Story: Octopus and squid
Page 1 – Head-footed animals
Octopus and squid represent the pinnacle of invertebrate evolution: they are the most intelligent, and contain some of the largest, heaviest and most aggressive of the invertebrates (animals without backbones).
How to tell a squid from an octopus
A popular misconception is that squid have eight arms and two long tentacles, while octopus have eight arms and no tentacles. But this is not always the case. The only real difference between the two groups is that squid have hooked or saw-like suckers, and octopus do not.
Octopus and squid belong to a group of marine invertebrates known as cephalopods. Cephalopod means ‘head-footed’ – the arms and tentacles are derived from the head. Their closest relatives are snails, slugs, chitons and shellfish, collectively known as molluscs. Cephalopods have a long fossil history and include extinct groups such as the ammonites and belemnites, both of which have been found as fossils in New Zealand.
Of the five surviving orders (major groups), four are represented in New Zealand: cuttlefish, squid, octopus and vampire squid. The group to which pearly or chambered Nautilus belong are absent from New Zealand waters, although their empty shells occasionally arrive on northern beaches, swept down from the Pacific on ocean currents.
Octopus and squid bodies
All species of octopus and squid have a mantle, head and eight arms (and in the case of some squid, two additional tentacles).
- The mantle is a tapered or cylindrical sac containing the organs. Most squid and some primitive octopus have an internal shell within their mantle. A few cephalopod species (true nautilus and the female paper nautilus) secrete external shells. Squid and some octopus also have fins at or near the rear of their bodies, which assist with movement through the water.
- The head contains the brain, two eyes, a mouth with a beak like a parrot’s and, attached to the underneath surface, a muscular funnel. When water is forced out of the funnel, the animal is propelled in the opposite direction.
- Both octopus and squid have eight arms encircling their mouths. Most octopus have suckers on the insides of their arms. Squid suckers are armed with a variety of hooks and claws. All squid possess two long tentacles at some stage of their life.
Techniques for disguise
The bodies of octopus and squid are covered in chromatophores – tiny contracting and expanding pockets of pigment that cause instantaneous colour change. They have the ability to change from solid red to completely transparent in a millisecond. Octopus can alter the texture and colour of their skin to perfectly match the surrounding seabed.
Ocean-dwelling squid and a few octopus species have light-producing (bioluminescent) organs that are used for counter-illumination, masking the animal’s silhouette against the bright light from above.
Most octopus and squid species can squirt out a cloud of ink from their mantle when disturbed. The ink is thought to function as a decoy to confuse predators. Some deep-sea species release luminescent ink.