Topic: Cephalopods – Cephalopoda

Blekksprut 10 01 2017 2 — Photo: Erling Svensen / IMR

eledoneblekksprut — *Eledone cirrhosa*.
Photo: Erling Svensen / IMR

sepiaskall (hvalsprø) fra sepiablekkspruten skylt opp på land — Shell from *Sepia*.
Photo: Erling Svensen / IMR

Cephalopods are an ancient group of animals that have existed for about 500 million years. Like bivalves and gastropods, they belong to the class Mollusca, although their relationship is not necessarily obvious.

Published: 03.06.2020 Updated: 04.06.2020

Most of today's cephalopod species have a soft body with no outer hard parts, but an inner shell that stiffens the body and regulates buoyancy.

The Greek word "Cephalopoda" means head-feet and refers to the feet or arms sitting directly on the head. There are around 800 species in the world, and it is an exclusively marine group. Species diversity is highest around the equator and decreases towards the poles. We can distinguish between eight- and ten-armed cephalopods. The ten-armed ones have two tentacles in addition to the eight arms, and these are often longer and thinner than the arms.

In Norwegian waters, we find about 25 species, some of which are very rare and verified by only one or a few individuals. They can be divided into four main groups:

eight-armed cephalopods with and without fins (at least 5 species)
ten-armed cephalopods with torpedo-shaped mantle (most species and individuals, can get large)
ten-armed cephalopods with round mantle (9 species, small species)
ten-armed cephalopods with oval mantle and calcified inner shell (at least 1 species)

Many species have commercial value and are exploited as food in other parts of the world.

The main groups of cephalopods in Norwegian waters:

Facts about cephalopods

Common names: Eight-armed cephalopods are often called octopus and the ten-armed are often called squid, but this does not necessarily reflect the biological systematics. Calamari on the menu is fried squid.
Size: From 1 cm to the giant squid which might reach 18 meters length.
Diet: Fish, molluscs and crustaceans.
Distribution: There are 800 species worldwide, none live in fresh water, but someone can occur in brackish water.

Cephalopods are dioecious, i.e. they have distinct male and female individual organisms.

Did you know ...

that some species have light organs, and can thus produce and radiate light? This is called bioluminescence, and it can be found in many different groups (e.g. firefly, fish, jellyfish, etc.). The photophores (light organs) can sit in the abdominal cavity, around the eyes, on the arm or tentacle tips or distributed throughout the mantle and arms.
all squid species have suckers that are very effective grasping tools? Their function is based on underpressure and adhesion. Some species have also developed hooks (e.g. Gonatus) that are converted suckers.
the Norwegian term "akkar" is often used for all large ten-armed cephalopods and thus includes species from taxonomically very different groups.
some cephalopod species are very intelligent? Of all invertebrates, they have the most complex nervous system and largest brain in relation to body mass.
most species have an internal shell? It can be large and calcified or relatively thin and chitinous. The cuttlebone is the calcified shell of species in the family Sepiidae. It is often found drifting on the water surface or stranded on the beaches. Since it is very light, it can drift over large distances and originate further south in Europe.