Download Echinoderms for Dummies

Echinoderms for Dummies
By Claudia Cristescu
Phylum Echinodermata is member of the kingdom Animalia and subkingdom Eumetazoa. They
are commonly known to have radial symmetry and are mostly marine.
Marine animal members of the Phylum Echinodermata include sea star, sea urchin, feather star
and sea cucumber.
Sea stars belong to the class Asteroidea, order Forcipulatida, family Asterinidae, genus Asterias
and species Rosy Starfish.
Sea urchins belong to the class Echinoidea, order Clypeasteroida, family Echinarachniidae,
genus Echinarachnius, and species Sand dollar.
Feather stars belong to class Crinoidea, order Comatulida, family Antedonidae, genus Antedon,
and species Antedon bifida.
Sea cucumbers belong to class Holothuroidea, order Aspidochirota, family Stichopodidae, genus
parastichopus, species P. parvimensis.
Sea Urchins
Sea urchins have a five-sided radially symmetrical body, also known as pentamerism, and an
external chitinous skeleton. Sea urchins have movable spines of various sizes and forms that are
attached to the body and have a pincer-like pedicellaria for grabbing small prey. Appendages of
sea urchins include multiple paired rows of tiny tube feet used in respiration and movement.
Sea Urchin’s Nervous System
The center of the sear urchin is a large nerve ring from which five nerves radiate underneath
the radial canals of the water vascular system and branch into even finer nerves to innervate
the tube feet, spines, and the pedicellariae. A sea urchin’s body works as one compound eye, as
it is sensitive to touch, light, and chemicals, even though it has no eyesight. Sea urchins have
light-sensitive molecules, mostly in their tube feet and in tiny stalked appendages found in
among their spines. Each spine can have about 140 light-sensitive cells. They have a diffused
and small nerve net as the animals are more or less made of rock which makes it hard to
examine their nervous system.
Sea Urchin’s Digestive System
Sea urchin’s digestive system consists of tube joining a mouth at the bottom to an anus at the
top. The mouth consists of a complex arrangement of muscles and plates surrounding the
circular opening. Sea urchins have a centrally located jaw (called Aristotle's lantern) with horny
teeth. Aristotle's lantern is made up of five hard plates that come together like a beak. The food
is grasped and then it is passed up through a complexly coiled gut and the waste is passed
upward through the anus. Sea urchins are mostly known to eat brown and green algae and
decayed matter.
Sea Urchin’s Respiratory System
Sea urchins have hundreds of transparent tubes that emerge which allow them to stick to the
bottom of the ocean. These tubes are called “tube feet” and are much longer than the spines
outlining their shells and they are also used by the sea urchin in respiration. A sea urchin
breathes through its tube feet, where gases are exchanged, instead of in gills or lungs.
“Aristotle's lantern," surround a sea urchin's mouth on the bottom of its shell.
Sea Urchin’s Movement
A sea urchin uses its many tube feet to hang on to the bottom of the ocean, move along rocks,
sand or other surfaces. The tube feet of the sea urchins allow them to move by way of water
vascular system and hydraulic pressure allowing it to extend or contract its feet. They can move
fast, walking on their feet, their spines, or even their teeth. Hydraulic powered tube feet are
unique to sea urchins and other echinoderms, which includes starfish, sea cucumbers, sea lilies
and brittle stars.
Sea Urchin’s Reproductive System
Sea-urchin species have separate sexes and release their gametes into the sea, mostly during
the spring. The gonads are the reproductive organs for both males and females. The female sea
urchin releases millions of tiny, jelly-coated eggs into the water that are then fertilized by the
sperm of the male sea urchin. Metamorphosis is usually complete within a week. The tiny sea
urchin eggs become part of the plankton and the sea urchin babies (larvae) do not hatch for
several months. The sea urchin young will not become large enough to retreat from the
plankton and down to the ocean floor until they are between 2 and 5 years old.
Fun Facts

Sea urchins can eat almost anything and their sharp teeth can scrape algae off rocks,
and grind up plankton, kelp, periwinkles, and sometimes even barnacles and mussels.

Sea urchin behavior can signal poor water quality, as they show stress in polluted water.

When sea stars get close to a sea urchin, the urchin moves its spines aside and lets the
sea star's arm get really close until the pincers chomp on the sea star's tube feet.

When a sea urchin dies, all its spines fall off, leaving only the spherical test.

The sea urchin gets its name from an old English word for the spiny hedgehog which is
similar to the American porcupine.
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