por The squid, also known as Teutids, are animals that belong to the order Teuthida, which, in turn, is found within the 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, some of which are still almost unknown to science. This is because many of them live in the depths of the seas, which makes it very difficult to observe them in their natural habitat and study them.
Some squids 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 blend in with the external environment in order to go unnoticed.
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taxonomy
The taxonomic classification of squids is as follows:
-Domain: Eukarya.
-Animalia Kingdom.
-Edge: Mollusca.
-Class: Cephalopoda.
-Subclass: Coleoidea.
-Superorder: 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 their embryonic development is studied, it is possible to affirm that they are triblastic and coelomate animals. This is because when they are forming inside the egg, they present the three known germ layers (endoderm, ectoderm and mesoderm). They also have an internal cavity known as 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 through the longitudinal axis of the animal, being able to observe that it is made up of two exactly equal halves.
Squids are heterotrophic organisms because they feed on other living beings or on substances produced by them. In this sense, they are purely carnivorous.
Taking into account their reproduction, they are dioecious organisms, which means that the sexes are separated, having female individuals and male individuals with their well-defined characteristics. They are also internally fertilized, oviparous and present a direct development.
Regarding its life time, specialists have estimated that it is quite short, with the standard life expectancy 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 a body elongated longitudinally.
– External anatomy
The squid body is divided into three regions: the visceral mass, the head, and the arms.
visceral mass
In most squids, 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 front part. In the middle of these, you can see the opening of the mouth, which contains a sharp, dark-colored beak.
Arms
They have eight arms and are covered by suction cups. In some species, the arms are also covered by spines. 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 their movement and precision.
– Internal anatomy
Digestive system
The digestive system of squids is complete. It begins in the mouth, which has a beak that allows it to cut the food. The mouth is followed by the pharynx and later the esophagus. Posteriorly is the stomach and then the cecum (intestine).
After the intestine comes the rectum, which culminates in the anal orifice.
Circulatory system
Squids have a closed-type circulatory system. Unlike other invertebrates, it has three hearts. Two of these hearts are responsible for sending blood to the gills for gas exchange to occur, while the other heart pumps blood for the rest of the body.
Nervous system
The nervous system of squids 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 in terms of transmission of information and nerve impulses.
Respiratory system
The type of respiration that squids present is gill. The gills are housed inside the paleal cavity. These are nothing more than lamellae in which the gas exchange between blood and water takes place.
Reproductive system
The reproductive system is located 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 canal (oviduct) emerges, which flows into the gonopore. It presents as accessory organs the nidamentary glands and the oviductal glands.
On the other hand, the male reproductive system presents the testicles, the Needham sac, which is where the spermatozoa are stored, the seminal vesicle, and a spermatic duct that also opens into the gonopore.
Habitat and distribution
Squid 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 breviswhich is found in waters with little saline content, near the mouth of rivers.
Likewise, squids can be found in any type of marine environment, regardless of temperature. They exist in warm tropical waters and in cold waters near the poles. For example, the largest squids 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 locate at great depths (more than 1000 meters). This reason has given rise to the creation of certain legends, with giant squids being the main protagonists of these.
Furthermore, squids are not limited to a single habitat, but are capable of moving 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 also lack a second eyelid.
Likewise, in the case of females, they have two types of nidamentary 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 Oegopsin
The individuals that belong to this suborder have some characteristics that distinguish them from the others, such as their eyes are not covered by any corneal membrane.
Likewise, the tentacles and arms present hooks and in the case of female individuals, they have oviducts arranged in pairs. Another of their distinctive elements is that they do not have a bag on their heads for the tentacles, which is a key aspect that allows them to be fully differentiated from other types of squid.
The suborder is made up of a total of 26 families, which group approximately almost 70 genera.
Reproduction
The type of reproduction that can be seen in squids is sexual. This involves the fusion of male and female sex cells (gametes). They also present internal fertilization, are oviparous and present a direct development.
The squid reproduction process is conditioned by the climatic seasons, being summer and spring the ones selected for this to happen. Of course, the squids that live in tropical waters are not influenced by this, since at all times of the year the conditions are given 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. Among these we can mention the intermittent change of color, as well as the movements when swimming.
Swimming is one of the most observed rituals in these animals. Here what happens is that the males begin to swim frantically from one side to the other, trying to get the attention of the females who remain expectant.
Finally, they enter the game and begin to swim with the males. Little by little, couples are formed so that copulation can begin.
fertilization and spawning
Once the different individuals have mated, copulation finally occurs. For this, the male generates and stores several spermatophores. A spermatophore is a kind of capsule in which the spermatozoa are contained. The number of spermatophores that the male can produce varies according to each species.
Also, according to the species, the copulation process may have some variants. There are species in which the male has a shorter tentacle known as a 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. She is she does not lay all the eggs in one place, but distributes them in different 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 more than 40,000 eggs.
However, most of them do not develop into adult squids, since environmental conditions can be adverse and predators are constantly on the lookout to feed on them…