Cephalopoda

The cephalopods (Cephalopoda, from the Greek κεφαλή (kephalé), "head" and ποδός ( podós), "foot" → feet on head) are a class of marine invertebrates belonging to the phylum Mollusks. There are more than 800 species today, commonly called octopus, squid, cuttlefish, and nautilus. All belong to the coleoid subclass, with the exception of the nautilus, which belongs to the Nautilina subclass.

Features

Reconstruction of Roads of the Ordovician.

NautilusA living fossil.

In cephalopods, the characteristic foot of mollusks appears next to the head, diversified into several appendages, from 8 in octopuses to 90 that nautiluses can have. In the latter there are no suckers on the tentacles. Some of these appendages (in colloids) have been modified into reproductive structures called ectocotyls and spadices, which fulfill the role of introducing spermatophores (sacs filled with sperm) into the female's paleal cavity.

In the mouth mass they have a structure typical of this group known as a parrot beak, which is a pair of strong beak-shaped jaws that they use to tear the prey so that they are later processed by the radula. The shell It tends to shrink, become internal, or disappear, depending on the species. When they have a well-developed shell, it is divided into chambers separated by septa, and the animal inhabits the last (most recent) chamber. In colloids, when it exists, it is internal and is divided into 3 zones; from the caudal to the cephalic region these are rostrum, phragmoconus (separate) and proostracus, each with variable development in each group. In nautiloids it is external, planospiral and septate in its entirety.

Cuttlefish, along with nautiluses, follow the same swimming system as their ancestors, filling certain parts of their shell with gas to float. Squid swim by means of dynamic buoyancy, similar to sharks, with highly tuned water jet propulsion. The rest of the cephalopods that live far from the surface developed a chemical flotation system, filling the spaces of their body with ammoniacal compounds or oils; Being these substances less dense than water, they float.

Cephalopods have pigment cells on their mantle called chromatophores. These cells have pigments that expand or condense at will by means of a muscular contraction controlled by the nervous system. In this way they can change color in a matter of seconds to blend in with the surrounding space and go unnoticed. They also use this ability to communicate with each other through their coloration and thanks to their keen vision.

They have a complex nervous system, with ganglia around the esophagus that form a true brain. The brain is divided into two parts, called supraesophageal mass and subesophageal mass according to its position with respect to the esophagus, although both parts are joined by connectives. A particular and exclusive feature of cephalopods is that the brain is surrounded by a cartilaginous mass or box in an "intent" evolutionary way of forming a skull. Many cephalopods have rapid flight behaviors that depend on a system of giant motor nerve fibers that control powerful, synchronous contractions of the mantle muscles, allowing water to be forced out of the paleal cavity. The focal point of this system is a pair of first-order giant neurons (formed by fusion of visceral ganglia) giving off second-order giant neurons, and these extend to a pair of large stellate ganglia. From these stellate ganglia, giant third-order neurons innervate the circular muscle fibers of the mantle. Neurologists from all over the world have experimented with octopuses throughout the 20th century and an intelligence superior to any other invertebrate has been detected in them; They are capable of finding their way out of a maze, opening boats, and even learning behaviors from their peers.

The eye of cephalopods is an organ similar to that of vertebrates, of different evolutionary and embryonic origin, but due to convergence both are very similar. Cephalopods have the most developed eye of all invertebrates and even rival that of vertebrates.

They have a muscular and flexible body, a property that is intensified in octopuses, which are capable of hiding in spaces 10 times smaller than their body.

They have low-frequency hearing, like marine mammals, which allows them to locate predators beyond their visual field.

They secrete a blackish liquid, ink, with which they cloud the water in order to hide. The ink is a pigment that is stored in the ink bag located above the rectum and can be expelled through the siphon.

They have a closed circulatory system with a dense network of capillaries. They have a systemic heart for the blood that carries oxygen from the gills to the rest of the body and two branchial hearts that provide greater pressure to the circulation, when the blood enters the gills. Your blood contains dissolved hemocyanin.

In the metabolism of this group, the importance of obtaining energy from the metabolization of proteins is noteworthy, which is not a great evolutionary advantage compared to other groups in their environment such as fish, which oxidize the fats of your adipose tissue. Even so, this seems to be one of the characteristics that has allowed them to conquer such exclusive habitats as great depths, where large specimens are preyed on by pilot whales, beaked whales and sperm whales. This metabolic characteristic is also related to the nutritional properties when it is from this group used for human consumption: low fat content, high protein content, and sometimes ammonia flavor due to the presence of nitrogenous bases, remains of the aforementioned protein metabolization.

Evolution

Amonite fossils.

Cephalopods diverged from the rest of the molluscs around 500 million years ago (Miaolingian), with the appearance of the first molluscs capable of filling certain parts of their shell with gas in order to float. This new swimming ability, which some species still retain today, allowed them to leave the seabed to which the molluscs were attached and access new, more superficial trophic routes.

The latest discoveries indicate that cephalopods originated much earlier than previously thought.

But these first cephalopods, still living near the coast, were displaced into the sea by more advanced organisms, such as fish and marine reptiles. Another problem arose for them: their superficial life prevented them from going down too far to the seabed since their shell could not withstand the pressure of the water. The descendants with smaller shells could go lower and have more nutritional possibilities, so natural selection stayed with those with small shells, which eventually became internal or disappeared. Approximately 470 million years before the present (Middle Ordovician) there were already coleoids, together with a wide range of cephalopods extinct today.^{[citation needed]}

According to the theory of the meteorite that fell on the Yucatan peninsula 65 million years ago (Late Cretaceous), at that time the earth suffered serious climatic changes that caused mass extinctions, like that of the dinosaurs. This meteorite could also be the cause of the extinction of most cephalopods, such as ammonites. Only the coleoids and the nautiloids survived. That is why it is said that the current cephalopods come from a true lineage of survivors. Although they are now a staple food for thousands of species, at the time they were at the top of marine food chains.

Taxonomy

Reconstruction of belemnites.

Squids drying the Sun in Shimane.

Currently, more than 800 current species survive, although their number increases every year, with some living species still to be discovered. Besides, it is estimated that the number of extinct species is around 11,000.^{[citation required]}

Cephalopods are subdivided into three subclasses:

Subclass Nautiloidea

Order Plectronocerida †

Order Ellesmerocercerida †

Order Actinocerida †

Order Pseudorthocerida †

Order Endocercerida †

Order Tarphycercerida †

Order Oncocerida †

Discosure †

Order Nautilida (only non-extinct order) - nautilos

Orthocerida †

Warrant Order †

Order Bactritida †

Subclass Ammonoidea † - ammonites

Order Goniatitida †

Order Ceratitida †

Ammonitide Order †

Subclass Coleoidea

• Cohorte Belemnoidea † - belemnites

Classified Order †

Order Belemnitida †

Order Hematitida †

Order Phragmoteuthida †

• Neocoleoid cohort (most live cephalopods)

Order Boletzkyida †

Sepid Order - Sepias

Sepiolida order - sepias

Spirulide Order - Sepias

Order Teuthida - squid

Order Octopoda - octopus

Vampyromorphide Order - Vampire Squid

Gastronomy

Cephalopods are one of the most prized shellfish. They are eaten alone (roman-style squid, grilled cuttlefish, cuttlefish, Galician octopus, etc.) or as ingredients in other dishes (seafood paella, fideuá, etc.)