Cephalopoda, squid, octopus, cuttle fish and nautilus

On this page, Cephalopoda overview - cephalopod body - Coleoidea (squid cuttlefish and octopus) - eyes - chromatophores and photophores - mating - life span

Class Cephalopoda overview

The Cephalopoda are considered to be the most sophisticated molluscs, and possibly the most intelligent invertebrates. There are about 660 species and a large size range, the Giant squid (Architeuthis sp.), can be over 20m long.

The cephalopod fossil record goes back to the Precambrian, and were similar to the nautilus extant today. It is believed that the Class of molluscs most closely related to the Cephalopoda is the Monoplacophorans. Cephalopods have a wide range of behaviours, a relatively large and complicated brain, and a great capacity for learning. All are predators.

Cephalopod body

Cephalopods, unlike us, have no internal skeleton to anchor their muscles, nor, unlike insects, do they have an external skeleton, apart from the nautiloids. Yet they can make the most delicate and intricate movements. They have what is termed a muscular hydrostat. We have something similar in the human tongue. You can stick out your tongue, move it about, but the volume of the tongue stays the same in all these movements. The shape changes as the muscles inside your tongue elongate and contract. This is the same method that the whole body of the cephalopod uses.

The cephalopod body has three main regions:

The foot of the standard mollusc has developed into a number of prehensile arms with suckers around the mouth, with one or two modified for sperm transfer (see mating below); and a muscular funnel in the mantle cavity. This funnel is used in movement when water is forcibly expelled through it, a form of jet propulsion.

All cephalopods, except nautilids, have either eight or ten arms. Nautiloids have around ninety arms that are suckerless, but do have adhesive ridges.

Cephalopods have a radula and a pair of beak-like jaws. They also have an ink gland which releases a cloud of ink through the anus when the animal is alarmed. The ink cloud can act as a decoy to a predator allowing the cephalopod to escape. The ink is a suspension of melanin stored in the ink sac until it is expelled. Cephalopods have blue blood as the chemical that binds the oxygen in the blood is copper based haemocyanin, which is blue, whereas in mammals it is iron-based haemoglobin which is red.

There are two Sub-classes, the Nautiloidea with six species, and Coleoidea which includes the squids, octopuses and cuttlefishes. All are marine.

Coleoidea (squid, cuttlefish and octopus)

The fossil record of the Coleoidea goes back more than 250 million years. They have no external shell. Cuttlefish have an internal calcareous shell, squids have a thin cartilaginous pen, and in octopods the shell is absent, see Eledone sp.

Octopuses tend to be solitary, but squid are often found in shoals making them a commercially viable species for fishermen to catch. Squid and cuttlefish have 10 arms - 8 short and 2 long. Octopods have 8 arms; all of the same length.

Coleoidea (squid, cuttlefish and octopus) eyes

Cepalopod eye cross section

The eyes are large, sensitive and share many features with vertebrate eyes (see above), e.g. iris, cornea, lens focused by muscles and retina. The pupil is slit-shaped and the slit is aligned so that it is kept horizontal.

The vertebrate eye arises from the development of the brain, whereas the cephalopod eye arises from the development of the skin. The Colossal squid, Mesonychoteuthis hamiltoni has a huge eye measuring 60cm in diameter. It is a deep sea squid, and the eye can glow in the dark, perhaps helping it catch its fast moving prey, such as the Patagonian toothfish. Few specimens have been caught, but one female from the Ross Sea weighed 150kg, was 5.4m long including her tentacles, and 2.5m excluding her tentacles.

Chromatophores and photophores

The cephalopod chromatophore cell is a small sac filled with pigment and surrounded by muscle cells. These muscle cells are capable of stretching the pigment cell out so that it displays the colour. When the muscles cells are relaxed the pigment is more-or-less invisible. This type of cell allows for rapid colour changes, also known as Rapid Adaptive Colouration (RAC). The change in colour can happen in a fraction of a second. RAC is used in camouflage, alarm, threat, escape from predators, and mate attraction.

Photophores are light producing, and are found mainly in the nocturnal and deep dwelling species. There are two types of photophore: 1) biochemical, 2) bacterial.

The bacterial photophores use symbiotic bacteria that live and grow in special chambers in the cephalopod. These bacteria are sometimes expelled with ink to create a glowing cloud.

Both bacterial and chemical photophores produce a blue/green light. Some photophores have a lid which can be opened or closed to produce flashes of light.

Mating in the cephalopoda

Some octopus males fertilize the female by inserting the hectocotylus (located at the end of one of their arms) into the female. In some cases the hectocotylus breaks off and stays in the female.

Before the mating habits were understood females found with a hectocotylus embedded in them were thought to have been parasitised.

Life span

Apart from Nautilids, cephalopods, even the huge giant squids, seem to live no more than five years at most. For smaller species their life span may be just a few months. However, much is still unknown, especially for those species living in the deep oceans.