Vertebrates are animals with a backbone and include mammals, birds and fishes.
Considering roughly three-quarters of the earth is covered in water, it should be no surprise that fish are the most numerous vertebrates in the world. According to the Florida Museum of Natural History, there are about 33,000 known fish species in the world, although thousands more likely have yet to be discovered.
One might think since 97 percent of the earth’s water is salty, an equal percentage of fish would be found in saltwater environments. However in reality only about half of all fish species are marine. Recent research has revealed that fish diversity only exploded in the last 100 million years or so. But strangely, all the marine ray-finned fish that we see today appear to be derived from a freshwater ancestor. This fish appeared about 300 million years ago.
Most saltwater fish live in the nearshore areas, less than 200 meters deep (roughly 600 feet). This is because sunlight can penetrate those waters, so there is more primary production (algae), more oxygen, more food and therefore more biodiversity.
Most fish are cold-blooded (also known as poikilothermic or ectothermic) meaning their internal temperature is the same as the environment around them. Some fast-swimming pelagic fish, meaning they use the entire water column (tunas, for instance) have heat-exchanging retia mirabilia to conserve heat produced by the fish’s metabolism. These fish also have large cutaneous arteries and veins for blood transport between the heart, gills and the heat exchanger.
All living fish are divided into three major groups. The first group, Agnatha, is the most primitive and includes hagfish and lampreys. Agnathids are known as jawless fish. They are essentially eel-looking fish with a suction mouth for a head. Hagfish and lampreys are slimy and lack scales. Most are temperate species where they are parasitic to other fishes. Fossil records indicate that this group of fishes is at least 500 million years old, although the living Agnatha date to about 350 million years ago.
The second group of fish, known as Chondrichthyes, includes sharks, rays skates, ratfishes and chimeras. Sharks, rays and skates comprise a group of Chondricthyes known as elasmobranchs. Chrondrichthyes means “cartilaginous fishes.”The skeletons of sharks and other chondrichthyans are made of cartilage instead of bone. Chondrichthyes are the earliest known jawed fish. Their upper and lower jaws contain a continuous supply of teeth and are made of cartilage like the rest of the skeletal structure.
Unlike most bony fishes that mass spawn, sharks and rays reproduce through internal fertilization. Many sharks and rays give live birth. In some cases, the eggs are held internally but receive no nourishment from the parent (ovoviviparous). Others receive nourishment from the mother (viviparous). Skates more frequently lay egg cases (oviparous) after internal fertilization. The egg cases are commonly called mermaid’s purses. Male sharks and rays have modified pelvic fins called claspers to hold onto the female during mating.
Sharks and rays have placoid scales (also called dermal denticles). The denticles are like teeth that are pointed to the rear. This makes a shark feel like sandpaper if you rub it from back to front. Fossil records indicate sharks date back 400 million years, which makes them older than dinosaurs. Rays and skates are about 150 million years old.
The last group of fish is the Osteichthyes or bony fish. This group includes the primitive (lobe-finned) lungfishes as well as ray-finned gars, sturgeons and the largest group, teleosts. This last group, also called modern fishes, comprises well over 25,000 species. Bony fishes, as its name implies, have a skeleton made of bone. Most bony fishes have scales.
Scales can be cycloid (smooth), ctenoid (feathery) or ganoid (like armor plating). Some, like the catfishes, have no scales and are considered naked. Most bony fish have two sets of paired fins (the pectoral fins, usually behind the gill covers, and the pelvic fins on the belly), in addition to three unpaired fins which aid in locomotion. They also have a swim bladder which they can inflate and deflate, allowing them to move up and down in the water column.
The swim bladder is also used to detect movement, as are ear stones (otoliths) and the lateral line. The lateral line is a sensory organ that allows a fish to detect objects in its environment. Otoliths are made up of calcium carbonate (same as coral). Fish lay down rings of calcium carbonate as they grow just like a tree develops rings. Fish scientists, also known as ichthyologists, age fish by slicing the otoliths and counting the rings.
So there you have it — a little fish vocabulary to start off your weekend of fishing.