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ECHINODERMS: THE SPINY ANIMALS!
© Jonathan Bird/ORG
For educational use only
The phylum Echinodermata , which contains about 6000 species, gets its name from the Greek, literally
meaning "spiny skin." Many echinoderms actually do have "spiny" skin, but others do not. This phylum
exists exclusively in the sea, and cannot be found on land or in fresh water. All echinoderms have one
thing in common: radial symmetry. This means that the creatures have appendages (or body
construction) which point outward from the center of the body like the spokes on a bicycle wheel.
Furthermore, these appendages usually occur in multiples of five, although there are a few exceptions.
There are several well known members of this group, like sea stars and sea urchins. The radial symmetry
is obvious in these creatures.
Perhaps not as obvious is the water vascular system, another trait common to all echinoderms. By
examining the oral (ventral or underside) side of a sea star, one will be able to see hundreds of tiny feet
usually arranged into several rows on each ray (appendage) of the star. These are called tube feet, or
podia, and are filled with sea water in most echinoderms. The water vascular system within the body of
the animal is also filled with sea water. By expanding and contracting chambers within the water
vascular system, the echinoderm can force water into certain tube feet to extend them. The animal has
muscles in the tube feet which are used to retract them. By expanding and retracting the right tube feet
in the proper order, the creature can walk. Many echinoderms can also form suckers on the ends of
their tube feet. These suckers can be used to capture and hold prey, or to hold onto rocks in a swift
current or tide.
Interestingly, although most mature echinoderms are benthic (meaning that they live on the bottom),
the larvae are usually planktonic with bilateral symmetry. During the process of maturing, the
echinoderm will change its body shape from bilaterally symmetrical to radially symmetrical, and in the
process, settle down on the sea floor.
The Stelleroids
Perhaps the most common echinoderm is the sea star. Although more well known as the star fish, sea
stars are not actually fish. The scientific community prefers to reserve the term "fish" for vertebrates
with fins. The subphylum Stelleroidea contains the two classes of sea stars. The class Asteroidea
contains the true sea stars and sun stars. The class Ophiuroidea contains the brittle stars and basket
stars. The distinction between the two classes lies in the way the arms connect with the body.
Ophiuroids have a distinct central body part (called a central disk) with arms radiating out from the
body. Adjacent arms do not connect with each other. Asteroids, on the other hand, have arms which
seem to connect together in such a way as to make it difficult to discern where the arms end and the
central disk begins.
The sea star's aboral (top) surface is spiny looking when closely examined. The rumpled skin, when
examined up close, contains several different types of formations. Some of the bumps on the surface
are called Dermal Branchiae, which are used to absorb oxygen from the water. Another type of
adaptation of the skin are called pedicellaria. These pincher-like pairs of organs can be used to pluck
things off of the skin of the sea star which might otherwise cause a problem. For example, the larval
form of a barnacle could settle onto a sea star and begin to grow if the sea star had no way of removing
it.
The single colored spot on the aboral surface of the sea star is called the madreporite (mad-ruh-POREite). This is a calcareous piece of the water vascular system filled with tiny holes, like a strainer. It is the
interface between the water vascular system and the ocean, acting as a filter.
The sea star has a light sensitive organ at the tip of each ray, called an eyespot. When moving across the
ocean floor, the sea star usually leads with one ray, probing the surface ahead. Although the star cannot
see in the way we do, it can detect the presence and direction of light, and does seem to have some idea
where it is headed.
Sea stars are capable of regenerating limbs in the event that one or more is severed or damaged. The
wound first closes off, and in time, the new limb will begin to grow. In a few species, the severed limb
can regenerate a new sea star, but in most species, the severed limb dies.Sea stars eat a variety of
different things, including barnacles, clams, mussels, snails, sea urchins, and in some cases, other sea
stars! Many sea stars, such as the Northern Sea Star, eat mussels and clams in a fascinating way. The sea
star first surrounds its intended victim. Then it applies outward force (with its suction cup equipped tube
feet) on the two mussel shells (called valves), to pull them apart. Contrary to popular belief, the sea star
does not need to apply force for a long time in order to tire out the mussel. The sea star can apply so
much force to the mussel valves (7 or more pounds!) that it will bend the shell. Seizing the moment, the
sea star then everts its stomach out through its mouth, and into the mussel (only a 1/100th of an inch
opening is required). Once the sea star begins to digest its victim within the victim's own body, the
victim dies. The sea star then finishes the meal by consuming the rest of the mussel. When the star is
finished with the mussel, nothing remains but a shell.
Echinoids
The class Echinoidea includes the sea urchins, heart urchins, cake urchins and familiar sand dollars. The
sea urchins are well known to most people, having a large number of sharp spines pointing out in all
directions. These spines offer protection from many would-be predators. The spines are joined to the
skeleton of the animal, called the "test," in a form of ball-joint. This coupled with the fact that there are
muscles attached to each spine enables the urchin to swivel its spines in the direction of a predator. The
test is an egg-like spherical structure constructed of rows of radially arranged plates fused together.
The creature has 5 paired rows of tube feet which, when extended, are long enough to reach past the
length of the spines. They contain suckers. The anus is on the top of the creature, while the mouth is on
the underside. The mouth contains five teeth which are arranged pointing towards the center of the
mouth. This structure looks and works like the jaws of a drill chuck and has been termed "Aristotle's
Lantern" because it was first described in detail in a book by Aristotle as looking like the top of an oil
lamp. The creature uses its tube feet to pull itself against the substrate so it can gnaw away at algae with
its mouth.
Holothuroids
The class Holothuroidea is composed of creatures called sea cucumbers. A sea cucumber is so named
due to the fact that many members of the group (but not all) resemble the garden variety of cucumber.
The similarity ends there.
Sea cucumbers are usually somewhat football shaped and lay on their side on the bottom. They have 5
rows of tube feet running lengthwise, like the seams on a football. Three of the rows of tube feet are
well developed and are in contact with the substrate. The other two rows are usually either
underdeveloped (and not used) or missing completely. At the oral end of the animal is a mouth
surrounded by tentacles. These tentacles, usually branched, are actually special tube feet and are part of
the water vascular system. The water vascular system is not filled with sea water in sea cucumbers, as in
sea stars and urchins, but rather with a special body fluid. There is, therefore, no direct interface
(madreporite) between the outside water and the internal organs of the animal.
The sea cucumber feeds in a fascinating way. It may position itself in a spot on the ocean floor where a
current will bring a steady supply of food (plankton and other organic particles) its way. The tentacles
are opened to collect the food. Then, the cucumber sticks each tentacle in its mouth, one at a time, and
licks them off. As soon as a tentacle has been licked, the cucumber takes it out of its mouth and holds it
out to collect more food while it licks the next tentacle. The sea cucumber will do this for hours at a
time. Other kinds of sea cucumbers use their tentacles to sift through sand on the bottom for particles
of food, rather than collect plankton from the water.
Many sea cucumbers are quite poisonous. These cucumbers, if injured, can kill fish in the same
aquarium. The poison of some sea cucumbers has shown promise as an inhibitor in the growth of cancer
cells.
The sea cucumber has another interesting (if not slightly disgusting) habit, called self-evisceration. When
a sea cucumber is attacked, it may expel some of its internal organs. This could either satisfy a predator,
or scare it off. The cucumber will then proceed to grow another set of organs.
Some sea cucumbers can secrete a sticky glue-like substance as a defense mechanism. This goo is so
sticky that it absolutely cannot be removed from the skin without shaving off any hair with which it has
come in contact. Historically, people have used this substance as a bandage to bind wounds.
The Chinese eat certain sea cucumbers, and consider them quite delicious. Some species have edible
muscular body walls. I haven't tried it myself, so I cannot offer any opinion on the taste. I have a friend
who says that the texture is somewhat "gelatinous."
No matter what you think about the looks of Echinoderms, they are very interesting and diverse animals
with amazing adaptability.
The Cushion Star, found in the Caribbean, is a classic example of an Asteroid.
This basket star, photographed in the Caribbean, is a good example of a Ophiuroid.
This Northern sea star, photographed in Massachusetts, is regenerating two lost arms. It will take over a
year, but they will be good as new!
The voracious Spiny Sun star of the North Atlantic stalks and eats other sea stars! It's an Asteroid with
up to 12 arms!!
This Purple sea urchin from California has hundreds of pointy spines for protection.
The Orange-footed sea cucumber lives in the North Atlantic. It resembles a football with tentacles at one
end for feeding.
This is a view of its feathery tentacles.It catches plankton in its arms and then licks them off one at a
time.
The Tiger Tail sea cucumber feeds on particles found in the sand on the bottom.
This sea cucumber is eviscerating a portion of its intestines to thwart an attack.
Watch the Emmy Award-winning free internet
series about the underwater world:
JONATHAN BIRD'S BLUE WORLD
All images on these pages for non-profit educational use only.
update 6/5/07