The basking shark, Cetorhinus maximus, is the second largest living shark, after the whale shark. It is a cosmopolitan species — it is found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder.
This shark is called the basking shark because it is most often observed when feeding at the surface and appears to be basking. It is the only member of the family Cetorhinidae. It was first described and named Cetorhinus maximus by Gunnerus in 1765 from a specimen found in Norway. The genus name Cetorhinus comes from the Greek, ketos which means marine monster or whale and rhinos meaning nose, the species name maximus is from Latin and means "greatest". It was later described as Squalus isodus by Macri in 1819, Squalus elephas by Lesueur in 1822, Squalus rashleighanus by Couch in 1838, Squalus cetaceus by Gronow in 1854, Cetorhinus blainvillei by Capello in 1869, Selachus pennantii by Cornish in 1885, Cetorhinus maximus infanuncula by Deinse & Adriani 1953, and finally as Cetorhinus maximus normani by Siccardi 1961.
The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters around the continental shelves. It has traditionally been observed in waters between 8 and 14° C (46 and 57° F) but recently they have been confirmed to cross the equator. It is often seen close to land and will enter enclosed bays. The shark will follow concentrations of plankton in the water column and is therefore often visible on the surface. They are a highly migratory species leading to seasonal appearances in certain areas of the range. The basking shark is found from the surface down to at least 910 metres (3,000 ft).
The basking shark is one of the largest known sharks, second only to the whale shark. The largest specimen accurately measured was trapped in a herring net in the Bay of Fundy, Canada in 1851. Its total length was 12.27 metres (40.3 ft), and it weighed an estimated 19 tons. There are reports from Norway of three basking sharks over 12 m (the largest being 13.7 m), but those are considered dubious since few if any sharks anywhere near such size have been caught in the area since. Normally the basking shark reaches a length of between 6 metres (20 ft) and a little over 8 metres (26 ft). Some specimens surpass 9 or even 10 metres (33 ft), but after years of hard fishing, specimens of this size have become exceedingly rare.
These sharks possess the typical lamniform body plan and have been mistaken for great white sharks. The two species can be easily distinguished, however, by the basking shark's cavernous jaw (up to 1 m in width, held wide open whilst feeding), longer and more obvious gill slits (which nearly encircle the head and are accompanied by well-developed gill rakers), smaller eyes, and smaller average girth. Great white sharks possess large, dagger-like teeth, whilst those of the basking shark are much smaller (5–6 mm) and hooked; only the first 3 or 4 rows of the upper jaw and 6 or 7 rows of the lower jaw are functional. There are also several behavioural differences between the two. Other distinctive characteristics of the basking shark include a strongly keeled caudal peduncle, highly textured skin covered in placoid scales and a layer of mucus, a pointed snout (which is distinctly hooked in younger specimens), and a lunate caudal fin. In large individuals the dorsal fin may flop over when above the surface. Colouration is highly variable (and likely dependent on observation conditions and the condition of the animal itself): commonly, the colouring is dark brown to black or blue dorsally fading to a dull white ventrally. The sharks are often noticeably scarred, possibly through encounters with lampreys or cookiecutter sharks. The basking shark's liver, which may account for 25% of its body weight, runs the entire length of the abdominal cavity and is thought to play a role in buoyancy regulation and long-term energy storage. In females, only the right ovary appears to be functional: if so, this is a unique characteristic among sharks.