Biology & Behaviour
Information
The basking shark, scientific name Cetorhinus maximus, Manx name Gobbag vooar
(big mouth) is the second largest fish in the world and we, in the Isle of Man,
are fortunate enough to have a large number of them coming through our coastal
waters from May to September. They are named basking sharks because of their
habit of 'basking' at the surface of the water. They feed by filtering out
plankton from the water in a similar manner to whales. Despite their huge size
they are completely harmless to man. Unlike bony fish that can produce millions
of young in a year basking sharks are viviparous (live bearing) and therefore
breed exceedingly slowly. They are, therefore, very vulnerable to
over-exploitation.
Basking sharks often bask at the surface apparently doing nothing! This one is
feeding.
If you have a picture of a basking shark just basking
we would be delighted to put it here!
Picture: Pauline Oliver.
Many scientific papers mourn the fact that little is known about these gentle
giants. This isn't completely true as this website draws on 37 scientific
reports about basking sharks and there are many more. Drs Matthews and Harrison
carried out some remarkable dissections in 1947 and their scientific papers
remain the classical works on basking shark anatomy[17,18]. Until recently
there were large knowledge gaps about what basking sharks did in the winter but
currently Dr Sims' team at Plymouth Marine laboratory[24-31] and researchers
working in New Zealand [10] are well on the way to remedying this situation. Dr
Colin Speedie [33] is working with the European Basking Shark
Photo-Identification Project (EBSPiP) to try to understand how far individual
basking sharks move. This work is complimentary to Dr Sims teams tagging work.
No one is sure where the females have their young. Hopefully the resurgence in
interest about basking sharks will encourage funding to help solve the
remaining mysteries surrounding these wonderful fish.
Classification
Basking sharks are, like all sharks, part of the family of fishes known as
elasmobranches. The skeleton of elasmobranch fish is made of cartilage, a
material not unlike the hard gristle found around mammalian joints. This might
be partially calcified, as it is in the basking sharks backbone but not to the
same extent as in the bony fish (teleosts). The scientific classification of
sharks is a little controversial. Rather surprisingly the harmless,
plankton-eating basking shark is in the same 'family', the Lamniformes, as the
great white shark! However, the basking shark is in a genus of its' own, the
Cetorhinidae.
Geographical distribution.
Basking sharks have a worldwide distribution in cool temperate seas of
temperature 8-14°C, being found in the Pacific, Atlantic and Indian Oceans. It
has only once been found in warm tropical water (off Florida) and this was
probably a sick stray. They are mentioned in the scientific literature as being
sighted in waters off Britain and Ireland, Florida, California, Japan, Norway,
New Zealand, Canada and Novia Scotia.
General Basking Shark Biology
Basking sharks are, like all sharks, part of the family known as elasmobranches
. The skeleton of elasmobranch fish is made of cartilage, a material not unlike
the hard gristle found around mammalian joints. This might be partially
calcified, but not to the same extent as in the bony (teleost) fishes. The
harmless, plankton-eating basking shark is in the same 'family', the
Lamniformes, as the great white shark! However, the basking shark is in a genus
of its own, the Cetorhinidae. If you find this hard to believe the pictures
below make this relationship a little more apparent!
This does look like a great white but it is actually a basking shark with its
mouth
half-shut. This photograph makes it possible to see that the basking shark
might be related to a great white, however improbable that seems at
first glance! This picture was taken about half a mile
off Fleshwick, the Isle of Man.
Picture: Andreas Perethoner.
Moi? Related to a great white? I hardly think so!
Picture: Maura Mitchell.
I can look completely different from the side!
Picture: Shane Stigant.
Geographical distribution.
Basking sharks have a worldwide distribution in cool temperate seas of
temperature 8-14°C, being found in the Pacific, Atlantic and Indian Oceans. It
has only once been found in warm tropical water (off Florida) and this was
probably a sick stray. They are mentioned in the scientific literature as being
sighted in waters off Britain and Ireland, Florida, California, Japan, Norway,
New Zealand, Canada and Novia Scotia.
External Features of Basking Sharks.
Most people who see basking sharks only see the fins and nose projecting above
the surface, as is shown in the diagram below. Historical documents suggest
that basking sharks can grow up to 13.7 metres long, as long as a double decker
bus. You are very unlikely to see one longer than 11 metres today but this is
still the length of single decker bus! There is a huge amount of debate about
basking shark maximum size [15,18] (see the next section for more on this).
The dorsal fin can be up to 2 metres tall. Note the notches in the dorsal,
second dorsal and tail fin. These are quite characteristic of the basking shark
as is the bulbous nose. The pectoral fins are very large. They enable the fish
to move up and down in the water column. Looking at the underside of the shark,
between the pelvic and anal fins, you can see the claspers. These are the male
sexual organs, used to introduce sperm into the female. Basking sharks often
have one or more long thin fish called lampreys attached to this area. The five
gill slits almost completely surround the head.
Imagine trying to study the anatomy of such a huge animal as a basking shark!
Dissecting an animal of such huge proportions would seem to be impossible.
However, in 1947 two scientists, Drs Matthews and Parker, accepted the kind
invitation of Major Gavin Maxwell, of “Ring of Bright Water” fame, to visit his
basking shark fishery on the Scottish Isle of Soay, off Skye. They were able to
observe the sharks from Gavin Maxwells’ hunting craft and to dissect 4 males
and 6 females on shore. These massive dissections were performed on the landing
slips using the very tools that were normally used to butcher the animals.
Pathology specimens were fixed in formalin and taken back to their laboratories
for later study. Dr Matthews wrote a scientific paper on reproduction in the
basking shark[17]. Drs Matthew and Parker wrote a joint paper about basking
shark general anatomy[18]. More than 50 years later these two scientific papers
are still the definitive works on basking shark anatomy and this section draws
heavily upon them.
Physical characteristics of basking sharks.
Picture: Maura Mitchell.
Basking sharks have five pairs of very large gill slits that almost completely
surround the head. They have a large, hoop-like sub-terminal mouth with
hundreds of tiny, vestigial teeth.
Picture: Maura Mitchell.
The pink gill rakers, used for filtering their planktonic shrimp food from the
water, are visible if you are too close! Note the small eye.
Picture: Maura Mitchell.
Their pectoral (side front) fins are large and angular. The basking shark uses
these rather like aeroplane wings to 'fly' up and down in the water column.
Their dorsal (back) fins are very large, up to 2 m tall and angular (see
photograph below). Their caudal (tail) fin is notched and lunate (moon shaped)
with a distinct lateral keel. (See photograph at top of this page ).
Picture: Maura Mitchell.
Adults such as the one in the photograph above have a characteristic bulbous
nose. Juvenile basking sharks are smaller and more slender with a nose
described as a “pointed prominence or beak”. The snout makes the transition
from the juvenile to the adult form at about 12-16 ft (about 3.6 - 4.8m). Smell
seems to be very important to the basking shark (see section on basking shark
brains further down).
Picture: Pauline Oliver.
Sometimes, when the basking shark is feeding or basking at the surface, three
points are visible, the pointed nose, the dorsal fin and the caudal or tail
fin. Note how the large dorsal fin has flopped over in this shark. This is more
common in large specimens.
Size, weight and sexual maturity.
Historical accounts give maximum sizes for basking sharks of 40-46 ft (12-13.72
m)[15]. It is notoriously difficult to estimate the length of a fish when you
see it in the water because of the magnifying effect of the water. Dr Matthews
found how easy it is to over-estimate the size of a basking shark when it is
still in the water. His estimate of a shark when it was along side the boat
turned out to be surprisingly exaggerated when they landed the animal. Some
down-to-earth facts come from Anthony Watkins who was a very experienced
basking shark fisherman operating off Scotland and Ireland. In his book ‘The
Sea My Hunting Ground’, published in 1958, he discusses the issues of length
and weight of basking sharks. He says that the biggest basking sharks his
fishery caught were about 29ft (8.7m). This is about the length of a large bus.
He estimated the weight of a 29 foot (8.7m) long basking shark by extrapolating
measurements obtained from a small basking shark he weighed whole. Watkins
reasoned that as the weight of a fish varies in proportion to the cube of its
length if you know the weight and the length of one fish of any species you can
calculate the weight of any fish of the same species from its length. On this
basis he calculated that a 29 foot one probably weighed about 6.2 tons. Given
that mature fish tend to be bulkier than young ones this might be a minimum
estimate. It seems reasonable to estimate a weight of about 7 tonnes for an
8.7m basking shark. This is about the weight of two fully grown elephants.
Several observers on the isle of Man have estimated basking shark lengths there
to be up to 12meters! Keep sending your basking shark sighting forms in and the
picture of basking shark size distribution on the island should become clearer.
Dr David Sims, is a prominent current basking shark researcher. He is working
mainly off the Plymouth coast. He and his team have produced many scientific
papers on basking shark feeding and seasonal movements[24-31]. Using the
scientific literature and his own experience Dr Sims estimated the ages of
sharks at different lengths. Newborn sharks are about 1.5m–2 m long. The
young-of-the-year sharks that appear inshore in June and July are 2-3 m.
Basking sharks reach 5m by age 3-4 years and 10m by approximately 8 to 15
years. It is not known how long basking sharks live but it may be 30-50 years,
so the possibility for bigger basking sharks does exist.
Basking sharks are immature up to 6m in length and become sexually mature
adults when they reach 6-9m.
Skin
“Basking sharks are dark grey, almost black, with an inconsistent amount of
light grey or white in the mid ventral line”. Colour variation occurs amongst
individuals. The colour is very patchy and has been described as being not
unlike mackerel skin markings on a grand scale.
Picture: Maura Mitchell.
The skin may have scars where lampreys have attached themselves. These white
scars are typically 2-3 inches (5-8cm) long and half as wide. The shark in the
picture above has some white scars behind its dorsal fin. Basking sharks often
have scars and bleeding regions around the cloaca (external genital region).
These are the result of their very rough skins rubbing together during mating.
There are often white scars around or on the dorsal (back) fin due to the male
biting the female's dorsal fin during the mating embrace. Basking shark mating
behaviour is far from gentle!
Basking shark skin is covered with small denticles. These are small tooth-like
placoid scales arranged with their points directed posteriorly, so that the
skin feels smooth to a hand passed over it from front to back, but exceedingly
rough in the reverse direction[18]. This account by Drs Matthews and Parker is
at odds with an account from Florida, which examined the skin from 2 specimens.
One of the sharks had denticles all pointing in the same direction but the
‘Sarasota’ specimen had skin with denticles pointing randomly. It was
exceedingly rough when stroked in any direction. The scientific paper which
reports this type of basking shark skin has 2 photographs of the Sarasota
basking shark’s skin with sharp denticles pointing in all directions . This
would be very unpleasant to rub against! It appears that basking shark skin
becomes very rough once tanned and made into leather. It was traditionally used
to sole fishermens boots as it provided such good grip on slippery surfaces. If
the 'Sarasota' specimen had been dead for a while it is possible that the skin
denticles could have become dry and disordered. Basking sharks skin is
criss-crossed with varying patterns of 2 mm-deep skin creases that are bare of
denticles. These creases correspond with lines of flexure (where the skin
bends).
Basking shark wrinkles! The skin in the gill region needs
to be able to expand a lot during feeding.
Picture Maura Mitchell.
Basking shark skin is plentifully supplied with mucous. During life the mucous
forms an even film over the entire surface of the skin below the surface of the
denticles. Scuba divers who have accidently come into contact with this mucous
say that it is thick and black, rather like axle grease, very fishy-smelling
and rather hard to clean off one’s wetsuit!
Basking sharks suffer from several external parasites typically seen around the
shoulder, flank and vent. The largest is the lamprey (Petromyzon marinus) an
eel-like primitive fish that has a sucker-mouth that, when attached to a soft
skinned whale, might feed from the flesh. It appears that this does not happen
in basking sharks since its jaws cannot penetrate the thick denticle-covered
skin. The lamprey probably just uses the shark as a mode of transport. Lamprey
are often seen clinging to the area around the vent in groups of up to 8.
This shark has a lamprey attached near its ventral fin.
Picture Maura Mitchell.
Drs Matthews and Parker found that basking shark skin was often infested with
three species of parasitic shrimp-like copepods, more commonly known as fish
lice.
Fish lice tend to attach themselves behind the basking shark gills, as in this
amazing
photograph. The photographer was in a kayak, letting the fish come to him.
The camera was put into the water next to the kayak and the shark (as you can
see)
swam very close. Niarbyl 2003.
Picture: Shane Stigant.
Basking Shark Brains.
Basking shark brains are very small, especially when compared to the size of
the animal. The basking shark needs to locate its preferred planktonic food and
the brain provides some evidence that smell is the most importance sence to
this species. Normally sharks detect their food by seeing it, smelling it or by
detecting electrical signals from it with its Ampullae of Lorenzini (of which
more later). The tiny eyes are probably not directly important in locating its
food but it is probable that they are important as light detectors, helping the
basking sharks to detect changing light levels when they are moving up and down
through the water column trying to locate food. Drs Matthews and Parker found
that the brains olfactory tract, where the sense of smell is processed, is 15cm
long whereas the whole of the rest of the brain is only 10.5 cm long! If
basking sharks allocate that much relative brain power to smell it would seems
to indicate that smell is VERY important to basking sharks! Interestingly, the
basking sharks brain cavity is much larger than its brain. Drs Matthews and
Parker found that the small brain was held in place in the large brain cavity
by fine strands of tissue! Its predatory relatives have proportionally larger
brains. Obviously the basking shark does not need a large brain like its
predatory cousins and its small brain with a relatively large portion for
smelling out its food source is all it needs to survive.
Feeding
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[19]. 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.
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.
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.
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.
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.
The plankton-laden water flows passively through the mouth and out through the 5
gill slits where the plankton is filtered out by its pinkish, filamentous,
brush-like gill rakers. The basking shark uses its gills to 'breath' extracting
oxygen and releasing carbon dioxide through them. The basking shark gill slits
also house the brush-like gill rackers that sieve the plankton from the water.
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 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!
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[7].
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[24]. To help you
visualise how much food is in 1gram of plankton per cubic meter; it has been
calculated[24] 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.
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.
When basking sharks find a good patch of plankton they make the most of
it by 'zigzaging' through the water.
Picture: Pauline Oliver.
How do they locate high concentrations of their favourite food?
It used to be thought that basking sharks were indiscriminate filter
feeders[18]. 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.
Oil “digestion”.
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.
Reproduction
Quite a lot is known about basking shark courtship but very little is certain
about their gestation or where they give birth. No scientist has ever dissected
a pregnant basking shark and only one live birth of six 1.5-2m long pups has
been observed. The terminology surrounding basking shark reproduction can be
quite confusing but this should clarify it-
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They are live bearing (this is called being viviparous).
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The young hatch from eggs inside the uterus (this is called being oviparous).
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The developing embryos nourish themselves by eating eggs in the uterus (they
are oophagous).
Picture: Pauline Oliver.
Courting basking sharks often congregate in groups of 2 or more off the West
coast of the Isle of Man. The sharks in the photograph above are off Peel
castle. You may be lucky enough to see courting behaviours such as paired
swimming, following nose to tail, close following, or, if you're really lucky,
breaching (leaping out of the water).
Basking Shark Courtship Behaviour, and Mating.
It is wonderful that something as exotic as basking shark courtship behaviour
can be seen from the shore on the Isle of Man. You will find an eyewitness
account in the Isle of Man basking shark stories section of this website.
Dr David Sims team has provided a marvellous up to date account of basking
shark courtship behaviour . Dr Sims’ team based their scientific paper on the
courtship behaviour of basking sharks on observations of 25 separate episodes
that occurred over a 5-year period. They concluded that courting behaviours
occur along oceanographic fronts off Plymouth from May to July while the
basking sharks are feeding off the rich plankton found at these fronts. The
same seems to be true off the Isle of Man coast. Another group of scientists,
travelling in a Coastguard helicopter off the coast of Novia Scotia, had a
wonderful bird’s-eye view of 13 basking sharks engaged in courtship behaviour
and of a possible mating. The sighting only lasted 5 minutes but they had video
footage and photographs that they were able to analyse later.
When do basking sharks become mature?
Previous to work done by Dr David Sims and his team it was thought that the
female did not become sexually mature until 7-9m[17,21] and males were
estimated to become mature at 4.6-6.1 m[17]. Dr Sims and his team found that
sharks exhibited sexual courtship behaviour when over 5m in length. Smaller
individuals who were present did not engage in such behaviour.
What is basking shark courtship behaviour?
Dr Sims describes basking sharks nose-to-tail following, close following,
approaching one another's flank, close swimming, parallel swimming, echelon
swimming (where sharks follow one another but are slightly offset, not in a
straight line) and breaching (leaping out of the water). They describe how only
large basking sharks in groups of 3 or more breach repeatedly, leaping clear of
the water. They propose that breaching is being used as male-male competitive
behaviour during courtship displays. However, the only shark that they were
able to definitely sex was a large female that breached 7 m from their boat,
which must have been interesting! They propose the alternate idea that female
basking sharks breach to signal their readiness for mating. The scientists in
Novia Scotia describe similar courtship behaviour to Dr Sims’ team. They also
mention that the basking sharks swam faster than usual and exhibited circling
behaviour by up to 10 sharks at a time.
This picture, taken off the West coast, shows three sharks exhibiting the
circling behaviour associated with courtship behaviour.
Picture: Pauline Oliver.
Basking shark mating.
There has only been one scientific account of a possible basking shark mating
but we have our own possible Isle of Man mating sighting documented in our
'Isle of Man Basking Shark Stories' section on this website!
The scientists in the Novia Scotia helicopter saw possible mating behaviour
where 2 sharks body curves matched one another. They saw one shark holding on
to another by biting down on its pectoral fin. They also observed
white-coloured marks on the dorsal area of the pectoral fins and behind the
dorsal fin. They interpreted these as being bite marks made by males on females
during mating. Although basking sharks are filter feeders they do have an
impressive array of small teeth (see the section of feeding for more details).
It would seem likely that these teeth are primarily used for holding on to
their sexual partner during mating, as there is no evidence of basking sharks
using teeth for catching any food items. When Dr Matthews dissected recently
mated basking sharks he found many abrasions on the genital cloaca regions of
sharks that were known to have been recently mated. He surmised that the claw
on the males’ claspers caused abrasions on and in the females and that
abrasions on both sexes were caused by the abrasion of the sharp sharkskin one
against the other. Evidently basking sharks are not gentle with one another!
It is possible to identify the sex of a basking shark if you can see its
ventral region (underneath). Dr Matthews found that the male has two long
scroll-like claspers, which are used to introduce about 4 gallons of sperm into
the female during mating. He dissected one female that had recently mated,
which is how he knew how much sperm was used. He measured one clasper as being
1.09m long. It would seem that everything about basking sharks is done on a
grand scale!
How do basking sharks nourish their unborn young?
Is not clear how the basking shark mother nourishes her young so it is worth
explaining some of the fascinating different methods that other shark species
use to nourish their live-born young[9]. Sharks are a very diverse group and
they exhibit many different forms of reproduction. Some sharks lay eggs, others
bear live young that are nourished inside the mother by egg yolk only. Others
provide egg yolk and a rich milk-like substance called uterine milk. Some shark
species are oophagous, the mother ‘lays’ many eggs into her uterus and her
developing young eat them. Other shark species cannibalise their womb-mates;
this is called adelophagy and can do little to improve the reputation of
sharks! Some shark species exhibit a combination of strategies, the live-born
young having been nourished by egg-yolk, intrauterine milk and by eating eggs
whilst in the uterus. Finally, some sharks have placental structures that
directly nourish their young.
When Dr Matthews dissected 6 female basking sharks he found that only the right
ovaries were functional. He didn’t understand why the half-metre across ovary
had 6 million eggs ready to be spawned when a female would only have a few live
births in her lifetime. It now appears that this is because the eggs are used
to feed the developing young. Dr Matthews also found that the uterus was lined
with structures called trophonemata or ‘feeding threads’. Trophonemata are
associated with shark species such as the butterfly ray in which these long
nutritive threads feed the developing sharks directly into their oesophagus. It
is possible that basking sharks adopt more than one strategy. Basking sharks
might use a combination of nutrition from oophagagy (eating eggs) AND
trophonemata (feeding threads) to nourish their unborn, developing young. As
discussed before, other species of sharks use a combination of methods to
nourish their unborn young but until a pregnant basking shark is examined this
will remain a matter for conjecture.
How long is a basking shark pregnancy?
It is not clear how long the gestation period of the basking shark is.
Gestation in some other viviparous (live-bearing) sharks is known to be about
22 months. There have been various suggestions about how long basking shark
gestations are. Some suggest 20 months others basing their estimate on an
analysis of the age/length data suggest a gestation period of 3.5 years,
another equally prominent scientist concluded that a 1-year gestation was more
probable. The conclusion? It is not known how long a developing basking shark
spends in its mothers uterus!
Where are basking sharks born and how big is a newborn basking shark? There are
only three accounts of female sharks being found with live young in
them[22,17,36], and one record of a basking shark birth. Given how many basking
sharks have been butchered for their livers the lack of stories about pregnant
basking sharks suggests that they might stay in deeper water or somewhere that
is not normally targeted by fisheries. Dr Sims and his team reasoned that as
young-of-the-year, (size 2-3m) do not appear inshore until mid-June that it was
probable that young of 1.5m length were born offshore in deep water in May and
June.
The oldest record of a pregnant basking shark female is by a T. Pennant writing
in 1769 who wrote “ They are viviparous, a young one about a foot in length
being found in the belly of a fish of this kind”. Dr Matthews also describes
how a fisherman told him that when he opened a female he found a young one 6
foot long inside. A Norwegian fisherman described catching a female basking
shark that gave birth to 5 live young and 1 stillborn which he estimated to be
between 1.5 and 2m long.
When Dr Matthews[17] dissected basking shark females he found that they had a
1-2m long uterus. The uterus size tallied with the very few accounts of the
size of basking shark young found inside female sharks. The smallest newborn
basking sharks are about this size so this is quite plausible. We have been
extremely lucky to get TWO picture of a newborn basking shark that has been
seen off the West coast of the Isle of Man during July 2005. The first picture
shows the remarkable snub nose. The second shows a full length shot of the
whole newborn shark.
This snubby pig-like nose with the underset lower jaw is so unlike the adult
that
unless it was so closely associated with the group of adults and so clearly
feeding on plankton with them you would think it was a different species!
Picture Shane Stigant July 2005 Four miles off Dalby.
Do basking sharks hibernate?
Until recently it was thought that basking sharks hibernate in the winter.
There were two main reasons for this hypothesis. The first reason was that
basking sharks apparently disappeared for most of the year. People normally see
them in the late spring and summer. Sightings at other times of year are very
much rarer. There has been one sighting of a basking shark off the west coast
of the Isle of Man this February (2005) so winter sightings are not unknown,
just rare. The second major reason for thinking that basking sharks hibernate
was that some winter-caught basking sharks have shed their gill rakers. This
obviously renders them incapable of filter feeding. This led to the general
belief that basking sharks probably hibernate on the bottom in deep water
whilst their rakers grow back and until the plankton levels came back to levels
that made it worthwhile for them to feed. It was thought that they lived off
the huge store of oil in their liver. A 4-ton basking shark may have a one ton
liver, which is almost totally composed of stored oil. The winter hibernation
theory was not unreasonable as the first spring sightings of basking sharks are
normally near deep water and winter-caught basking sharks were caught in deep
water.
Basking sharks are sometimes found to be without gill rakers when they are
caught in the
winter. This, combined with the fact that they are not normally seen at the
surface
in winter lead to the theory that they hibernated on the sea bottom and didn't
feed
in the winter. This shark was photographed off Niarbyl, the Isle of Man.
Picture: Shane Stigant.
The studies that disprove the basking shark hibernation theory.
A couple of studies by Drs Francis and Duffy in New Zealand and Dr Sims and his
team in Plymouth, England, have laid the hibernation theory to rest. Drs
Francis and Duffy were looking at incidental trawl-caught basking sharks caught
in fishing nets off New Zealand. They found that 203 basking sharks were
accidentally caught over a 13-year period. They found that many basking sharks
were caught in winter, particularly on the West Coast of New Zealand. Although
it was not possible to determine the exact depths that the fish were caught
there was no doubt that some of the sharks were caught in midwater trawls. They
concluded that hibernating sharks were unlikely to hover in midwater and that
these sharks were not hibernating. The study by Dr Sims and his team showed
that the basking sharks which were tagged off Plymouth did not hibernate but
actually travelled huge horizontal distances of up to 3400 km, actively diving
in all water depths from the surface down to 750m deep during summer, autumn
and winter. This study clearly showed that the basking sharks were not lying on
the bottom in the winter but were very actively tracking the areas with the
highest quantity of plankton in order to feed there.
As winter basking sharks have been shown to travel actively, to move to the
areas with the highest plankton levels, and to be caught apparently actively
swimming in midwater, the hibernation theory can be assumed to be disproved:
basking sharks do NOT hibernate. It now seems more probable that basking sharks
shed their gill rakers at irregular intervals in the winter in order to renew
worn feeding apparatus rather than not needing them to feed because they are
hibernating.
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