Is a squid a fish?

In this blog post, we will learn if a squid is a fish. Additionally, we will understand the general characteristics and adaptations of the squids. And also meet the top two most mysterious squid species.

Is a squid a fish?

No, squids are not fish, they are molluscs. The term “squid” refers to cephalopod individuals that belong to different genera: Loligo, Architeuthis, Alloteuthis, Histioteuthis, etc. They all have a pair of gills and are decapods (5 pairs of legs).

Squid of the Loligo genus has a pinkish to reddish skin tone and can change its tone very often. Their body is elongated and ends in a triangular shape fin with a concave posterior edge inserted in the last two-thirds of the mantle.

Squids are molluscs

Squids are molluscs and belong to the cephalopod class. They live in marine environments and breathe through gills. The Cephalopoda word has a Greek origin, from kephalé = head and podos = feet. The squid body is divided into a head, visceral mass and tentacles. Unlike their close relatives, the clams (bivalves) and slugs (gastropods), squids do not have a hard outer shell, but an inner shell with the outer part of their body is very soft.

The Cephalopods class is divided into Octopoda (with eight arms) and Teuthida (eight arms and two tentacles). The squid is a teuthid as it has ten tentacles. Its visceral mass is elongated and its body is surrounded by the mantle, which has a muscular cavity found behind the squid’s head. It is through this cavity of the mantle that the water circulates, passing through the gills making it possible for the mollusc to breathe.

At the back of its visceral mass, the squid has two triangular-shaped fins. There are approximately three hundred different species of squid widespread around the world, in shallow and deep water, coastal shallow or open oceanic deep water.

The squids produce ink, also known as sepia, that assists the animals when chased by predators or threats. This ink and the circulating water in the respiratory cavity are expelled 

through a tube called a funnel, located below the squid’s head. 

This jet of water is generated by the contraction of the mantle and expelled by the funnel. This results in the squid being projected to swim backwards by jet propulsion. Between the cephalopods, squids are the fastest and most agile swimmers.

Squids can change colour and even texture due to the pigment cells present in their tentacles, called chromatophores. Despite their speed, agility and a camouflage system used for defence, squids are vulnerable prey due to their soft body. 

Squid are great hunters, they prey on small fish, shrimps, crabs, and other smaller squids. They use the tentacles to hunt, by driving the prey into their beak-shaped jaws, where they rip and cut the prey.

The total size of a squid refers to the size of the visceral mass (part of the body that houses the internal organs), the head, and the tentacles. This measurement is not the best, once the animal can be stretched. There are reports that the first Architeuthys spp. described, found stranded on a beach, was 22 meters long. This was because of the elasticity of its tentacles, so it is extremely important not to analyse the information incompletely.

Giant Squid

The Giant Squid (Architeuthis spp.) is a cephalopod of the order Teuthida. They are known as the second-largest invertebrate existing on Earth, the first is the Colossal Squid. 

The eight species of the genus inhabit the depths of the oceans and can reach lengths up to 10 metres for males and 13 metres for females, measured from the tail fin to the tip of the tentacles. The Giant Squid has one of the biggest eyes of all living creatures, surpassed only by the Colossal Squid. 

Tentacle suction cups can reach up to 5 centimetres in diameter. Several examples of these suckers marks have been found embedded in the heads of sperm whales, which are predators of Giant Squids.

First observations of live specimens

In September 2004, the team of Japanese researchers managed to photograph for the first time in history a live specimen of the Giant Squid in the North Pacific, near the Ogasawara Islands. The eight-meter-long animal grabbed a bait that was attached to a rope and launched 900 meters deep. The specimen struggled for four hours to free itself, amputating one of the tentacles in the process. The tentacle measured about 5.5 metres long and was rescued by scientists, still alive and moving.

In July 2012, Japanese scientists filmed for the first time a live Giant Squid in its natural habitat in a joint project with the Discovery Channel and Japan’s National Museum of Science and Nature. The animal was located at a depth of 630 meters by a submersible with three crew members on board, off the island of Chichijima.

The observations carried out by these scientists led to the conclusion that Giant Squids do not just float, but, on the contrary, are active swimmers and attack their prey horizontally, trapping them in their arms and tentacles.

Colossal Squid

The Colossal Squid (Mesonychoteuthis hamiltoni) is probably the largest extant species of squid and the only member of the genus Mesonychoteuthis. The Colossal Squid is considered the largest known invertebrate in terms of mass on the planet and may exceed 15 meters in length. 

It inhabits the depths of the Antarctic Ocean. The distribution of the Colossal Squid ranges from Northern Antarctica to Southern South America, Southern South Africa, and Southernmost New Zealand.

The Colossal Squid has the largest head of all squid species, exceeding the Architeuthis in size and robustness. The Colossal Squid also has the largest eyes in the animal kingdom reaching the size of an ordinary plate, around 30 centimetres. It has also two huge, sharp beaks.

Little is known about the life of this creature. It is a hunter like other squids and in the deep sea, it uses bioluminescence to find its prey. Based on the analysis of squid beaks in sperm whale stomachs, the size of adult animals (since few were captured) is estimated to be at least 22 metres, while younger animals are estimated to be around 10 metres.

Many sperm whales carry scars caused by the tentacles of Colossal Squids. In addition to the suckers, they have hooks that can cause deep wounds on the whales’ bodies. The Colossal Squid is one of the main prey for sperm whales that feed in the Southern Ocean; around 14% of the squid tentacle marks found in these whales are from Colossal Squids.

New Zealand fishermen recently found a Colossal Squid over 14 metres long in Antarctic waters. The individual weighed 495 kilograms and had eyes 25 centimetres in diameter. The animal was accidentally hooked, brought aboard and kept on ice. Then it was sent to be studied at the Auckland University of Technology in New Zealand. This is the second-largest specimen of Colossal Squid ever found and is on display at the New Zealand Museum.

The Colossal Squid, unlike the Giant Squid, as it grows acquires a round shape on its head. The Colossal Squid’s tentacles are great for catching prey in icy seas. So its body just floats while its tentacles seek out prey. Because of this ability to float, a dying squid rises to the surface. This is a very common way to find Giant or Colossal Squid.

Differently from the Giant Squid, which has arms and tentacles lined with tiny teeth, the Colossal Squid has limbs with sharp hooks. Its body is robust and wider, and consequently heavier than the body of the Giant Squid.

The Colossal Squid has abyssal gigantism. One specimen collected had an eyeball with 27 centimetres and a pupil with 9 centimetres in diameter. It is believed that when alive, the eyeball was between 30 and 40 centimetres in diameter.

Conclusion

Here, we discussed if a squid is a fish. Additionally, we outlined the squid general characteristics and adaptations. We also pointed out the top two most mysterious squid species with their specificities.

Hope you enjoyed this post! Feel free to drop us a comment below!

Frequently Asked Questions (FAQs): Is a Squid a fish?

How many species of squid are there?

To date, there are about 812 known squid species in the world.

How many tentacles does the Giant Squid have?

Giant Squids are gigantic cephalopods with eight arms and two tentacles.

What is the largest octopus that ever lived?

The largest known octopus that ever lived is Giant Pacific Octopus. This species inhabits the northern Pacific Ocean. The largest individual found measured around 10 metres and weighed more than 270 kilograms.

How many hearts does a squid have?

All the squid species have three hearts.

What is the difference between squids and octopuses?

There are many differences, they are listed below:

–       Squids have an elongated body; Octopuses, rounded bodies;

–       Squids have eight arms plus two tentacles and fins; Octopuses, only eight arms;

–       Squids inhabit the water column; Octopuses craw on the ocean bottom;

–       Squids are classified as Teuthida; Octopuses, as Octopoda.

What is the smallest squid in the world?

The smallest known squid species is the Thai Bobtail Squid, scientifically known as Idiosepius thailandicus. When full-growth, a female of Thai Bobtail Squid has a 10 millimetres mantle and a male, 7 millimetres.

References 

Rosa, R., & Seibel, B. A. (2010). Slow pace of life of the Antarctic colossal squid. Journal of the Marine Biological Association of the United Kingdom, 90(7), 1375-1378.

Hochberg, F. G., & Brusca, R. C. (1998). Class Cephalopoda.

Remeslo, A. V., Yakushev, M. R., & Laptikhovsky, V. (2015). Alien vs. Predator: interactions between the colossal squid (Mesonychoteuthis hamiltoni) and the Antarctic toothfish (Dissostichus mawsoni). Journal of Natural History, 49(41-42), 2483-2491.

O’SHEA, S., & BOLSTAD, K. S. 2008. Giant squid and colossal squid fact sheet. The octopus news magazine online.

Vasconcelos, Y. 2018. Qual a diferença entre polvo e lula? https://super.abril.com.br/mundo-estranho/qual-a-diferenca-entre-polvo-e-lula/

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