Sea-urchins are not stationary
creatures, but move around on shore and seabed using their spines and sucker
like tube feet. Some can climb up vertical rocks, and even bore into them. Well,
Sea-urchins belong to a group of marine animals aptly named echinoderms,
meaning spiny-skinned. Starfish, brittle-stars, sea-cucumbers and feather-stars
are also members of this group. Many of the sea-urchins around the British
coastline live both above and below the low tide mark, either singly or in
groups.
Some species, such as the common
sea-urchin, green sea-urchin and edible sea-urchin, live under stones, among
seaweeds or in rock pools and crannies. But the heart-urchin, or sea potato as
it is commonly called, burrows in sandy shores. Although the living creatures
are often hidden from view, you may often find the round, hollow and colorful
shells of dead sea-urchin washed up along the strandline.
Spiny test, the shell tests look
like an external skeleton. But for the living urchin it is also an internal
one, covered in a thin layer of living tissue which contains the digestive and
reproductive organs. The test is made up of layers of white limy plates. As the
animal grows, it adds more layers each year with layers of pigment in between. You
can tell the age of a sea-urchin by counting the numbers of layers. The edible sea-urchin
for example, can live for 8 years or more. The edible sea-urchin (Echinus
esculentus) is found on rocks and among seaweeds along the English Channel and
Western coasts, from the low tide mark to a distance. As with other sea-urchin
its mouth is armed with a set of five sharp teeth operated by a structure
called Aristotle’s lantern named after the Greek philosopher who first
described the animal in detail, the Aristotle’s lantern may be found intact
inside empty tests. The cut away contains the gut rising from the center and
coiling inside. Five yellow sex organs are visible, and are edible hence its
name. The test is normally 10cm (4 inches) in diameter and when empty and
cleaned is a beautiful mixture of reds and purples, with small white knobs
where the spines about 1.5cm long were attached. The spines which cover the test
an soon break off when the animal dies not only protect the sea urchin against
predators, but also help it to feed and to move about. In some species, the
spines enable the animal to burrow into sand or bore into rock.
On the move Sea-urchins also have
a number of tube-feet interspersed among the spines, which they use chiefly for
movement and also for respiration and waste disposal. The tube feet make p part
of a complex structure known as the water vascular system. Sea-urchins pump
fluid through their bodies to help them move about. The fluid, largely sea
water is pushed through a series of tubes to the tube-feet which then extend.
The sucker-like tip of a tube foot grips firmly once it touches a rock or other
firm surface. The muscular wall of the foot then contracts, and the water is
forced back through the canal system. As the foot shortens, the animal is
pulled along. Once attached to a rock, Sea-urchins are difficult to dislodge.
The tube-feet project through
holes in the Sea-urchins test, in a characteristic pattern of five double rows
running from top to bottom round the body. You can clearly see the tube-feet
holes on empty, spine free tests. The Sea-urchins mouth and teeth are on the
underside of its body. The edible Sea-urchins and other rocky shore species,
move slowly over the rocks to graze on the vegetation especially soft young
seaweeds.
Colorful Varieties
Sea-urchins common around the
British Isles vary in size from five to fifteen cm their shapes differ from
rounded to heart shaped, and the colors alter dramatically from black to bright
purple and to the pea green of the green Sea-urchins. The common Sea-urchins
sometimes called the rock-urchin, forms groups in or among rocks between the
high and low tide marks. If it cannot find a rock pool or suitable cranny, this
urchin makes its own shelter against the waves, wind and sun by boring into the
rock. Its teeth bore deep into the rock, while the spines widen the hole. The
much larger edible Sea-urchins live down on rocky shores. It is only uncovered
at spring tides, when the low water mark reaches its lowest point; but the
slightly conical empty test may be found
higher up the beach. The tests are a colorful reddish brown and are often sold
as souvenirs.
Sea Potato
This Sea-urchin is also known as
the heart-urchin because of its fragile heart shaped test. It differs from
other Sea-urchins in that it lives in burrows on sandy shores near the low tide
level. Like many animals in this habitat, the sea potato burrows to find
somewhere to live and to exploit its food source in the sand, as well as
protecting itself against the drying effects of the sun and wind, or the force
of the waves.
A group of spines on the
underside near the mouth which are longer and spoon-shaped are specifically for
digging down into the sand. A star-shaped depression on the surface shows where
a heart-urchin has recently burrowed. Once underground it feeds with the use of
special tube-feet and collects and grains from the burrow floor. Spines around
the mouth then scrape off the food particles.
Non-Mating
Like several animals, Sea-urchins
do not mate but shed their eggs and sperm into the sea (groups of sea potatoes
gather together to help ensure the maximum numbers of eggs are fertilized).
Spawning takes place in spring and the fertilized eggs develop into minute
rounded larvae, with four arms that stick out and up. The larvae float among
the plankton, later settling on the seabed to transform into adults.
Note:
The green sea-urchin if often
found under stones or seaweed on the parts of the shore that are covered by the
tide lower shore. Green Sea-urchins (Psammechinus miliaris) greenish test up to
4cm across violet spines 15mm long.
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