SQUIDS: CHARACTERISTICS, HABITAT, SPECIES, REPRODUCTION, NUTRITION - BIOLOGY - 2025

The squid, also known as teutid, are animals belonging to the order Teuthida, in turn, it is within cephalopods. Morphologically they have a certain resemblance to other cephalopods such as octopuses.

This order was first described by the Swiss zoologist Adolf Naef in 1916. It has approximately 300 described species, of which some still remain almost unknown to science. This is because many live in the depths of the seas, which makes observation in their natural habitat and study very difficult.

Examples of squid. Source: Miguel Hermoso Cuesta

Some squid are endowed with an excellent defense mechanism. When they feel in danger, they release a kind of dark ink, which misleads their potential predators and allows them to flee. Some also have the ability to camouflage themselves with the external environment in order to go unnoticed.

Taxonomy

The taxonomic classification of squids is as follows:

-Domain: Eukarya.

-Animalia Kingdom.

-Filo: Mollusca.

-Class: Cephalopoda.

-Subclass: Coleoidea.

-Superorden: Decapodiformes.

-Order: Teuthida.

characteristics

Members of the order Teuthida are animals whose cells are eukaryotic. They are also multicellular because they are made up of various types of cells, which perform a wide range of organic functions such as reproduction.

If its embryonic development is studied, it is possible to affirm that they are triblastic and coelominated animals. This is because when they are being formed inside the egg, they present the three known germ layers (endoderm, ectoderm and mesoderm). They also have an internal cavity known as a coelom, in which their organs are contained.

In this same order of ideas, squids belong to the group of animals that have bilateral symmetry. This is verified by drawing an imaginary line along the longitudinal axis of the animal, and it can be seen that it is made up of two exactly equal halves.

Squids are heterotrophic organisms because they feed on other living beings or on the substances made by them. In this sense, they are clearly carnivorous.

Taking into account their reproduction, they are dioecious organisms, which means that the sexes are separated, with female individuals and male individuals with well-defined characteristics. They are also internally fertilized, oviparous and have a direct development.

Regarding its life span, specialists have estimated that this is quite short, with the standard expectation of a squid being around 1 year. The species that live the longest usually reach up to 2 years.

Morphology

Squids are animals that can sometimes reach large sizes. Some are so small that they only measure 2 cm, while others are so large that they can reach 20 meters. They are characterized by having an elongated body in the longitudinal direction.

- External anatomy

The body of squid is divided into three regions: the visceral mass, the head and the arms.

Visceral mass

In most squid, the visceral mass is the largest portion of the body. It is covered by a kind of layer that has its embryonic origin from the ectoderm. It has fins that allow the squid to speed up its movement.

The mantle has some openings, such as the paleal opening, which gives rise to the gills and the siphon, which is used to propel itself through a jet.

Head

It is small in size compared to the visceral mass. On both sides are the two eyes of the squid, which are large. It also has arms or tentacles in its anterior part. In the middle of these, you can see the opening of the mouth, which contains a sharp, dark-colored beak.

Squid. The visceral mass, the head and the arms are observed. Source: Betty Wills

arms

They have eight arms and are covered by suction cups. In some species, the arms are also covered by thorns. Squids have two tentacles that serve them for their feeding process. In the arms there are a large number of muscle fibers, which are responsible for its movement and precision.

- Internal anatomy

Digestive system

The digestive system of squid is complete. It starts in the mouth, which has a beak that allows food to be cut. The mouth is followed by the pharynx and later the esophagus. Later is the stomach and then the cecum (intestine).

After the intestine comes the rectum, culminating in the anal opening.

Circulatory system

Squids have a closed type circulatory system. Unlike other invertebrates, it has three hearts. Two of these hearts are in charge of sending blood to the gills for gas exchange to occur, while the other heart pumps blood to the rest of the body.

Nervous system

The squid's nervous system is very well developed. It has some ganglia and a brain. Its cells (neurons) are large, constituting the largest in the animal kingdom. Its operation is similar to the human nervous system with regard to transmission of information and nerve impulses.

Respiratory system

The type of respiration that squid have is gill. The gills are housed inside the paleal cavity. These are nothing more than lamellae in which the gaseous exchange between blood and water takes place.

Reproductive system

The reproductive system is in the part of the body known as the visceral mass. As already mentioned, they are dioecious, so the sexes are separate.

In the case of female individuals, the reproductive system is made up of an ovarian sac, from which a channel (oviduct) emerges, which empties into the gonopore. It presents as accessory organs the nesting glands and the oviductal glands.

On the other hand, the male reproductive system presents the testes, Needham's sac, which is where the sperm are stored, the seminal vesicle and a spermatic duct that also leads to the gonopore.

Habitat and distribution

Squids are animals that are widely distributed throughout the planet and are present in most bodies of water.

These have been determined to have a preference for saltwater habitats, although there are a few species that inhabit or near freshwater bodies. Such is the case of Lollinguncula brevis, which is found in waters with little saline content, near the mouth of rivers.

Likewise, squid can be found in any type of marine environment, regardless of temperature. There are them in warm tropical waters and in cold waters, close to the poles. For example, the largest squid are found in regions close to Antarctica, which indicates that low temperatures are not a limiting element for them.

The location of these has meant an inconvenience for specialists, since many species prefer to be located at great depths (more than 1000 meters). This reason has given rise to the creation of certain legends, with giant squid being the main protagonists of these.

In addition, squids are not limited to a single habitat, but are able to move from one to another according to the availability of food.

Classification

The Teuthida order is made up of two suborders, within which there is a limited number of families.

Suborder Myopsina

Among the distinctive characteristics of this suborder it can be mentioned that their eyes are covered by a corneal membrane that is transparent and they also lack a second eyelid.

Likewise, in the case of females, they have two types of nesting glands: the main ones and some accessory ones. Its size is very varied, there are very small species that fit in the palm of the hand, while others are as big as a child.

This suborder in turn is made up of two families: Australiteuthis and Loliginidae.

Suborder Oegopsina

The individuals that belong to this suborder present some characteristics that distinguish them from others, such as, for example, their eyes are not covered by any corneal membrane.

Likewise, the tentacles and arms have hooks and in the case of female individuals, they have oviducts arranged in pairs. Another of its distinctive elements is that on the head they do not have a bag for the tentacles, being a key aspect that allows them to be totally differentiated from other types of squid.

The suborder is made up of a total of 26 families, which group approximately 70 genera.

Reproduction

The type of reproduction that is appreciated in squid is sexual. This involves the fusion of male and female sex cells (gametes). They also present internal fertilization, are oviparous and have a direct development.

The squid reproduction process is conditioned by the climatic seasons, with summer and spring selected for this to occur. Of course, the squid that live in tropical waters are not influenced by this, since the conditions are in place at all times of the year for them to reproduce.

Mating rituals

Squids are one of the groups of animals that present the most interesting mating rites in the animal kingdom. These include intermittent color change, as well as swimming movements.

Swimming is one of the most observed rituals in these animals. What happens here is that the males begin to swim frantically from one side to the other, trying to attract the attention of the expectant females.

Eventually, they enter the game and begin swimming alongside the males. Little by little, couples are formed so that copulation begins.

Fertilization and spawning

Once the different individuals have mated, that is when copulation finally occurs. For this, the male generates and stores several spermatophores. A spermatophore is a kind of capsule in which sperm are contained. The number of spermatophores that the male can produce varies according to each species.

Also, according to the species, the copulation process can have some variants. There are species in which the male has a shorter tentacle known as the hectocotyl, which he uses to take the spermatophores and introduce them into the female's body.

Once fertilization occurs, the eggs are expelled by the female. It does not place all the eggs in one place, but rather distributes them in various places, trying to hide them from predators.

The number of eggs that each female can lay is abundant, although it varies according to the species. It is generally accepted that a female can lay up to more than 40,000 eggs.

However, most do not develop into adult squid, since environmental conditions can be adverse and predators are constantly on the lookout to feed on the small squid or the eggs.

Importantly, in most species, once the female lays her eggs, she dies.

Nutrition

Squids are known predators of the seas in which they live. They feed on a large number of animals such as fish and small invertebrates (shrimp, crabs, mollusks). There are species in which cases of cannibalism have even been recorded.

The essential tool of squid to capture their prey are their tentacles, which, with the help of their suction cups, help to fix the prey and thus prevent them from getting loose.

Once the prey has been captured, it is directed towards the squid's mouth and ingested. From the mouth, food is transported into the pharynx and then into the esophagus. From there, food passes into the stomach, which is where digestion actually takes place. This is so because it is in the stomach where the food is subjected to the action of the various digestive enzymes.

Subsequently, the already processed food is absorbed at the level of the intestinal cecum. As in any digestive process, there are food particles that are not assimilated, constituting these waste substances.

These unabsorbed substances are expelled through the final part of the digestive system, the anus.

Featured species

Giant squid

Giant squid are not a single species, but a genus: Architeuthis. Within this there are a total of eight recognized species. As their name implies, they can reach a large size, in the case of males, they can measure up to 10 meters and the females even about 15 meters. Exceptionally, specimens that have measured up to 20 meters have been described.

Representation of a giant squid. Source: Locutus Borg

The giant squid has a predilection mainly for waters with low temperatures. They are generally located very close to the seabed and can rarely be seen on the surface. They are very efficient predators and roam freely in the sea, since they have only one predator: the sperm whale.

Mesonychoteuthis hamiltoni

This is known as the colossal squid and is the largest squid species on record so far. The studied specimens have reached lengths of up to almost 15 meters in length and almost a ton in weight.

Its tentacles have suction cups that have hooks, which allow it to adhere firmly to prey and to various surfaces. They also have the largest eyes that have been described in any living animal.

They are located in the southern hemisphere of the planet, specifically in the Antarctic Ocean. However, so few specimens of this species have been studied that many aspects remain unknown to science.

Taonius borealis

This is one of the most curious squid species. Its body is translucent, allowing you to visualize the organs inside. Its tentacles are small in size compared to its body.

It also has wide eyes, which it can move from the lateral position, on both sides of the head, to place them next to each other. Likewise, this squid has photophores throughout its anatomy, which allows it to emit light. Its habitat is the North Pacific, so it is adapted to cold temperatures.

References

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4. Laptikhovskii, V. (2002). Ecology of decapod reproduction (Cephalopoda: Teuthida, Sepiida). Zoologicheskii zhumal. 81 (11)
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