Basking Shark Fact Sheet
Many biological aspects of the basking sharks are
still a complete mystery. We do not know where the animals spend
the cold season, and their gill rakers - a shedable filter structure
in their mouths designed to strain plankton from the water - is
a puzzle to scientists.
Biology of basking sharks (Cetorhinus maximus)
Description
The basking shark's most important characteristics are its long
gill slits which almost encircle the head and its pointed snout.
Additional recognizable features are its lunate caudal fin and
well-formed lateral keels.
Size and Age
The maximum size of these animals is approximately 10 meters.
Reports on larger specimans have to date not been confirmed. Their
maximum age is unknown, although efforts are being made to determine
this by counting the number of vertebral spines/Wirbelringe (similar
to the growth of tree trunks) and comparing them to the size of
animals in known populations.
Feeding
Basking sharks belong to the few plankton eaters in the shark
world. The only other filter-feeding species known to prefer plankton
are whale sharks (Rhincodon typus, Fact Sheet Shark Info 3/00)
and megamouth sharks (Megachasma pelagios, Fact Sheet Shark Info
1/99). Although other species such as the blue sharks (Prionace
glauca) also have the necessary filter structure in the gill region
which allows them to strain water and thus feed on plankton (krill)
as their basic food, they only do this in rare cases. Called gill
rakers, they are the main reason why basking sharks swim through
the water almost permanently with wide open mouths. Compared to
whale sharks, basking sharks are passive filterers, meaning they
do not actively suck in water but rather let it flow in while
swimming.
Reproduction
This species' method of reproduction has not been widely researched.
As with other representatives of the same group (white sharks,
mackerel sharks, thresher sharks, etc.), they too bear living
young, but the embryos are not connected by placenta to the mother
(aplacental viviparous or ovoviviparous). The embryos nourish
themselves by feeding on eggs produced by the mother (oophagy,
the eating of eggs). Their size at birth is approximately 1.7
meters long. Based on counts made of the number of animals and
their sex, the species appears to mate in schools prevailed by
females. In addition they seem to have the longest gestation period
of all sharks.
Distribution
This species prefers cool and temperate waters and is distributed
almost worldwide, although they appear to be limited to regions
on the continental shelf. They are seen both in immediate proximity
to the shore as well as in open waters. They also appear to be
migratory, periodically appearing in certain places. Basking sharks
can be found on the northern hemisphere off the coast of China,
Korea and around Japan, on the East coast of the U.S., from Newfoundland
down to Florida, around Great Britain and Norway, but also in
the Mediterranean. In the southern hemisphere they are found primarily
in Southern Australia, parts of South America and the tip of South
Africa.
Behavior
Basking sharks migrate mostly in schools, accordingly they also
feed together. Their feeding behavior and migrations appear to
be connected with the blooming of plankton. One theory is that
when no plankton is available the animals periodically retreat
to deeper regions where they shed their gill rakers. Based on
this hypothesis the gill rakers are then regenerated by the time
the next plankton blooms. Thanks to deposits of fat in their huge
liver, they have no problem in overcoming this time period without
plankton. Other theories say the animals who have shed their gill
rakers do not fast when no plankton is available, but revert to
feeding on organisms living on the ocean floor. More research
will be necessary before these behavioral patterns are fully understood.
Encounters with humans
Basking sharks are harmless. Years ago the animals were hunted
for their huge liver with its high content of Vitamin A and oil,
the latter of which was used as lamp oil. Because of their very
low reproduction rates and the strong pressure put on their populations
by the fishing industry, these animals are today found on the
IUCN list of endangered shark species.
Mythology
Basking sharks were the basis for many sea monster reports. They
have a habit of swimming directly underneath the surface of the
water ("basking" means to sun oneself or bask in the
sun) and are frequently observed swimming in a row so that their
dorsal fins and the upper lobes of their caudal fins penetrate
the water.
