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Echinodermatasea stars, sea urchins, sea cucumbers, and relatives(Also: echinoderms)

By Renee Sherman Mulcrone

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)

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)

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)

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. (Barnes, 1987; Brusca and Brusca, 2003)

Reproduction

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)

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)

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)

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)

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. (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)

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
  • 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)

  • IUCN Red List [Link]
    Not Evaluated

Other Comments

Since they are almost exclusively marine species, echinoderms are probably osmoconformers, with little ionic regulation. (Brusca and Brusca, 2003)

Contributors

Renee Sherman Mulcrone (author).

Glossary

Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

detritivore

an animal that mainly eats decomposed plants and/or animals

diurnal
  1. active during the day, 2. lasting for one day.
ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

external fertilization

fertilization takes place outside the female's body

fertilization

union of egg and spermatozoan

filter-feeding

a method of feeding where small food particles are filtered from the surrounding water by various mechanisms. Used mainly by aquatic invertebrates, especially plankton, but also by baleen whales.

food

A substance that provides both nutrients and energy to a living thing.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

keystone species

a species whose presence or absence strongly affects populations of other species in that area such that the extirpation of the keystone species in an area will result in the ultimate extirpation of many more species in that area (Example: sea otter).

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

nocturnal

active during the night

omnivore

an animal that mainly eats all kinds of things, including plants and animals

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

planktivore

an animal that mainly eats plankton

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

radial symmetry

a form of body symmetry in which the parts of an animal are arranged concentrically around a central oral/aboral axis and more than one imaginary plane through this axis results in halves that are mirror-images of each other. Examples are cnidarians (Phylum Cnidaria, jellyfish, anemones, and corals).

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

scavenger

an animal that mainly eats dead animals

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

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.

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