What do basking sharks eat?
Basking sharks eat plankton. Plankton is composed of very small animals (zooplankton) and microscopic plants (phytoplankton). Their preferred food is zooplankton, which is a rich mixture of shrimps, very small fish, animal eggs and larvae. When scientists have examined the pink soup-like stomach contents of basking sharks they have found a macerated mass of zooplankton, small fish, fish eggs and, in one case, an eel. We are used to seeing basking sharks feed at the surface but plankton occurs throughout the water column and as pelagic deep-sea shrimps have been found in basking shark stomachs in Japan, it is apparent that they feed at depth as well as at the surface. Dr David Sims and his team of scientists at Plymouth University have been studying the basking shark since 1996. They are particularly interested in their feeding behaviour. This section draws heavily on their work.
This basking shark is feeding on planktonic shrimp. The camera flash reflects off tiny particles such as plankton and shows them as little flecks in the water. Niarbyl, Isle of Man 2003. Picture: Shane Stigant.
The huge basking shark feeds on zooplankton such as these planktonic shrimp. These are photographed through a microscope, magnified about 15 times bigger than real life. They are actually
about half the size of an uncooked rice grain. Picture: Marine Conservation Society.
Scientists had assumed that basking sharks fed indescriminately but Dr Sims and his team found that basking sharks foraged preferentially and for longer in areas where their favourite shrimp prey species, Calanus helgolandicus, was more abundant than other species in the plankton. C.helgolandicus is a species of small (about 1-2mm long) shrimp called a calenoid copepod. It is about the same size as as half a gain of uncooked rice. When the scientists looked at the type of plankton present where basking sharks were feeding they found more and bigger plankton, with more of their preferred prey species, C.helgolandicus, than they did when sampling plankton where sharks were just swimming, not feeding.
Basking shark feeding knowledgebase
Click on the links below to find out more about basking shark feeding, how, where, the anatomy of the mouth and much more.
- How do basking sharks feed?
There are few more dramatic sights than a 7m long feeding basking shark coming towards you! When feeding at the surface the basking shark swims slowly along with the mouth wide open.
A basking shark typically swims with its mouth open for 30 to 60 seconds before closing its mouth and swallowing. Divers have reported that if the water is clear, indicating low plankton concentrations, the swallowing rate is slower, occurring every few minutes. When the shark closes its mouth to swallow, it gulps 3-5 times in a characteristic fashion which Maura Mitchell, an Isle of Man scuba diver, describes as ‘Kermit-like”. For those of you who are not familiar with Kermit he was a frog puppet from the Muppet show, popular in the 1980s!
This wonderful photograph, taken off Niarbyl, the Isle of Man, shows the five gill slits rather well! The gullet is closed so that water does not pour into the stomach. Picture: Shane Stigant.
This close-up shows the pink gills in the gill slits. The photographer did not chase or harrass the shark but waited patiently in the water until it swam past. Picture: Maura Mitchell.
A basking shark with its mouth closed, swallowing. Picture: Maura Mitchell.
This type of feeding is called obligate ram filter-feeding. The other large filter-feeding sharks, the whale shark (Rhincodon typus) and the megamouth shark (Megachasma pelagios) feed by a different method, suction-feeding. This means that they don’t have to swim forward fast through their prey in the same way as the ram-feeding basking sharks.
Dr Sims found that the basking shark typically swims slower, at 0.85meters per second (1.9miles per hour) when feeding as compared to 1.08 meters per second (2.4 miles per hour) when swimming without feeding. This was not anticipated by mathematical models that predict fish swimming speeds. Bony teleost fish such as the anchovy swim faster when feeding than when not feeding. Dr Sims concluded that this was because bony fish have just one gill slit opening and filtration apparatus for the water to be forced through, whereas the basking shark has five. As you can imagine this must create considerable drag, thus reducing the swim speed. Do not imagine that 2.4 miles per hour is the top speed of basking sharks. This is just a cruising speed. They are capable of generating huge amounts of power as can be seen when they breach (leap clear of the water during courtship displays).
- The anatomy of a basking sharks mouth and feeding apparatus
Drs Matthews and Parker describe the mouth of an average size basking shark as being 70 cm from the central mid-lip to the angle of the gape and rather more across the mouth from side to side. The palate was 1m long, the tongue 0.8 m long. Up to 1,500 small teeth, approximatly 5mm by 3mm are present on 4cm wide tooth cartilage which looks rather like a pair of thin lips! There are six rows of teeth on the upper jaw, nine on the lower. These teeth are not known to be used for feeding but they are certainly important for grasping mating partners.
If you look closely at the mouth edge of this shark you can see the thin, lip-like area that has about 1,500 very small teeth on it, used only for holding on during mating. Picture: Shane Stigant.
In the mouth, near the rakers, are cells that secrete huge quantities of mucous. Drs Matthews and Parker suggested that plankton filtered out by the rakers become entangled in the mucous. When the mouth is closed the rakers collapse, squeezing the plankton-laden mucous into the mouth where it can be swallowed.
- Where basking sharks feed
Dr David Sims and his team are interested in where and why basking sharks choose to feed where they do. They have found that basking sharks prefer to feed at current ‘fronts’ where two water masses of different temperature meet. When the sea is calm less mixing occurs and the water stratifies into different layers, typically warmer on top, cooler below. This may result in the plankton experiencing low nutrient levels. Therefore plankton levels are higher where waters of different temperatures mix, such as at a thermal front. An excellent definition of a thermal front is given by a scientist named Le Fevre
A thermal front is a region characterised by a larger than average horizontal gradient in water temperature which forms a boundary between warm, stratified and cold, mixed waters.Basking sharks accumulate at tidal fronts where their microplantonic food gathers. Macroplankton such as this lions-mane jellyfish appear at the same places.
Picture: Maura Mitchell.
These ‘fronts’ can be visually identified as narrow ribbons of still water occurring next to an area with small ripples. These fronts may have an oily looking slick of fish eggs floating on the surface. There might be large quantities of floating seaweed debris and Aurelia jellyfish (moon jellies). This has been described as looking like an in-the-water strandline. Plankton concentrations are higher here than in the general water body.
- How dense does the plankton have to be for basking sharks to feed?
Dr Sims has shown that basking sharks do not feed when the plankton concentration is less than 1 gram of plankton per cubic meter of water, presumably because it is energetically not worthwhile so to do. To help you visualise how much food is in 1gram of plankton per cubic meter; it has been calculated that 0.62 gram of plankton per cubic meter equates to about 400 copepod shrimps, so 1 gram of plankton per cubic meter of water equates to approximately 650 planktonic shrimp.
This basking shark is feeding off Niarbyl, on the West coast of the Isle of Man. The water is quite clear and plankton concentrations are probably low. Picture: Shane Stigant.
When basking sharks find a good patch of plankton they make the most of it by ‘zigzaging’ through the water. Picture: Pauline Oliver.
The higher the plankton concentration, the longer the sharks feed. When the plankton reach concentrations of 3 grams of per cubic meter of water the sharks will feed for up to two and a half times longer than when it’s at 1 gram. When the basking sharks find a good place to feed they are seen to stay in the area of high plankton concentrations by zigzagging backwards and forwards in approximately 50m ‘legs’. The technical term for this is area restricted searching, more commonly known as ARS. Dr Sims found that the sharks followed particularly productive frontal areas for up to 20km over two days.
- How do they locate high concentrations of their favourite food?
It used to be thought that basking sharks were indiscriminate filter feeders. It was a commonly held belief that they ate pretty much any plankton that happened to be in their path. This was not an unreasonable assumption as it is hard to understand how they could select which planktonic species to filter from the seawater! Dr Sims and his team have, however, shown that basking sharks elect to feed in waters which contain higher concentrations of their preferred prey species of planktonic shrimp. It is not known how sharks locate high concentrations of plankton and their favourite prey. Their eyes are very small and it is not apparent that they are important in prey selection. Dr Sims and his team hypothesize that they might smell the dimethyl sulpide (DMS) given off by the plant plankton (phytoplankton) when it is being grazed by zooplankton. Drs Matthews and Parker state that “The olfactory organ…..is arranged so that a continuous stream of water enters at a scoop like funnel, passes over the nasal mucosa spread out on a number of plates”. So they have a very specialised ‘nose’ to ensure that they are continuously sampling the smell in the water. If you have read the section on basking shark brains you may recall that the brain region responsible for smell (olfaction) was bigger than the whole of the rest of the brain put together, so smell is obviously very important to them. The other theory that Dr Sims and his team propose is that the basking sharks can detect the muscle activity of their prey using their electroreceptors. Sharks are able to detect electrical activity in the water using their sensory pits, which are called Ampullae of Lorenzini. Drs Matthews and Parker describe how a large mass of these sensory pits fills the whole space above the nose cartilage and under the basking shark’s eye. Clearly electrical detection is very important to the basking shark.
It is known that basking sharks feed at considerable depth as deep-sea shrimp have been found in their stomachs. Dr Sims and his team think that basking sharks undertake deep dives through the various stratified water layers to sample the food potential (smell and electrical signals) of each layer. This enables them to detect whether any sampled water layer contains a food source worth exploiting. The stratification (layering) that occurs between waters of different salinity or temperature would otherwise mean that the sharks would be unable to detect food signals from one water layer whilst they were in another.
- How far do basking sharks travel to find food (and mates)?
It has been known for a long time that basking sharks undertake seasonal migrations from wherever they spend the winter months to their summer feeding and courtship grounds. The few studies which have tracked basking sharks haven’t done it for long enough to be able to see how far they do travel. Dr Sims and his team tagged 5 basking sharks and found that they undertake long journeys both horizontally (up to 3400km) and vertically (up to 750m). Shark Bay Films have made a film called ‘E.Mail from a Shark’ about this innovative research. See the references and contacts section for how to order it. The tags, which cost £3000 each, are shot through the base of the shark’s dorsal fin. This is done with a harpoon fired from the surface. The placement of the tag has no lasting effect on a shark’s health. The tags contain a mini-computer data logger. This collects information about where the shark has been. It also records water temperature and light levels. They are designed to release themselves from the shark after a pre-set period. They then pop up to the surface and send the data information via a satellite to Dr Sims e.mail! The sharks’ tags stayed attached for between 2 and 7 months, covering summer, autumn and winter. One shark travelled 500km in just 10 days with a directed movement towards the continental shelf edge. Another shark, a sub-adult, travelled 1900km in 76 days, tracking the plankton along the shelf edge from the English Channel to Scotland. They found that the sharks, which were tagged off the southern coast of Britain, had huge ranges extending from the sea off France to the Scottish North-Western Isles. They correlated this information with data about plankton levels and found that the sharks moved to, or stayed in, the areas with the highest plankton levels. These areas tended to be around continental shelf and shelf-edge habitats. Dr Sims concluded that the basking sharks he tagged around Britain were ‘local’ British sharks, not seasonal migrants from foreign shores moving into inshore British waters.
- Anatomy of a basking shark stomach
The stomach, as you might expect, is enormous. Drs Matthews and Parker dissected a 7.03 m long basking shark with a stomach 1.8m long and 0.6m wide. In his paper on the reproduction of the basking shark Dr Matthews describes the devastating results of accidentally spilling the “better part of a ton of semi-digested plankton” over his dissection!
The stomach can be described in two halves; first the cardiac sac with contents of approximately half a ton of plankton mixed with mucous. By the time that all the mashed and churned plankton has reached the pyloric limb of the stomach, much of the water has been removed.
- Basking shark liver oil
It is at the next point that the digestive process becomes very different from that found in mammals. Anyone who has observed planktonic shrimp down a microscope will notice that the shrimp contains a shiny globule of oil. Drs Matthews and Parker came to the conclusion that this oil was forced out of the food mass by pressure at the end of the stomach. The oil is then diverted into an organ called the bursa entiana from where it is probably absorbed through the epithelium (cell lining). This could not be more different from the mammalian digestive process. Drs Matthews and Parker were clearly puzzled and they say “it is not obvious how the oil is separated from the rest of the food; nor how the solid matter can pass from the pyloric limb through the bursa to the duodenum without the oil accompanying it. It is remarkable that oil should be absorbed in the gut, proximal to the pylorus, before the bile and pancreatic secretion have acted upon the food”.
Why is basking shark liver so rich in oil: for energy or for buoyancy?
This oil is vitally important to the shark. A four-tonne basking shark will have a one tonne liver full of oil! It was this liver, so rich in commercially valuable oils, that caused the basking shark to be hunted so enthusiastically. The liver is divided into two lobes that extend the whole length of the abdominal cavity. The basking shark does not have solid fat stored under the skin like marine mammals. Its lipid reserves are all stored as oils in the massive liver.
It used to be thought that shark-liver oil was an energy reserve for times of starvation but research performed on many shark species indicates that its main purpose is to keep the shark neutrally buoyant in the water. The teleost bony fishes control their buoyancy with air-filled swim bladders but sharks do not have these. Oils are less dense than seawater, allowing the animal to remain neutrally buoyant. Basking shark liver oil is so effective in this role that the net weight of a 2 tonne basking shark supported in seawater is likely to be just a few kilograms. Sharks control their position in the water by tilting their pectoral fins like aeroplane wings whereas the bony fish change the amount of gas in their swim bladders.