If you were to only examine a squid and an octopus on a cursory basis, you might believe that they are members of similar species. After all, they both live in the ocean, they both are cephalopods, and they both have protruding arm-like features.
That, however, is where the similarities end. One of the biggest differences between octopus and squid is their level of intelligence. We spend countless hours every day observing octopuses, their intelligence, and their abilities and know this first hand.
Here is a video of Cliff, one of the residents here at Octolab during a mirror test.
Another important difference involves the number of protrusions that each has. An octopus can be distinguished by having eight arms. Hence the origin of its name, octopus, which means eight feet.
Squid, on the other hand, will have ten of these appendages. Eight are classified as arms and two as tentacles. The difference between arms and tentacles has to do with the suckers along the length of each. An arm will be covered with suckers throughout its underside. Tentacles will only have these suckers at the tips.
Another difference between squid and octopi involves the structure of their respective heads. An octopus will have a roundish head. The head of a squid is more triangular in shape and has two fins attached to each side. These fins help the squid navigate its direction as it swims.
Squid have a semi-rigid internal structure that runs along its mantle. This is known as the gladius or “pen.” This is made of chitin — the same material which makes up the exoskeleton of insects and the scales of fish. While not a skeleton, it does provide stability for the squid’s body while swimming and also serves as an attachment point for certain muscles associated with the animal’s locomotion. Octopi do not have any such internal structures.
There are approximately 300 species of octopi, roughly the same for squid. Octopi species can vary in size from as small as half an inch to as large as 15 feet. Squid species range from two feet to as large as 60 feet.
Observation of octopuses demonstrates that they are solitary creatures. They do not live in groups preferring a “hermit” lifestyle. A few squid species are also solitary but most live in groups, traveling the ocean in schools.
The way that squid and octopuses catch and eat their prey is also different. Squid will hunt for fish and free swimming crustaceans, such as shrimp, and catch them with their two tentacles. They will then consume their prey by using a beak-like structure known as a radula at their mouth opening to cut into the prey. The radula has multiple rows of “teeth” which help the squid grind the food before ingesting it. Many squid species hunt in groups and coordinate their movement to maximize their catch.
Octopi are solitary hunters. They normally catch their prey on the ocean floor with their arms. They then inject the prey with a venom which paralyzes it. Digestive enzymes are also introduced into the catch which begin to soften it up for ingestion by the octopus.
— In Short… —
Squid and Ocopti are both sea mollusks belonging to the cephalopod class. Octopi have eight arms, squids have 10 appendages (two being tentacles, eight being arms).
Squid species are more prone to living in groups, octopi are solitary.
The octopus has a round head, the squid a triangular one.
