On a superficial level, all species of fish are aquatic vertebrates with a head, body, and tail. For classification purposes, it is useful to divide fish into three main groups: jawless fish, cartilagenous fish, and bony fish.
Jawless fish are considered the oldest species of fish from an evolutionary standpoint. The two main varieties of jawless fish are hagfish and lampreys. Both are essentially parasites, latching on to other sea creatures and sucking nutrients from them directly.
Cartilagenous fish include sharks, skates, and rays. Their skeletons are composed entirely of cartilage; however, sharks are able to regenerate lost teeth. Many sharks shed thousands of teeth over their lifetimes. Unlike sharks, rays and skates do not depend on teeth in order to feed but rather on bony plates that serve the same function. Many species of rays contain a venomous barb on their tails which they use to paralyze prey.
Another interesting point of shark anatomy is the presence of a large, oil filled liver which helps keep the shark submerged. As almost all species of sharks live in salt water (which is more buoyant than freshwater), so remaining submerged requires more adaptations compared to a freshwater habitat. A further adaptation observed in sharks and rays is their ability to concentrate salt in their tissues in order to maintain an osmotic balance with their environment. This metabolic process is not carried out by kidneys, which fish generally lack, but instead by specialized intestinal cells called the spiral valve.
Bony fish represent the majority of all fish species. Their general characteristics include eyes, a rudimentary brain, a bony skeleton, a two chambered heart, gills for gas exchange, and an extensive digestive tract. Most fish release ammonia directly into the water through their gills and therefore have no biological need for a urea cycle or kidneys. In most fish, reproduction occurs by means of external fertilization. Some fish do build nests, and sea horses carry their eggs until they hatch, but few (if any) fish care for their young afterward.
When it comes to hunting and defense, some species of tropical fish in the scorpion fish family produce venom, for example stone fish and lion fish. These venoms usually act as toxins which paralyze or kill their target. Others species, most notably eels, have evolved jelly-like electric organs filled with specialized cells called electroplaques. These cells are arranged in series like rows of miniature batteries. When these cells discharge, their voltages are additive. Although an individual electroplaque may generate a voltage of only 0.15 volts, 5000 of these cells arranged in series can produce a discharge of 750 volts - enough to kill a person.