Did you know that fish are the most abundant vertebrates on the planet? Although we cannot see them or distinguish their species as easily as it happens with other groups of animals (such as mammals or birds), the biological richness and diversity of fish in the world is truly amazing. Thanks to their great ability to adapt and move to other habitats, fish occupy all aquatic environments on the planet. From the polar seas to the tropical reefs, passing through areas of estuaries and continental environments such as rivers, lakes and lagoons.
The taxonomy of fish is the scientific classification of this group of animals. This follows the following scheme in order:
- Super Kingdom Eucaryota (organisms made up of eukaryotic cells, with true nuclei)
- Kingdom Animalia (they have embryonic development and the ability to move. They lack chloroplasts)
- Subkingdom Eumetazoa (They present tissues themselves)
- Superphylum Deuterostomy (During the first stages of embryonic development, in them the anus develops first and then the mouth)
- Chordata phylum (They have a dorsal “cord” made up of turgid cells, as well as a hollow neural tube in a dorsal position, gill slits and tail)
- Subphylum Vertebrata (Tinen spine or vertebral column formed by vertebrae)
- Classes Agnatos (jawless fish) / Chondrichthyans (cartilaginous fish) / Osteictians (bony fish)
This class of fish includes those groups of Gnatostomates (fish with a jaw) that have cartilage in their skeleton instead of bones, and are therefore considered chondrichthyan organisms. These fish are also characterized by having their gill slits very developed and extremely visible on either side of the body, or on the ventral side.
Worldwide known, sharks, rays and mantas are the cartilaginous fish par excellence, although this group also includes chimeras and torpedoes or scrapiers. Thus, some of the best known species of cartilaginous fish are:
- Whale shark ( Rhyncodon typus).
- Butterfly stingray (Genus Gymnuridae).
- Chimera ( Hydrolagus colliei ).
- Puelcha torpedo (Genus Tremolina).
The bony fish , as its name suggests, have a skeleton bone , ie, consisting of bones. They are also characterized by having protected gills thanks to the structure of an operculum, also bony, that covers them. This class of fish includes the following groups of fish:
- Actinopterygii: bony finned fish with rays. They constitute the largest group within vertebrates, inhabiting any aquatic environment on the planet, both marine and fresh and brackish water. This group includes species such as salmon (Genus Salmo), trout (Genus Oncorhynchus), sardines ( Sardina pilchardus ), carp (Genus Cyprinus) and tuna (Genus Thunnus) , among many others.
- Sarcopterygians: those fish whose fins are lobed. This group is the “sister group” of tetrapods (four-legged vertebrates), so it was established that the first amphibians originated from primitive sarcopterygian fish. Among the current species that survive in the wild (although always considered as living fossils), the scaly salamander fish ( Lepidosiren Paradoxa ) and the Queensland lungfish ( Neoceratodus forsteri ) stand out.
The cyclostomes or agnatos (jawless fish) fish are characterized by a circular mouth without jaws but with tongue (adapted to suction) and several teeth, one aspect vermiform (worm – like), paired fins underdeveloped and fins odd of the dorsal and anal regions well developed. On the other hand, they are easy to identify because they completely lack scales on their skin and they swim in a meandering way. The gills of cyclostomata fish are shaped like sachets (6 to 7 pairs), and have a single nostril.
It is probably the strangest and least known subclass of all fish, as only lampreys and hagfish belong to the group of cyclostomes .
- Lampreys: includes species such as the Argentine flag ( Geotria chilensia ) and the bag lamprey ( Geotria australia ). Both can reach about 60 centimeters in length, their eyes are highly developed and their only nostril is on the dorsal part of the head. They tend to go up the rivers to reproduce in them, even in the case of marine species.
- Mixinas: highlights the barbosa fish ( Myxine glutinos ), a species that lives on the sandy shores of the Atlantic. It measures between 30-60 centimeters in length, its eyes are hidden under the skin and its nostril is located in the anterior terminal part of its body.
Within the great diversity of morphological, evolutionary, behavioral and habitat characteristics that the more than 27,900 species of fish from all over the world present , we could summarize the main characteristics of these aquatic animals as follows:
- They are poikilothermic animals (“cold-blooded”, they cannot significantly regulate their body temperature), with fin-shaped limbs and appendages, the body generally covered with scales and they breathe mainly through gills.
- They have very variable sizes and shapes: such as Perciformes (groupers), fusiform (tuna), round (puffer fish), elongated (eels and moray eels), flattened (sole, rays and stingrays), or even aberrant shapes, such as the of the abyssal fish, which inhabit the abyssal plains .
- Coloration: due to the fish’s ability to camouflage itself, its coloration will always depend on the ecosystem in which it inhabits and its main predators, thus being able to present blue, bright, dark, metallic, transparent colorations, with stripes or bands, etc. In addition, the diversity of spines that they present is also highly variable.
- Reproduction: most species of fish have separate sexes and fertilization is internal, in which females (and even males) care for and protect their eggs (which can number in the millions), to ensure the survival of the progeny.
- Behavior: there are fish that swim in large groups (schools of fish), others that are territorial and live in caves, and some that present a certain degree of commensalism with other fish or animals with which they live.
- Food: there are large carnivorous or piscivorous predatory fish, as well as plankton filter feeders, omnivorous species and other highly specialized ones. Some become parasites and feed on other organisms (even female or male of the same species).
- Regarding individual or collective defense mechanisms used by fish, they are capable of producing poisonous substances, light, electricity and / or sounds.
- Habitat: fish occupy each and every one of the different aquatic habitats that exist on the planet.