Cephalopoda

Cephalopoda is an incredible group of animals that includes the Octopus, Cuttlefish, Squid and Nautilus. Octopus and Cuttlefish are often heralded as the smartest invertebrates known to man. They have the largest brain to body mass ratio out of all invertebrates, and highly developed eyes and sensory organs. Learning through trial and error, and by example, they can solve complex puzzles. They also have a good memory. Cephalopods have been around for 500 million years and have developed into not only the smartest, but also the fastest, largest and deadliest of the Mollusca Phylum.^1,2

Many consider the eye of Cephalopods to be one of the best eyes in the invertebrate world. It is similar in complexity to the vertebrate eye, although they evolved independently.  They contain many of the same structures as in the vertebrate eye. An iris, pupil and lens, although most do not possess a cornea (Octopus are the only Cephalopods to possess a closed cornea).  Without a cornea, the majority of Cephalopod eyes are in direct contact with the saltwater. The Octopus have a distinct, slit-shaped, rectangular pupil, compared to a “W” shaped pupil in Cuttlefish and round pupil in Squid. The eyes of the Nautilus are much less complex and mounted on stalks. Most likely used only for light detection as they have no lens and a very small pupil (1-2mm only), suggesting a poor resolution image.^1,2

The definition of arms and tentacles seems to confuse most people. Octopus, Squid and Cuttlefish possess 8, non-retractable arms, however only Cuttlefish and Squid have tentacles. Little fleshy extensions, called Cirri, and often suckers, line the arms. To allow them to catch prey, or stick to a substrate, these arms often have hooks on the underside too. Tentacles, on the other hand, are longer than arms and retractable.  Cuttlefish and Squid possess two tentacles each, usually with a flattened tip, called a club, which is covered in suckers. These are used to strike prey quickly, hence the name club.^1,2

Unlike most Molluscs, Cephalopods have separate sexes and mating usually begins with courtship and flashes of colour. They will then interlock tentacles and the male will used a specialised arm, called a hectocotylus, to pass a packet of sperm (or spermatophore) to the female through an opening in her mantle. Most females then lay large eggs, which develop into full swimming juveniles before they hatch.  Most adults then die, shortly after reproducing.^1,2

Cephalopods also have the ability to change the colour and texture of their skin. Highly specialised cells, called chromatophores and iridescent dermal tissue make this possible. The chromatophore cells can expand and contract to hide or exhibit small dots of colour and can be so numerous that 200 cells may fill a space the size of the blunt end of a pencil.  “The Passing Cloud Display” is one of their favourite moves and involves passing waves of light down their body. In cuttlefish, the waves run from their head down their body, whereas in Octopus, it runs from the arms towards the head.  They use this move when hunting, to confuse their prey, but it also helps them blend in as the sunlight passes overhead.^1,2

It is also well known that Cephalopods have the ability to squirt ink.  This confuses predators, but may also affect their senses of taste and smell.²

You can find a list of all of the groups of Cephalopoda found on The Great Barrier Reef, below.

Order – Fancy science word used to group organisms with similar characteristics

• Nautiloids – Nautilus

• Octopoda – Octopus

• Sepioidea- Cuttlefish

• Teuthoidea – Squid

• Vampyromorpha – Vampire Squid