There are so many types of fish in this world. Fish is a group of animals that breathe with gills. This type of animal only lives in water. This animal can be found both in seawater and in freshwater. In general, there are currently three types of fish in their grouping, mainly from their physical characteristics. The three types of fish are jawless fish, cartilaginous fish, and bony fish. Previously, there were also fish of the armored fish and spiny sharks, but both of these species from the group have become extinct.
Jawless fish has another name; Agnatha from its superclass classification. As the name implies, this type of fish does not have a jaw on the head. Most fish from this group are extinct and only two types of fish are still in existence today. The two types of fish are lamprey and hagfish.
Hagfish, from the Myxini class, are fish-shaped like an eel that secretes mucus on its body and lives in seawater. This type of fish is known as the only animal that lives with a skull but without a spine. Lamprey, from the Hyperoartia class, is the oldest type of fish that still exists from the jawless fish group. An adult lamprey, who also has a shape like an eel, has a circle-shaped mouth like a vacuum and toothed. Most of the lamprey fish are parasitic, by sticking to their host fish by sucking up their blood and even their flesh.
Cartilaginous fish, or fish from the Chondrichthyes class, have the characteristics of a pair of fins, a pair of real nostrils, scaly, and have a heart that has two chambers. However, this fish still has a body frame that consists of cartilage, not true bone. This type of fish is divided into two sub-classes, namely Elasmobranchii and Holocephali.
Sharks, skates, and rays are types of fish originating from the Elasmobranchii subclass. Fish from this subclass has a total of five to seven-gill slits that open one by one. This type of fish is mostly spread in tropical and temperate water climates. Holocephali subclass is a subclass that still survives in its Ordo classification. The type of fish moves by moving its pectoral fins like a sweeping motion. Usually, this type of fish lives at the bottom of the water. Chimaera or ghost shark is one of the Holocephali fish.
Bony fish, or fish from superclass Osteichthyes, are a type of fish that have true bones — as the name implies: bony. Nearly the majority of fish in the world are in this group, around 28,000 species. Bony fish are divided into two groups, namely ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii).
Actinopterygii fish are called ray-finned fish because their fins are webs of skin supported by bony or horny spines (or it called rays). Fish from the group can be found almost everywhere, both seawater and freshwater, from the deepest to the waters in the mountains. The examples are tuna, salmon, and cod. Sarcopterygii fish have two dorsal fins with separate bases, as opposed to the single dorsal fin of Actinopterygii. Unlike Actinopterygii, fish from Sarcopterygii are mostly extinct. The famous species of these types of fish are the coelacanth and lungfish.