What is a fish?
In the most basic biological terms, a fish can be defined as a marine or aquatic animal that has a backbone, gills, and fins. However in fisheries, the term fish is used to classify any marine or aquatic animal that is harvested, this includes invertebrates like crustaceans (https://t.me/espedition/988) and mollusks, which are collectively identified as shellfish. Fish that conform to the biological definition are often referred to as true fish or finfish.
1️⃣ the Superclass Agnatha (jawless fish, 105 known species).
These are the most primitive fish that still live today. The fish lack a jaw, have cylindrical and elongated bodies (like Eels or snakes), lack scales, and have no paired fins. Agnathan fish like the Hagfish and Lamprey use suction to feed with their round, muscular mouth and primarily live along the seafloor.
2️⃣ the Class Chondricthyes [kondrˈɪkθiz] (cartilaginous fish, at least 928 species).
This group of fish includes Sharks, Rays, Skates, and Ratfishes. These fish are characterized by a cartilaginous skeletal structure, moveable jaws with well-developed teeth, lateral fins, and tiny scales. The tiny scales give the fish sandpaper like skin. Chondricthyan fish lack swim bladders, a gas-filled sac which the fish is able to adjust to prevent from sinking or rising. Instead, the fish have large livers that hold a large volume of oil, which helps to prevent the fish from sinking to the seafloor. Chondricthyan fish are found throughout the world’s oceans at almost all depths. Both commercial and recreational fisheries exist for Sharks and Skates.
3️⃣ the Class Osteichthyes [ɑstiˈɪkθiz](bony fish, over 27,000 species)
It makes up 96% of all known fishes and nearly half of all vertebrates. These fish have skeletons that are composed partially, if not entirely, of bone. Other characteristics of these fish include thin, flexible overlapping scales that develop from bone, the presence of an operculum that protects the gills, fins composed of bony spines rather than flesh, and a swim bladder. Osteichthyan fishes are extremely diverse and have adapted to nearly every type of marine environment. These fish make up most of the commercial and recreational fisheries around the world.
gill = the organ through which fish and other water creatures breathe
fin = a thin, triangular part on a fish, which helps it to swim
cartilaginous = made of cartilage = firm, flexible connective tissue
scales = each of the small, thin horny or bony plates protecting the skin of fish and reptiles, typically overlapping one another
operculum = gill cover