European squid propulsion. The posterior border is slightly concave. The head is relatively small and has large eyes which are covered with a transparent membrane. Like almost all squid, this species has ten limbs surrounding the mouth and beak: eight are relatively short arms, and two, which form the tentacles, are long, as they are used to catch prey. The fourth left arm of males is a hectocotylus. The European squid can grow up to 30–40 cm in the mantle length, but more usually they are 15–25 cm long. The males are generally bigger than the females and exhibit more rapid rates
Humboldt squid fins. Their tentacles bear 100–200 suckers, each lined with razor-sharp teeth, with which they grasp prey and drag it towards a large, sharp beak. Although Humboldt squid have a reputation of being aggressive towards humans, some disagreement exists on this subject. Research suggests these squid are only aggressive while feeding; at other times, they are quite passive. Their behavior while feeding often extends to cannibalism and they have been seen to readily attack injured or vulnerable squid of their own shoal. A quarter of squid stomachs analyzed contained remains of other squid. This behavior may account for a large proportion