Discovery Science: Earth Biology – Animals – Vertebrates

Discovery Science: Earth Biology – Animals – Vertebrates

Vertebrates include many of the best known species; how- ever, they make up only a small percentage of all animals. All vertebrates have a spinal column.

This is the central element of the body and connects all parts of the skeleton together. Fish, amphibians, reptiles, birds, and mammals are all vertebrates.

Earth Science: Biology – Animals – Fish

Fish are the oldest and most varied group of vertebrates with over 30,000 different species. About 60 percent of fish species live in saltwater; the rest live in freshwater.

Fish exist in all aquatic habitats and are cold-blooded animals that breathe through gills. The majority have bony structures but a few-such as sharks and rays-have flexible cartilaginous skeletons and usually live in oceans.

Bony fish inhabit environments from freshwater lakes and rivers to the ocean depths as deep as 13,000 feet (4,000 m). Some migrate between fresh and saltwater habitats.

Anatomy of a fish

Like all vertebrates, a fish’s body is built around its backbone. Support is provided by ribs and free “fish bones.” Their body shapes vary greatly and are adapted to their particular habitat. Some bottom- dwelling species, like the flounder, are flat, whereas the pike and other swift hunters have torpedo-shaped bodies.

Fish swim by flexing their bodies and pushing forward with their tail fins, using the paired pectoral and pelvic fins to maneuver. The dorsal, anal, and caudal (tail) fins help to stabilize them.

The lateral line on their bodies has a row of organs to sense water currents and detect the positions of objects such as the seafloor and other creatures. Most have a thin skin covered with protective scales, but some bony fish have skin embedded with small, overlapping bony plates.

Nutrition and reproduction

Fish feed off plants, plankton, or other fish—the position of their mouths is a good indication of their diet. Fish tend to reproduce outside their bodies. The female deposits her eggs in the water while a nearby male releases his sperm at the same time.

A few fish, including the guppy and some sharks, give birth to fully formed young. The male fertilizes the eggs inside the female using an organ called a gonopodium and the eggs remain inside the mother until they are ready or almost ready to hatch, a process called ovoviviparity. After gestation, their offspring can usually swim within 24 hours.


Aquatic animals have gills for breathing. Due to the slow dispersion of water, the thin gill filaments need to be flushed through with water all the time.

Tiny cell projections (cilia) are used for ventilation to achieve flushing of the gills. Sharks and rays have a pumping mechanism for this purpose. Gills are also used for salt regulation, excretion of waste products, and, in some species to take in food particles