver to see underwater!
The smallest species in the world is Octopus aborescens, at only
50mm; and our common New Zealand species, Octopus maorum falls somewhere
in the middle with a body 230mm long and an arm spread of 1.2m.
Predators:
The octopus is prey to many species at all stages
of its life. Eggs are eaten by crabs, the juveniles by fish, and
the adults by a variety of seabirds, including penguins; seals,
large fish, and especially moray and conger eels, which are ideally
adapted to hunt through the cracks and holes the octopus like to
occupy.
Man is also a major predator, taking octopus for food, fish bait,
and as a by-catch.
Defense:
Colour Change:
The main form of defence used by the octopus is its
remarkable ability to change colour and blend in with its background.
Colour change in the octopus is achieved by special cells in the
skin called chromatophores. These are elastic sac's of pigment which
are surrounded by radial muscles; which when they contract, enlarge
the chromatophore, increasing the apparent concentration of colour.
(fig.2,appendix)
There are two sets of chromatophores, one set varying from black
to red, the other from red to pale yellow.
Below these are a layer of iridiocytes, (reflective bodies) which
break up white light into green or blue, giving the octopus a huge
number of potential colour combinations and patterns. The octopus
also has the ability to change the texture of its skin to match
its background by raising papillae,
At birth an octopus has only 70 chromatophores, compared to the
2,000,000 cells of an adult.
As well as being used for camouflage, colour change also occurs
with mood change, fear, and courtship.
Interestingly, a blind octopus is still able to blend in with its
background, raising the question of how background recognition is
achieved?
Ink
Release:
A further option the octopus has to confuse a predator
is the ability to squirt out a cloud of ink. This ink is a dark
fluid with a high concentration of melanin, and is passed from the
ink sac into the rectum, then is expelled out of the funnel with
water pressure.
It does not dissipate readily in salt water, and it is thought the
ink may also be objectionable to some predators due to its alkaloid
nature, and may inhibit their chemoreceptive senses.
Dymantic
Display:
The octopus employs this 'threat display' when approached
by a moving object larger than itself. It flattens against the bottom
and spreads itself out, changing to a uniform pale colour
except for rings around the eyes and interbrachial web. This is
done to exaggerate the animal's apparent size.
Reproduction:
Sperm from the male is packaged in a gelatinous sheath
called a spermatophore and loaded into the sperm groove by the penis.
Muscular contractions carry spermatophores along the groove of the
male's specialised hectocotylised arm, (third arm from the right)
to its spoon shaped tip, which is inserted into the female's mantle
cavity at the mouth of the oviduct. Once inside the spermatophores
rupture, insuring fertilisation.
The female lays her eggs a few at a time through the funnel; it
can take a week to lay several thousand in grape like clusters inside
her cave.
The female will stay to clean and oxygenate the eggs by blowing
water over them with her funnel, during this time she does not feed,
and dies soon after the eggs hatch.
The juveniles hatch as miniature adults and spend several weeks
in the plankton before settling to the bottom.
