Introduction to the Echinodermata
. . . from starfish to sea cucumbers. . .
A nyone who has been to the beach has probably seen starfish or sand dollars. The more intrepid
beachcomber may find brittle stars, sea cucumbers, or sea urchins. These and many other
organisms, living and extinct, make up the Echinodermata, the largest phylum to lack any
freshwater or land representatives.
Most living echinoderms, like this sand dollar from Baja California, are pentameral; that is, they
have fivefold symmetry, with rays or arms in fives or multiples of five. However, a number of
fossil echinoderms were not pentameral at all, and some had downright bizarre shapes.
Echinoderms have a system of internal water-filled canals, which in many echinoderms form
suckered "tube feet", with which the animal may move or grip objects.
Echinodermata
By Renee Sherman Mulcrone
Kingdom: Animalia
: Echinodermata
Members of this
Diversity
Echinodermata has approximately 7000 described living species and about 13,000 extinct species
known from the fossil record. This phylum is the largest without any freshwater or terrestrial
forms. (Brusca and Brusca, 2003; Waggoner, 1999)
Geographic Range
Mainly a marine group, echinoderms are found in all the oceans. (Brusca and Brusca, 2003)
Biogeographic Regions
arctic ocean
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ocean; atlantic ocean
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ocean /site/glossary/popup/20020917114731.html/site/glossary/popup/20020917114731.html;
mediterranean sea
Habitat
Except for a few species which inhabit brackish waters, all echinoderms are benthic organisms
found in marine environments. Echinoderms inhabit depths ranging from shallow waters at tide
lines to the deep sea. (Barnes, 1987; Brusca and Brusca, 2003; University of Alabama Center for
Communication and Educational Technology, 2000; Waggoner, 1999)
Habitat Regions
temperate
/site/glossary/popup/20020904145595.html/site/glossary/popup/20020904145595.html; tropical
/site/glossary/popup/20020904145598.html/site/glossary/popup/20020904145598.html; polar
/site/glossary/popup/20020904145371.html/site/glossary/popup/20020904145371.html; saltwater
or marine /site/glossary/popup/20020904145582.html/site/glossary/popup/20020904145582.html
Aquatic Biomes
brackish water
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Other Habitat Features
intertidal or littoral
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Physical Description
Larvae range from a few millimeters to a few decimeters, while adults can range from less than 1
cm to 2 m. While adult forms are radially symmetrical, larval forms are always bilateral. The
radial symmetry is secondarily derived. The pentaradial form, whether it has arms or not, has a
central disc.
An internal skeleton is present throughout members of the phylum. Ossicles, which make up the
skeleton, are below an outer dermal layer. The skeletal and muscular arrangement varies among
groups.
Pedicellariae produced by the skeleton, are pincer-like structures. Found mainly in echinoids and
asteroids, their function is debatable. They may be used to capture prey, clean, or hold items to
disguise from predators.
Echinoderms have a water vascular system consisting of a network of radial canals, which
extend through each of the five extensions (arms or rays) of the animal. Each canal has a lateral
connection which leads to a tube foot, which may be composed of three parts. Internally is the
ampulla and externally is the podia. At the end of the podia is usually a sucker.
Grooves with rows of podia extending from the mouth are called the ambiculacra . Between each
ambiculacra is the interambulacrum. For groups of animals with "arms" (sea stars, for example),
the interambulacrum is just the space between the ambiculacra. For other animals without
furrows (sea cucumbers, for example), the areas are like the ambiculacra, but usually lack holes
for the tube feet.
The water vasuclar system opening, called a madreporite, lies on a particular interambulacrum.
Letters are used to describe parts of echinoderms. The ambulacrum opposite the madreporite is
section A. Moving clockwise, other parts are coded B through E. Sections C and D are termed
the bivium while all the others are collectively termed the trivium. Interambulacrum sections are
named using the letters of the ambulacra sections they are between (e. g. AB). (Barnes, 1987;
Brusca and Brusca, 2003; Waggoner, 1999)
Other Physical Features
ectothermic
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heterothermic
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symmetry
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symmetry
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Development
Echinoderms are deuterostomes. The larvae, which are planktotrophic or lecithotrophic, have 3part paired coeloms. Embryonic coelomic structures have specific fates as the bilaterally
symmetrical larvae metamorphose into radially symmetric adults. Adult pheromones may attract
larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered
by adult pheromones. (Barnes, 1987; Brusca and Brusca, 2003)
Development - Life Cycle
metamorphosis
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Reproduction
Mating System
polygynandrous (promiscuous)
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Echinoderms are mainly gonochoristic (having separate sexes), with exceptions among the
asteroids, holothurians and ophuroids. Holothurians possess a single gonad, crinoids lack distinct
gonads, while asteroids and echinoids have multiple gonads. Echinoderm reproductive strategies
vary from free spawning and indirect development to brooding and direct development.
Spawning is probably a noctural event. (Barnes, 1987; Brusca and Brusca, 2003)
Key Reproductive Features
seasonal breeding
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gonochoric/gonochoristic/dioecious (sexes separate); simultaneous hermaphrodite; sequential
hermaphrodite; sexual
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fertilization
/site/glossary/popup/20020904145546.html/site/glossary/popup/20020904145546.html(External
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oviparous
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Parental investment ranges from no care after the release of eggs for free spawning to brooding
the young. Brooding is found in polar and boreal echinoderms and some deep sea echinoderms,
where environments are more difficult for the larvae. (Barnes, 1987; Brusca and Brusca, 2003)
Parental Investment
pre-fertilization (Provisioning); pre-hatching/birth (Provisioning: Female, Protecting: Female)
Behavior
Most radially symmetric animals are sessile, however, echinoderms are able to move. The water
vascular system originally functioned for collection and transport of food, but evolved to
function for locomotion as well. (Barnes, 1987; Brusca and Brusca, 2003)
Key Behaviors
diurnal /site/glossary/popup/20020904145414.html/site/glossary/popup/20020904145414.html;
nocturnal
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sedentary /site/glossary/popup/20020904145585.html/site/glossary/popup/20020904145585.html
Communication and Perception
The non-centralized nervous system allows echinoderms to sense their environment from all
sides. Adult pheromones may attract larvae, which tend to settle near conspecific adults.
Metamorphosis in some species is triggered by adult pheromones. (Brusca and Brusca, 2003)
Communication Channels
chemical /site/glossary/popup/20020904145606.html/site/glossary/popup/20020904145606.html
Other Communication Modes
pheromones
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Perception Channels
tactile /site/glossary/popup/20020904145500.html/site/glossary/popup/20020904145500.html;
chemical /site/glossary/popup/20020904145606.html/site/glossary/popup/20020904145606.html
Food Habits
Different groups have different feeding habits. Members of the Crinoidea sit with arms
outstretched parallel to the currents and filter feed on passing particles. Most Asteroidea are
predators or scavengers, everting their stomach (called a cardiac stomach), which secretes
digestive enzymes on their prey. Some asteroids are also suspension feeders. Brittle stars of the
Ophiuroidea are predators, deposit feeders, scavengers, and suspension feeders, which feed by
outstretching their arms to capture prey. Ophiuroids lack an intestine and anus, and therefore
have an incomplete digestive system. The members of Echinoidea are suspension feeders,
herbivores, detritivores, and predators. Many have a group of hard plates which retract and grasp
like teeth, commonly called Aristotle's lantern. This allows most sea urchins to graze on algae.
Most Holothuroidea are suspension or deposit feeders. Holothurians may also eviserate their
digestive and other organs in response to predation or seasonal events. (Brusca and Brusca, 2003;
Waggoner, 1999)
Foraging Behavior
filter-feeding
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Predation
Known Predators
otters (Lutrinae)
flounders (Pleuronectidae)
haddock (Melanogrammus)
Echinoderms in general are most vulnerable in their larval stage. As adults, asteroids have an
anti-predator adaptation where they can lose an arm to a predator and the arm is later
regenerated. Holothurians discharge sticky tubules, known as Cuvierian tubules , at a potential
predator. Otters prey mainly on sea urchins. (Brusca and Brusca, 2003)
Ecosystem Roles
Echinoderms are usually intricate parts of their ecosystems. Many asteroids are keystone species.
Sea urchins, if not controlled by predators, may overgraze their habitat. Asteroids have several
commensals, including polychaetes that feed on leftovers from the sea star's prey items. (Barnes,
1987; Brusca and Brusca, 2003)
Ecosystem Impact
keystone species
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Economic Importance for Humans: Positive
Research on echinoderms has contributed to the overall knowledge of animal fertilization and
development. Many echinoderms are easy to culture and maintain in a lab setting, and produce a
large amount of eggs. Sea urchin eggs are also edible and often served in sushi bars. (Brusca and
Brusca, 2003; University of Alabama Center for Communication and Educational Technology,
2000)
Positive Impacts
food /site/glossary/popup/20020904145418.html/site/glossary/popup/20020904145418.html;
research and education
Conservation Status
The European edible sea urchin, Echinus esculentus, is listed as endangered by the IUCN.
Isostichopus fuscus, a holothurian, is listed by CITES. It occurs on the coasts of Ecuador,
Galapagos, Mexico and Peru. (UNEP-WCMC, 2005; World Conservation Monitoring Centre,
2004)
Other Comments
Since they are almost exclusively marine species, echinoderms are probably osmoconformers,
with little ionic regulation. (Brusca and Brusca, 2003)
For More Information
Find Echinodermata information at
Encyclopedia of Life
Contributors
Renee Sherman Mulcrone (author), .
References
Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
UNEP-WCMC, 2005. "Isostichopus fuscus" (On-line). UNEP-WCMC Species Database:
CITES-Listed Species. Accessed January 21, 2005 at
http://www.cites.org/eng/resources/species.html.
University of Alabama Center for Communication and Educational Technology, 2000. "Phylum
Echinodermata – echinoderms" (On-line). Accessed January 16, 2005 at
http://www.ccet.ua.edu/expedition/scsstarsurcbrit.htm.
University of Paisley, 1998. "Echinodermata" (On-line). Accessed January 16, 2005 at
http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/echi1.htm.
Waggoner, B. 1999. "Introduction to the Echinodermata" (On-line). Accessed January 16, 2005
at http://www.ucmp.berkeley.edu/echinodermata/echinodermata.html.
World Conservation Monitoring Centre, 2004. "Echinus esculentus" (On-line). 2004 IUCN Red
List of Threatened Species. Accessed January 21, 2005 at
http://www.iucnredlist.org/search/details.php?species=7011.
Chordates are defined as organisms that possess a structure called a notochord, at least during some
part of their development. The notochord is a rod that extends most of the length of the body when it is
fully developed. Lying dorsal to the gut but ventral to the central nervous system, it stiffens the body
and acts as support during locomotion. Other characteristics shared by chordates include the following
(from Hickman and Roberts, 1994):
bilateral symmetry
segmented body, including segmented muscles
three germ layers and a well-developed coelom.
single, dorsal, hollow nerve cord, usually with an enlarged anterior end (brain)
tail projecting beyond (posterior to) the anus at some stage of development
pharyngeal pouches present at some stage of development
ventral heart, with dorsal and ventral blood vessels and a closed blood system
complete digestive system
bony or cartilaginous endoskeleton usually present.
Kingdom: Animalia
Phylum: Chordata
Members of this Phylum