Overview

Brief Summary

Echinodermata is an entirely marine taxon, occurring throughout the world’s oceans and includes sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies. Echinoderms are tricoelomate deuterostomes, with a simple hemal/excretory system, decentralized nervous system, and (in most species) separate sexes, with planktonic larvae settling to become benthic adults.

Distinguishing characteristics:

1. Pentamerous symmetry in adults, though larvae are bilaterally symmetrical.

2. Hard endoskeleton of calcareous ossicles, either loosely connected (e.g., brittle stars) or tightly bound (e.g., sea urchins).

3. Water vascular system involved in feeding, locomotion, and respiration.

4. Mutable dermis and connective tissue that, under the control of the nervous system, can fluctuate between extremely rigid and essentially fluid.

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Echinoderms are invertebrate marine animals that are found in all oceans of the world and at all depths. There are no echinoderms in freshwater environments.

Members of the Phylum Echinodermata include many easily recognizable creatures such as sea stars (or starfish), brittle stars, sea urchins, sea cucumbers, and sea lilies. All of these animals are radially or biradially symmetric, and they fill a variety of niches in marine environments as particle feeders, browsers, scavengers, and predators.

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Comprehensive Description

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.

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Description of Echinodermata

Metazoa with water vascular system, tube feet, and ciliated larvae. Marine.
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Distribution

Geographic Range

Mainly a marine group, echinoderms are found in all the oceans.

Biogeographic Regions: arctic ocean ; indian ocean; atlantic ocean ; pacific ocean ; mediterranean sea

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Physical Description

Morphology

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).

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry ; radial symmetry

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Ecology

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.

Habitat Regions: temperate ; tropical ; polar ; saltwater or marine

Aquatic Biomes: brackish water

Other Habitat Features: intertidal or littoral

  • Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
  • 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.
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Trophic Strategy

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.

Foraging Behavior: filter-feeding

Primary Diet: carnivore (Eats non-insect arthropods, Molluscivore , Scavenger ); omnivore ; planktivore ; detritivore

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Associations

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.

Ecosystem Impact: keystone species

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Predation

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.

Known Predators:

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Known predators

  • R. T. Paine, Food webs: linkage, interaction strength and community infrastructure, J. Anim. Ecol. 49:667-685, from p. 670 (1980).
  • Link J (2002) Does food web theory work for marine ecosystems? Mar Ecol Prog Ser 230:1–9
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web (online). Accessed February 16, 2011 at http://animaldiversity.org. http://www.animaldiversity.org
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Known prey organisms

Echinodermata preys on:
detritus
benthic macrofauna
Bivalvia
algae
Ostreoida

Based on studies in:
USA: Rhode Island (Coastal)
USA: Alaska, Torch Bay (Littoral, Rocky shore)
USA: Washington, Cape Flattery (Littoral, Rocky shore)
USA, Northeastern US contintental shelf (Coastal)

This list may not be complete but is based on published studies.
  • J. N. Kremer and S. W. Nixon, A Coastal Marine Ecosystem: Simulation and Analysis, Vol. 24 of Ecol. Studies (Springer-Verlag, Berlin, 1978), from p. 12.
  • R. T. Paine, Food webs: linkage, interaction strength and community infrastructure, J. Anim. Ecol. 49:667-685, from p. 670 (1980).
  • Link J (2002) Does food web theory work for marine ecosystems? Mar Ecol Prog Ser 230:1–9
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Life History and Behavior

Behavior

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.

Communication Channels: chemical

Other Communication Modes: pheromones

Perception Channels: tactile ; chemical

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Life Cycle

Development

Echinoderms are deuterostomes. The larvae, which are planktotrophic or lecithotrophic, have 3-part 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.

Development - Life Cycle: metamorphosis

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Reproduction

Mating System: polygynandrous (promiscuous)

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.

Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); simultaneous hermaphrodite; sequential hermaphrodite; sexual ; fertilization (External ); oviparous

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.

Parental Investment: pre-fertilization (Provisioning); pre-hatching/birth (Provisioning: Female, Protecting: Female)

  • Brusca, R., G. Brusca. 2003. Invertebrates. Sunderland, Massachusetts: Sinauer Associates, Inc..
  • Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
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Evolution and Systematics

Functional Adaptations

Functional adaptation

Pressure allows movement: echinoderms
 

Legs and tubes in echinoderms such as starfish allow movement and feeding by use of hydrostatic pressure.

   
  "Their [echinoderms'] bodies work by unique exploitation of hydrostatic principles. Feet, each a thin tube ending in a sucker and kept firm by the pressure of water within, wave and curl in rows along the arms. The water for this system circulates quite separately from that in the body cavity. It is drawn through a pore into a channel surrounding the mouth and circulated throughout the body and into the myriads of tube feet. When a drifting particle of food touches an arm, tube feet fasten on to it and pass it on from one to another until it reaches the gutter that runs down the upper surface of the arm to the mouth at the centre." (Attenborough 1979:49)
  Learn more about this functional adaptation.
  • Attenborough, David. 1979. Life on Earth. Boston, MA: Little, Brown and Company. 319 p.
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Functional adaptation

Tube feet attach in marine environment: echinoderms
 

The tube feet of echinoderms attach to surfaces via suction adhesion.

     
  "Besides mollusks, echinoderm tube feet make use of suction adhesion, as do a wide variety of other aquatic systems--either as the only attachment mechanism or in combination with others. Among terrestrial systems one thinks first of wet ones--frogs for instance. But the mechanism finds use even in arboreal mammals." (Vogel 2003:427)
  Learn more about this functional adaptation.
  • Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Functional adaptation

Skeleton components arranged efficiently: starfish
 

The skeletons of some echinoderms arrange their calcium carbonate plates efficiently using pentaradial symmetry.

     
  "Starfish have five arms; sand dollars have five radial food grooves on their undersides--this arrangement of five elements radiating from a center point ('pentaradial symmetry') is widespread among the echinoderms but unknown elsewhere in nature…Early echinoderms were covered with a skeleton made up of discrete plates of calcium carbonate. Now one can pave a floor with triangles, squares, or hexagons, but using pentagons alone inevitably leaves gaps. One can't make an array of squares close on itself to form a hollow solid unless at eight special locations the apices of three rather than four squares touch, a distinct complication. And one can't make any array solely of hexagons close on itself at all. Conversely one can get a closed, space-enclosing structure from triangles (tetrahedrons are the simplest, but others such as twenty-sided icosahedrons are possible) and pentagons (the simplest being the twelve-sided dodecahedron). Among the pentagons (fig. 4.13) hexagons can be intercalated practically without limit, but twelve basic pentagons must remain. In the most symmetrical arrangement, these pentagons are in six pairs with members of a pair at the opposite extremities of the solid. If we run an axis between members of one pair, the ten other pentagons then arrange themselves in two nearly equatorial rings. If enough hexagons are intercalated, these can form the key elements of five arms. And a look at any book treating the paleontology of echinoderms reveals a host of hexagonal plates. Perhaps a pentaradial symmetry is, in fact, a 'natural' or easy way to organize a radially symmetrical creature built of a shell of little solid elements!" (Vogel 2003:87-88)
  Learn more about this functional adaptation.
  • Steven Vogel. 2003. Comparative Biomechanics: Life's Physical World. Princeton: Princeton University Press. 580 p.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
Specimen Records: 30518
Specimens with Sequences: 23535
Specimens with Barcodes: 21795
Species: 2023
Species With Barcodes: 1646
Public Records: 16508
Public Species: 968
Public BINs: 1471
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Barcode data

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Genomic DNA is available from 1 specimen
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Conservation

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.

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Relevance to Humans and Ecosystems

Benefits

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.

Positive Impacts: food ; research and education

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