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Echinoderms
Echinoderms: An Introduction
Echinoderms are marine animals that are often found attached to rocks. Echinoderm
means ‘spiny skin’ and includes sea stars, brittle stars, featherstars, sea urchins, sand
dollars, and sea cucumbers. Echinoderms are characterized by a water vascular
system with tube feet and radial symmetry.
Symmetry:
As adults most, but not all, echinoderms exhibit five-sided radial symmetry. Their body acts as a hub with five
arms, or rays, projecting outwards. For sea stars and brittle stars the symmetry is easy to see, but if you
observe other echinoderms carefully, the symmetry is there as well. At the center of the sand dollar, there is
what looks like a flower with five petals. If you remove the spines from a sea urchin, you can observe five
bands radiating outward. Similarly, if you stand a sea cucumber on its end, you will observe five rows of tube
feet extending down.
A Sand Dollar with ‘Flower’ in
the Center
A Sea Urchin with Five Bands
Radiating Outwards
A Sea Cucumber with Five
Bands of Tube Feet
Although five-sided radial symmetry is common among echinoderms, some echinoderms such as sun stars
have 20 or more arms projecting outwards. As larvae, echinoderms exhibit bilateral symmetry.
Water Vascular System and Movement Using Tube Feet
Along with radial symmetry, another defining characteristic of echinoderms is a water vascular
system. This water vascular system is used for gas exchange, feeding, and locomotion. The water
vascular system extends to their tube feet called podia. When echinoderms move, they force water
into the podia, which causes them to expand. Echinoderms also have muscles attached to the podia,
which they can use to retract the podia. This system of expansion by forcing water into the podia and
then contraction by muscle allows echinoderms to move.
Sea Stars (Starfish)
Sea stars fall under the class Stelleroidea. There are two main subclasses of sea stars: Asteroidea and
Ophiuroidea. Asteroids are the true sea stars and sun stars. Ophiuroids are composed of brittle stars and
basket stars. The main difference between asteroids and ophiuroids is the way that the rays, or arms, attach to
the body. In ophiuroids, the arms are not joined to each other and the body and arms are clearly separated, but
in the asteroids the arms are joined to each other and the boundary between arms and the central disc is
blurred.
Sea stars eat many things including clams,
mussels, barnacles, coral, sea urchins and
even other sea stars. Some sea stars have
a remarkable way of eating shells. They
pry the shell open just slightly with their
powerful arms and then they insert their
stomach into the shell. When they are
finished, there is nothing but an empty
shell left.
Sea Urchins and Sand Dollars
Sea urchins and sand dollars are echinoids from the class Echinoidea.
Spines radiating outwards protect them from predators. The spines are
connected to the skeleton by a ball joint and muscle, which can swivel
towards a predator. Echinoids are herbivores, grazing on algae on rocks
and the sea floor. They have five bands of tube feet, which they use to
move around and cling to rocks.
A Sea Urchin with Spines
Sea Cucumbers
Sea cucumbers belong to the class Holothuroidea. They get their
name because some sea cucumbers look like vegetable
cucumbers. Sea cucumbers have five rows of tube feet along their
sides and tentacles around their mouths. The tentacles filter
seawater or sand for plankton and other organic matter.
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Regeneration
Sea Stars and Sea cucumbers have a special characteristic: they can regenerate (re-grow) lost body parts. If
you cut an arm off of a starfish, it will be able to regenerate that arm. Sometimes, even the separated arm can
become a whole new sea star. This makes sea stars very hardy and can even cause some problems. For
example, the crown-of-thorns starfish damages coral reefs, but because of its regenerative properties, it is very
hard to kill. Sea cucumbers have the amazing ability to eviscerate (expel) their internal organs, when attacked.
The predator is often satisfied with this easy meal and moves on. The sea cucumber then regenerates the
expelled organs.
Echinoderm Fact Sheet
10 Facts About Starfish (Sea Stars)
Sea stars are not fish.
Not all sea stars have 5 arms.
Although sea stars live
underwater and are
commonly called
"starfish," they are not
fish. They do not have
gills, scales, or fins like
fish do and they move
quite differently from
fish. While fish propel
themselves with their
tails, sea stars have tiny
tube feet to help them
move along.
While the five-armed varieties of
sea star are the most well
known, not all sea stars have 5
arms. Some have many more.
Take the sun star for instance,
which has up to 40 arms!
There are thousands of sea star species.
There are about 2,000 species of sea stars.
Some live in the intertidal zone, some in
deep water, some in tropical areas, some in
cold water.
Sea stars can regenerate a lost arm.
Sea stars have eyes.
Amazingly, sea stars can regenerate lost
arms. This is useful if the sea star is
threatened by a predator - it can drop an
arm, get away and grow a new arm. Sea
stars house most of their vital organs in
their arms, so some can even regenerate
an entirely new sea star from just one arm
and a portion of the star's central disc. It
won't happen too quickly, though. It takes
about a year for an arm to grow back.
While they can't see as
well as we do, sea
stars have an eye spot
at the end of each
arm. This is a very
simple eye that looks
like a red spot. The
eye doesn't see much
detail, but can sense
light and dark.
Sea stars do not have blood.
Instead of blood, sea stars
have a water vascular system,
in which the sea star pumps
sea water through its sieve
plate, or madreporite, into its
tube feet to extend them.
Muscles within the tube feet
retract them.
Sea stars eat with their
stomachs inside-out.
Speaking of prey, sea stars
have a rather unique way of
eating theirs. A sea star's
mouth is on its underside. They
prey on bivalves like mussels
and clams, as well as small
fish, snails, and barnacles. If
you've ever tried to pry the
shell of a clam or mussel open,
you know how difficult it is. Sea
stars wrap their arms around
the animal's shell and pull it
open just enough. And then it
does something we could never
imagine - it pushes its stomach
through its mouth and into the
bivalve's shell. It then digests
the animal and slides its
stomach back into its own
body. This unique feeding
mechanism allows the sea star
to eat larger prey than it would
otherwise be able to fit into its
tiny mouth.
Sea stars are Echinoderms.
Sea stars belong to the Phylum
Echinodermata. That means they
are related to sand dollars (yes,
they are a real animal), sea
urchins, and sea cucumbers. All
echinoderms have five-point radial
symmetry, which means that their
body plan has five sections (or
multiples thereof) arranged
around a central disk. Next time
you're in a beach-themed store,
see if you can find a dried sea
star, sand dollar and sea urchin
and find the 5 sections in each.
Sea stars are protected by armor.
Depending on the species, a sea
star's skin may feel leathery, or
slightly prickly. Sea stars have a
tough covering on their upper side,
which is made up of plates of calcium
carbonate with tiny spines on their
surface. A sea star's spines are used
for protection from predators, which
include birds, fish and sea otters.
Sea stars move using their tube feet.
Sea stars move using hundreds of tube
feet, which are located on their
underside. The tube feet are filled with
sea water, which the sea star brings in
through the sieve plate, or madreporite,
on its top side. Sea stars can move more
quickly than you might expect. If you
ever get a chance, try visiting a tide pool
or aquarium and take a moment to
watch a sea star moving around. The sea
star's tube feet also help the sea star
hold its prey, which includes bivalves like
clams and mussels.
Some Facts About Sea Cucumbers
There are A LOT of sea
cucumbers.
There are some 1000 or more
species of sea cucumbers and
most of them are shaped like
soft bodied cucumbers, thereby
giving them their name of sea
cucumbers. All of them are
ocean floor dwellers, with many
of them living in extremely deep
waters. In fact, sea cucumbers
form up to 80% of the entire
animal mass that occupy the
deep ocean floor.
Sea Cuc’s really ARE green!
Sea cucumbers play an
important role in reef
"recycling". They feed by
gathering organic detritus and
even bacteria from the water
or the sand. They speed the
breakdown of these bits of
plant and animal debris -extracting energy for their
own survival and recycling
materials that seaweeds can
absorb as fertilizers. The sand
that sea cucumbers process
along with the detritus they
consume is eliminated as
strings of sandy beads. This
processing helps "turn over"
sediments. People are even
planning on growing sea
cucumbers in areas of fish
aquaculture to help “process”
all the extra nutrients that the
fish produce (in the form of
feces) in order to help the
water quality.
Sea cucumbers
can regenerate
their organs.
Sea cucumbers may
lack spines and the
protection of a
skeleton, but they
have many different
means of defense
from predators. With
soft, flexible bodies,
they are able to
crawl under rocks
and into reef
crevices. Some
species have
repellant or toxic
chemicals in the skin
that make them
distasteful. Others
eject sticky threads
from the anus -these threads
entangle and
immobilize potential
predators and the
cucumber crawls to
safety. Still other
sea cucumbers can
eject part of their
digestive system as
a defensive
mechanism,
regenerating lost
parts later.
Sea Cucumbers don’t have many predators.
There are few marine animals that feed on sea
cucumbers, particularly since almost all sea
cucumbers have some form of chemical defence
that makes them disgusting to their predators
and sometimes dangerous as well. However, that
didn’t prevent humans from becoming the single
largest predator of sea cucumbers. In fact, some
species of sea cucumbers have been so over
harvested by humans that they face dwindling
populations and some becoming endangered
even.
Sea Cucumbers reproduce sexually and
asexually.
Sea cucumbers can reproduce very well; even in
the absence of other sea cucumbers, they can
reproduce asexually. In places where human
activity has not impacted sea cucumber
populations, sea cucumbers flourish, especially
in the deep oceans. Sea cucumbers usually
reproduce sexually though, but they do so by
releasing their sperm and eggs into the water in
large quantities (aka broadcast spawning) as do
most echinoderms. The egg is fertilized when it
meets a sperm in the water, but in certain
species, fertilized eggs are taken into an adult
sea cucumber body and develops within it. When
the young sea cucumber grows large enough,
they are expelled from the adult.
They have five-part radial symmetry.
Sea cucumbers lack some features
common in other echinoderms. The 5part body design you can see in sea stars
and even sea urchins is harder to find in
many sea cucumbers. Some sea
cucumbers have 5 rows of tube feet
running the length of the body. But, in
many sea cucumbers, you have to look
internally to see the pattern -- five
muscle bands run the length of the body.
A few sea cucumbers have 5 "teeth"
around the anal opening. These "rump
teeth" are not used in feeding, but may
protect the cucumber from parasitic fish
that shelter in the cucumber's body.
Name
Date
Block
Echinoderm Crossword
1
Down
2
3
1 Another name for sea star. (8)
4
5
2 Liquid that fills an echinoderm’s
vascular system. (5)
3 Type of symmetry exhibited by
echinoderm larvae. (9)
6
7
5 Type of symmetry exhibited by
adult echinoderms. (6)
8
9
10
11
8 A sea star that damages coral
reefs. (5,2,6)
12
9 Asteroidea and Ophiuroidea.
(12)
13
10 Echinoderm vegetable?! (3,8)
14
15
11 The group of brittle stars and
basket stars. (11)
12 Re-grow a limb like a sea star.
(10)
16
13 What echinoderm means. (5,4)
14 A sea star with many legs. (3,4)
17
Across
4 Type of habitat that echinoderms live in? (6)
6 The group of true starfish. (10)
7 A favorite food of sea stars. (7)
10 Echinoderm money?! (4,6)
15 Kind of feet echinoderms have. (4)
16 Echinoderm with long spines. (3,6)
17
Expelling internal organs like a sea cucumber. (10)
Echinoderm Factsheet
Crossword
Across:
3. What does a sea star pump into its tube feet?
4. What type of symmetry do echinoderms have?
7. What percentage of the ocean floor's biomass does the sea
cucumber make up?
9. The type of sea star with many arms.
11. What are the sea star's eyes called?
Down:
1. How long does it take for a sea star to
regenerate an arm?
2. What does the sea star have covering it for
protection?
3. What does echinoderm mean in Latin?
5. How do sea cucumbers reproduce sexually?
14. How do sea stars move?
6. What phylum do sea stars and sea
cucumbers belong to?
15. What are the teeth around the anus of the sea cucumber
8. What do sea stars eat?
called?
16. After expelling its internal organs, what can a sea
cucumber do?
10. What is the sea cucumber shaped like?
12. What does a sea star push into a bivalve
when it's eating it?
13. What do sea cucumbers eat?