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Originally, fish referred to any animal that spent its life in the water – for example, cuttlefish, starfish, and jellyfish. Today, fish usually refers to a group of animals with gills throughout their life, a backbone (or vertebra), and limbs in the shape of fins. Fish are the oldest and largest group of vertebrates in the animal kingdom, and scientists believe that all terrestrial vertebrates–amphibians, reptiles, birds, and mammals–evolved from fish to live on the land. According to the fossil record, fish have been around for over 500 million years! Interestingly, some of these animals later returned to the ocean to become today’s sea-turtles, whales, dolphins, and sea snakes. Fish are the most diverse and species-rich group of vertebrates; there are more fish species (around 32,000) than mammal, reptile, amphibian, and bird species combined. From the very small cyprinid fish of Indonesia which reaches a maximum length of 1/3 inch (10mm), to the large whale shark which reaches lengths of 40 feet (12 m), the tremendous fish diversity on our planet comes from having such a long time to diversify and the fact that they live in a great variety of habitats.
Living in water where sounds, chemicals, and electricity are more easily dispersed than in air, fish rely less on vision and more on hearing, taste, smell, and for some species, the sensation of electrical charges. Many fish have a special row of scales and sensors–called the lateral line–which they use to detect motion in the water. Fish are typically separated into four groups: cartilaginous fish, bony fish, jawless fish, and hagfish. Since jawless fish and hagfish are rarely seen on Hawaii’s reefs, we will focus on cartilaginous and bony fish. Cartilaginous fish, also known as Chondrichthyes, are characterized by the presence of cartilage tissue rather than bone tissue and includes sharks, skates, sawfish, rays, and chimaeras. Bony fish, also known as Osteichthyes, are characterized by the presence of bone tissue and include the majority of the fish in the world. Aside from their different skeletons, bony fish have an external flap of skin covering their gills, called an operculum, achieve neutral buoyancy using an internal swim bladder, have scales on the skin, and usually use external fertilization to reproduce. Contrastingly, cartilaginous fish have exposed gills, achieve neutral buoyancy using their lighter skeleton, more hydrodynamic exteriors, and an oily liver, have dermal denticles covering their skin, and use internal fertilization. On top of these differences are the contrasting heart, blood, jaw, and digestive structures. When referring to fish anatomy, a full set of fins includes two dorsal fins, a caudal, anal, pelvic, and pectoral fin. For more information about fish anatomy, follow this link from Manoa, University of Hawaii.