Difference Between Squid and Octopus

The main difference between squid and octopus is that squid has a stiff backbone-like structure, eight short and two long tentacles, and two fins on their triangular head whereas octopus has no backbone, eight tentacles, and a round head.

Squid and octopus are cephalopods or head-footed animals whose head is the most prominent structure of the body. It is surrounded by a ring of tentacles with suckers. Both move by jet propulsion. Squids live in the open ocean while octopus lives on the sea floor in dens.

Key Areas Covered

1. Squid
     – Definition, Facts, Characteristics
2. Octopus
    – Definition, Facts, Characteristics
3. What are the Similarities Between Squid and Octopus
    – Outline of Common Features
4. What is the Difference Between Squid and Octopus
    – Comparison of Key Differences

Key Terms: Backbone, Fins, Head, Octopus, Squid, Tentacles

Squid – Definition, Facts, Characteristics

Squid refers to an elongated, fast-swimming cephalopod mollusk with eight arms and two long tentacles, typically able to change color. It has a triangular head. Also, it has a flexible, stiff backbone called the pen. Two fins are present on the head, which are used to swim at low speed. 

Figure 1: Loligo vulgaris

The behavior of squids depends on the species. Some squids such as Humboldt squid can be aggressive; they can even attack a shark. Two long tentacles are used to capture the prey quickly. The largest invertebrate in the world is the colossal squid. Squids mate in large groups and attach their egg capsules to the seaweed or ocean floor.

Octopus – Definition, Facts, Characteristics

Octopus refers to a cephalopod mollusk with eight sucker-bearing arms, a soft sac-like body, strong beak-like jaws, and no internal shell. The head is prominent and it is round. Octopus uses jet propulsion to swim fast. During this, the water is sucked into the body cavity called the mantle, quickly expelling it out a narrow siphon. 

Figure 2: Coconut Octopus

The male octopus uses the hectocotylus, a specialized arm for the transferring of sperm into the mantle cavity of the female. The female lays strings of fertilized eggs on the roof of the den. It aerates, guards, and cleans the eggs with water expelled from the siphon until hatching. After that, the female may die inside the den.

Similarities Between Squid and Octopus

• Squid and octopus are cephalopods with a prominent head surrounded by tentacles.
• They are cephalopod mollusks that live in the sea.
• Both do not a shell outside the body.
• Both have blue blood since the oxygen-carrying pigments contain copper.
• Each tentacle contains suckers.
• They have beaks that help to kill and tear the prey.
• They move by jet propulsion. They can swim in any direction and can change course quickly.
• Poisons of most squid and octopus are too weak to harm humans.
• The common predators of them are whales, seals, fishes, moray eels, sea stars and several species of birds.
• Both can change their colors to blend with their environment by a mechanism called camouflage.
• They expel a dark cloud of ink from their ink sac to confuse predators.
• They have excellent vision, sensitivity, smell pits, and taste receptors.
• Both undergo internal fertilization.
• Both die shortly after reproducing.

Difference Between Squid and Octopus

Definition

Squid: An elongated, fast-swimming cephalopod mollusk with eight arms and two long tentacles, typically able to change color

Octopus: A cephalopod mollusk with eight sucker-bearing arms, a soft sac-like body, strong beak-like jaws, and no internal shell.

Order

Squid: Teuthida

Octopus: Octopoda

Superorder

Squid: Decapodiformes

Octopus: Octopodiformes

Number of Species

Squid: 300

Octopus: 289

Habitat

Squid: Lives in the open sea; solitary or may live in groups

Octopus: Lives in the dens on the sea floor; solitary animals

Size

Squid: 1 cm – 20 m

Octopus: 1cm – 5 m

Lifespan

Squid: 9 months to 5 years

Octopus: 1-3 years

Backbone

Squid: Has a stiff and flexible backbone called the pen

Octopus: No backbone

Head

Squid: Triangular head

Octopus: Round head

Fins

Squid: Has two fins used to swim at low speed

Octopus: Lacks fins

Tentacles

Squid: Eight short tentacles and two long tentacles

Octopus: Eight tentacles

Prey

Squid: Fish and shrimps

Octopus: Bottom-dwelling crustaceans

Attacking

Squid: Injects poisons into the prey to paralyze it and then releases salivary enzymes to loosen the meat

Octopus: Tears the flesh by beaks and scraps into the mouth by holding the prey by tentacles

Reproduction

Squid: Mates in large groups

Octopus: Uses hectocotylus to transfer sperm to the mantle cavity of the female

Conclusion

Squid has a backbone, triangular head, two, long tentacles, two fins, and a triangular head. Octopus has a round head, no backbone and fins. Bot squid and octopus are cephalopods with eight tentacles with suckers. Both live in the sea and move by jet propulsion. The main difference between squid and octopus is basically their anatomy.

Reference:

1. “Octopuses & Squids.” Vancouver Aquarium, Available Here

Image Courtesy:

1. “Loligo vulgaris” By © Hans Hillewaert (CC BY-SA 4.0) via Commons Wikimedia
2. “Coconut Octopus (Amphioctopus marginatus) (6079648725)” By Bernard DUPONT from FRANCE – Coconut Octopus (Amphioctopus marginatus) (CC BY-SA 2.0) via Commons Wikimedia