Author: Bee Smith
Basking Shark
Cetorhinus maximus
This species of shark is the ultimate riddle: It can be found basking in the sunshine but can also be found below 1000m, it swims slowly but can also jump completely out of the water, it has teeth but doesn’t use them and feeds on plankton, and it is the second largest shark species but has a brain the size of a walnut… it’s the basking shark!
All Images Credit: Andy Murch
Key Features & Appearance
The basking shark is the world’s second largest species of shark, after the whale shark. It is said that they can reach 40ft/12m but typically they reach a maximum of 30ft/9m. And like the whale shark, they are one of the only three species of shark that are filter feeders (the third being the megamouth). They have a wide jaw with long gill slits that go all the way under the throat and they swim with their mouth open so that water can flow through and their gill rakers can filter zooplankton from the water. It is named after its habit of appearing to bask in the sunshine whilst slowly feeding on plankton near the ocean surface. It is generally a slow moving species, but does have the ability to push its body entirely out of the water, known as breaching. They have the smallest brain-to-body size of any shark, likely due to their passive lifestyle.
Habitat & Distribution
They inhabit warm-temperate waters around the world and are a coastal-pelagic species, spending time near shore but also undertaking open ocean migrations.
Though they are named after their habit of hanging out at the surface, they are known to dive beyond 1000m, with a record of 1,504m. Some of these dives follow diel vertical migration or reverse diel vertical migration, suggesting they are following the daily movements of zooplankton up and down the water column. They are also known to do yo-yo dives, which are rapid dives from surface to depth repeatedly with little time at top or bottom, as well as spending weeks or months in deeper water.
Diet
As stated, basking sharks are filter feeders and use passive feeding where they pass water through their mouth by swimming forwards, also known as obligate ram feeding. Their diet consists of zooplankton, predominantly copepods (with their favourite being calanoid copepods) but also fish eggs and larvae.
They actually do have teeth, hundreds of them! But they are all very small at about 5mm. These teeth may be vestigial and just left over from evolution or they might have a purpose in mating…
Reproduction
Basking sharks are usually solitary but sometimes they come together to form large aggregations at the surface, anywhere from a few sharks up to hundreds of individuals. When they do this they are known to form a three dimensional ring structure, called a ‘torus’, formed of nose-to-tail following, parallel swimming and echelon swimming (where one shark is to the side and slightly behind the other) as well as touching fins. And it is thought that these might be courtship behaviors. However, they may also be for feeding-related hydrodynamic advantages.
Sharks mate by internal fertilization and so male and female sharks have to lie stomach to stomach, which can be tricky underwater, so male sharks often bite females to hold onto her and this could be a function for the basking sharks teeth. We have never actually seen basking sharks mate, or give birth. We know that they are ovoviviparous, meaning that the babies grow inside eggs that are inside the mum, then hatch from these eggs and are given birth to later. And it is believed that basking sharks may live for 50 years!
Threats
Fisheries for basking sharks existed in the North-East Atlantic from the 1700s to 1900s, and they were targeting them for their large oily livers in order to use the oil for lighting. From the mid 1940s to when the fisheries closed in 1995 over 100,000 basking sharks were killed. Basking sharks used to be so abundant that they were considered pests due to getting tangled in fishing gear targeting other species. In Canada this led to a federal eradication program in the 1950’s and 1960’s, involving patrol vessels using a blade mounted on the bow to slice basking sharks in half, killing around 400.
Today basking sharks are protected in many countries and their populations are recovering, though they are likely still much lower than they were historically. The main threat facing them today is shark finning, though the trade of their fins is regulated by the Convention on International Trade in Endangered Species of Wild Fauna and Flora. They still face the threat of entanglement in fishing gear as well as new threats such as consuming microplastics when they filter feed.
Status
The basking shark is classified as “Endangered” by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and it is on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix 2 which controls the trade of its products.
Fun Fact
In 2023 we learnt that the basking shark is warm blooded (it has regional endothermy). Some other sharks also have this trait, the white sharks like great whites and makos and the thresher sharks. However these species are fast-swimming apex predators, and this trait is thought to enable a fast, predatory lifestyle. So we are still working out why basking sharks have this trait…
Works Cited
Dolton, H. et al. (2023) ‘Regionally endothermic traits in planktivorous basking sharks Cetorhinus Maximus’, Endangered Species Research, 51, pp. 227–232. doi:10.3354/esr01257.
Sims, D. W. et al. (2022). Circles in the sea: annual courtship “torus” behaviour of basking sharks Cetorhinus maximus identified in the eastern North Atlantic Ocean. Journal of Fish Biology, 101(5), 1160–1181. https://doi.org/10.1111/jfb.15187
Sims, David. (2008). Sieving a Living: A Review of the Biology, Ecology and Conservation Status of the Plankton-Feeding Basking Shark Cetorhinus Maximus. Advances in marine biology. 54. 171-220.
Matthews, L.H. and Parker, H.W. (1950), Notes on the anatomy and biology of the Basking Shark (Cetorhinus maximus (Gunner)).. Proceedings of the Zoological Society of London, 120: 535-576. https://doi.org/10.1111/j.1096-3642.1950.tb00663.x