Is this a shark? And, if yes, what kind of shark is it?
On this and linked pages you will find general and simplified information about the class of cartilaginous fishes. Specific information about the orders, families and genera of sharks with their typical characteristics give you an overview of the family tree of sharks and facilitate the identification of an unknown shark.
The class of cartilaginous fish separated from the class of bony fish about 440 million years ago. The over 1,200 species of cartilaginous fish are divided into sharks, skates, rays and chimeras. Sharks and rays occur in the sea from the deep sea to the shallow water zones. Some species also live temporarily or permanently in freshwater. Chimeras live primarily in the deep sea. Most species are hunters, but some feed on plankton. Due to their slow reproduction, well over half of all cartilaginous fish are on the Red List of Endangered Species of the IUCN.
Chimaeras are grouped in the subclass Holocephali. There are only a little over 50 chimaera species worldwide. The last common ancestor of chimaeras, sharks and rays lived about 420 million years ago.
The name chimaeras fits this class of cartilaginous fish, because they also have features of bony fish. Thus, they are the only cartilaginous fish that do not have 5 to 7 gill slits but a gill plate (like bony fish) with a gill opening. Chimaeras have long bodies with a dominant head and grow up to 150 cm long. However, in many species the very long and thin tail, with the cartilaginous spine ending in the upper lobus, makes up a large part to its length. The pectoral fins are large and expansive. In front of the first dorsal fin is a large, venomous spine for defense. The upper jaws are fused to the skull. They have one tooth plate in the lower jaw and two in the upper jaw. The placoid scales or skin teeth typical of cartilaginous fish are almost completely absent, making their bodies very smooth. Skin teeth are found only on the reproductive organs (claspers) of males. Chimaeras all have internal fertilization and the females lay eggs, which are protected by a leathery egg case. Chimaeras are inhabitants of the temperate zones and most species live at depths up to 2,600 m.
Skates and rays, along with sharks, belong to the subclass of Elasmobranchii. Today, over 630 species of skates and rays are known, almost all of which are endangered. Skates are of the order Rajiformes, rays are composed of the orders sawfish (Pristiformes), stingrays (Myliobatiformes) and electric rays (Torpediniformes). Skates are egg-laying (oviparous) and reproduce with eggs in a species-specific, leathery egg case. Rays are ovoviviparous, with the young maturing in eggs but hatching in the female and being born alive. Externally, the simplest distinguishing feature is the tail. Skates have a long, whip-like and fleshy tail without spines, while rays all have spines on their tails. Skates are also usually smaller than rays and their dorsal fins are clearly visible.
In general, skates and rays are very flat and broad and their large pectoral fins are fused directly to the head. They have 5 gill slits on each side. Bottom-dwelling species often suck water through a spiraculum (a type of snorkel) on the top of the body through the gills. They move around by flapping or undulating their large pectoral fins. Many are bottom-dwelling and distributed worldwide as far north as the polar regions. Some species live in the deep sea, others are also found in brackish water and freshwater.
The main reason for their endangerment is national and international fishing. The meat and fins of skates and rays are even traded as delicacies in some countries.
Sharks have adapted to their aquatic habitat over 440 million years. Today, over 500 species of sharks are known, many of which are threatened due to their slow reproductive rate. They inhabit the deep sea as well as reefs, high seas and some species are even found in fresh water. Like skates, rays and chimaeras, they have a cartilaginous skeleton that is secondarily ossified only in highly stressed areas. Their best external identifying feature is the 5 to 7 gill slits. After that, it gets tricky because sharks have become extremely diversified. Sharks can grow from a few centimeters (lantern shark) to about 14 m (whale shark) long. They feed on fish, a few (tiger shark or white shark) on mammals and others on mollusks or crustaceans. Megamouth sharks, whale shark or basking sharks are pure plankton feeders. Some are more sluggish, often well-camouflaged bottom-dwellers, while others, like many deep-sea sharks, are streamlined, high-performance swimmers. To learn more about shark biology, click here.
Below you will find the main external characteristics of the 8 orders. From there you will continue to the families and genera:
External characteristics of the 8 different shark orders