The Basking shark (Cetorhinus maximus) is a shark belonging to the family Cetorhinidae, and is the sole extant species of its family. The Basking shark is the second-largest living shark, after the Whale shark, and one of three planktivorous shark species, along with the Whale shark and the Megamouth shark. Adults typically reach 20–26 feet in length. They are usually greyish-brown, with mottled skin. The caudal fin has a strong lateral keel and a crescent shape.
The Basking shark can be found world-wide, and is a highly migratory species following coastal and oceanic currents in cold to warm temperate seas. Basking sharks have can be found in aggregations and have complex social behaviors. It is a slow-moving, ram filter feeder. Its common name derives from its habit of feeding at the surface, appearing to be basking in the warmer water there. It has anatomical adaptations for filter-feeding, such as a greatly enlarged mouth and highly developed gill rakers.
Family: Cetorhinidae – Basking Sharks
Genus: Cetorhinus
Species: maximus
Phylum– Chordata
Class– Chondrichthyles
Common Name– Mackerel Sharks
Family– Cetorhinidae
Common Name– Basking Sharks
Genus– Cetorhinus
Species– maximus
Status: IUCN Red List VULNERABLE
Average Size and Length: They are born between 150-170 cm/4.9-5.6 feet. Mature males are around 570 cm/18.7 feet. Mature females have been measured at around 800 cm/26.2 feet. The maximum recorded has been over 1000 cm/32.8 feet. One account claims 40.3 feet in length.
Average Weight: The largest recorded was 38,000 pounds. They typically do not reach these weights today. Typically, they weigh around 5.2 tons or 10,400 pounds.
Current Rare Mythical Sightings: Johan Ernst Gunnerus first described the species as Cetorhinus maximus, from a specimen found in Norway. The genus name Cetorhinus comes from the Greek ketos, meaning “marine monster” or “whale”, and rhinos, meaning “nose”. The species name maximus is from Latin and means “greatest”. Following its initial description, more attempts at naming included: Squalus isodus, in 1819 by Italian Zoologist Saverio Macri (1754–1848); Squalus elephas, by Charles Alexandre Lesueur in 1822; Squalus rashleighanus, by Jonathan Couch in 1838; Squalus cetaceus, by Laurens Theodorus Gronovius in 1854; Cetorhinus blainvillei by the Portuguese Biologist Felix Antonio De Brio Capello (1828–1879) in 1869; Selachus pennantii, by Charles John Cornish in 1885; Cetorhinus maximus infanuncula, by the Dutch Zoologists Antonius Boudewijn Deinse (1885–1965) and Marcus Jan Adriani (1929–1995) in 1953; and Cetorhinus maximus normani, by Siccardi in 1961.
The largest accurately measured Basking shark was found in a herring net in the Bay of Fundy, Canada, in 1851. Its total length was 40.3 feet, and it weighed an estimated 19 tons or 38,000 pounds.
On 23 June 2015, a 20-foot, 7,716-pound Basking shark was caught accidentally by a fishing trawler in the Bass strait near Portland, Victoria, in southeast Australia, the first Basking shark caught in the region since the 1930s, and only the third reported in the region in 160 years. The shark was donated to the Victoria Museum for research.
Teeth and Jaw: The mouth is very large with tiny teeth. The Basking shark’s cavernous jaw measures up to 3.3 feet in width. The teeth are numerous and very small, and often number 100 per row. The teeth have a single conical cusp, are curved backwards, and are the same on both the upper and lower jaws. The teeth of the Basking shark measure between 5–6 mm. Only the first three or four rows of the upper jaw and six or seven rows of the lower jaw function.
Head: The snout is pointed and conical. In juveniles, the shout appears hooked. The eyes are small.
Denticles: The skin of the Basking shark is highly textured, covered in placoid scales and a mucus layer. Its large dermal denticles have been known to inflict damage on scientists and divers due to the roughness of their nature.
Tail: The tail is lunate. There are strong lateral keels on the caudal peduncle.
Demographic, Distribution, Habitat, Environment and Range: The Basking shark can be found world-wide in cold to warm, boreal temperate waters from the surface down to 2,990 feet. The Basking shark prefers temperatures of 46 to 58 °F, but has been confirmed to cross the much-warmer waters at the equator. They are considered coastal-pelagic. They can be found on the coast to the continental shelf edge and the slope. They have been recorded to occasionally enter brackish waters, and have even been spotted in bays with narrow openings. They are associated with oceanic fronts and coastal fronts.
Basking sharks are highly migratory. During winter they have been documented performing diel vertical migrations from depths as deep as almost 3,000 feet, following zooplankton. Argos system satellite tagging of 20 Basking sharks in 2003 confirmed Basking sharks move thousands of kilometers during the summer and winter, seeking the richest zooplankton patches, often along ocean fronts (Sims, DW; Southall, EJ; Richardson, AJ; Reid, PC; Metcalfe, JD (2003). “Seasonal movements and behaviour of basking sharks from archival tagging: no evidence of winter hibernation” (PDF). Marine Ecology Progress Series. 248: 187–196).
A 2009 study tagged 25 Basking sharks off the coast of Cape Cod, Massachusetts, and indicated at least some migrate south in the winter. Remaining at depths between 660 and 3,280 feet for many weeks, the tagged sharks crossed the equator to reach Brazil. One individual spent a month near the mouth of the Amazon River. They may undertake this journey to aid reproduction (Skomal, Gregory B.; Zeeman, Stephen I.; Chisholm, John H.; Summers, Erin L.; Walsh, Harvey J.; McMahon, Kelton W.; Thorrold, Simon R. (2009). “Transequatorial Migrations by Basking Sharks in the Western Atlantic Ocean“. Current Biology. 19 (12): 1019–1022.).
Diet: The Basking shark is a planktivore. They feed on deepwater plankton concentrations during the winter. During winter they follow the plentiful zooplankton. They will also filter tiny fish and invertebrates.
Samples taken in the presence of feeding Basking sharks recorded zooplankton densities 75% higher compared to adjacent non-feeding areas. Basking sharks feed preferentially in zooplankton patches dominated by small planktonic crustaceans called calanoid copepods, on average 1,700 individuals per cubic meter of water.
White sharks have been reported to scavenge on the remains of Basking sharks. Killer whales have been observed feeding on Basking sharks off California and New Zealand. Basking sharks are often noticeably scarred, possibly through encounters with lampreys or Cookiecutter sharks.
Ram-Suction Index: The are ram feeders, moving slowly forward with open mouths, collecting plankton.
Unlike the Megamouth shark and Whale shark, the Basking shark relies only on the water it pushes through its gills by swimming. The Megamouth shark and Whale shark can suck or pump water through their gills.
Aesthetic Identification: The Basking shark has a large, typical Lamniform-shaped body. The color of the Basking shark varies. They are darker dorsally and lighter ventrally. There is often a mottled pattern on the back and sides. There are white blotches under the head. The huge gill slits almost encircle the head, with gill rakers. The gill rakers, dark and bristle-like, are used to catch plankton as water filters through the mouth and over the gills. In large individuals, the dorsal fin may flop to one side when above the surface.
Biology and Reproduction: They have huge livers that provide them buoyancy, and help them store energy; 25% of its weight is its liver. The liver runs the entire length of the abdominal cavity.
The Basking shark has the smallest weight-for-weight brain size of any shark, reflecting its relatively passive lifestyle (Kruska, DC (1988). “Brain of the basking shark (Cetorhinus maximus)“. Brain Behav. Evol. 32 (6): 353–63.).
There is much unknown about their reproduction. More than likely they are ovoviviparous. One litter of 6 pups has been reported. They presumably engage in oophagy. In females, only the right ovary appears to function, and it is currently unknown why only one of the organs seems to function. Gestation is thought to span over a year, maybe even two to three years. Mating is thought to occur in early summer and birthing in late summer. The age of maturity is thought to be between the ages of 6 and 13 and at a length of 15–20 feet. Breeding frequency is thought to be two to four years.
The exact lifespan of the basking shark is unknown, but researchers estimate to be about 50 years.
Behavioral Traits, Sensing and Intelligence: The Basking shark is often seen feeding on the surface in aggregations of plankton, moving slowly forward with open mouths, sometimes in very large groups of over 100, but typically in sex-segregated shoals of 3 or 4 sharks. In the northeastern U.S., as many as 1,400 were spotted congregated together.
The Basking shark sheds and renews its gill rakers as an ongoing process, but there is no evidence of winter hibernation.
Complex courtship behaviors have been reported. Small schools in the Bay of Fundy and the Hebrides have been seen swimming nose to tail in circles (Sims, DW; Southall, EJ; Quayle, VA; Fox, AM (2000). “Annual social behaviour of basking sharks associated with coastal front areas“. Proceedings of the Royal Society B. 267 (1455): 1897–1904.).
They have also been reported leaping out of the water. This behavior could be an attempt to dislodge parasites or commensals, but there isn’t enough research or data to support any hypothesis at this point.
Speed: They are slow swimmers, but have been reported to leap out of the water. They have been tracked feeding at about 2 knots or 2.3 miles per hour (Sims, DW (2000). “Filter-feeding and cruising swimming speeds of basking sharks compared with optimal models: they filter-feed slower than predicted for their size“. Journal of Experimental Marine Biology and Ecology. 249 (1): 65–76.).
A 16-foot Basking shark has been calculated to filter up to 450 tons of water per hour swimming at an observed speed of 2.8 ft/s. (Sims, DW (1999). “Threshold foraging behaviour of basking sharks on zooplankton: life on an energetic knife-edge?“. Proceedings of the Royal Society B. 266 (1427): 1437–1443.).
Basking Shark Future and Conservation: They are considered vulnerable, and even endangered in some regions. In these regions their populations have been depleted by target fisheries for meat, oil, and their fins. In the Mediterranean and several countries, they are protected. It is fully protected in the United Kingdom and the Atlantic and Mexican Gulf regions of the United States. Since 2008, it has been illegal to fish for, or retain if accidentally caught, Basking sharks in waters of the European Union. In Norway, it is partially protected.
In New Zealand, Basking sharks had been abundant historically; however, after the mass by-catches recorded in 1990s and 2000s, confirmations of the species depleted significantly. In June 2018 the Department of Conservation classified the basking shark as “Threatened – Nationally Vulnerable” under the New Zealand Threat Classification System.
More recently, the basking shark has sparked an interest in eco-tourism operations.
Basking Shark Recorded Attacks on Humans: Not a threat to humans. Contact with its skin should be avoided, as its large dermal denticles have been known to inflict damage on divers and scientists.