Echinoderms (Sea Stars, Sea Cucumbers and Sea Urchins): Characteristics, Behavior, Reproduction

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Echinoderms (member of the phylum Echinodermata, distinguished by their radial symmetry): starfish, brittle stars, sea urchins, sand dollars, sea cucumbers and crinoids (sea lilies or "stone lilies")

Echinoderms (meaning "spiny skinned") are a phylum of animals that includes sea urchins, sea stars and sea cucumbers. They are invertebrates with no head. Their internal organs — and often their outer organs — are arranged in five symmetrical parts around a central stomach. The creatures have no front or back. Humans and other mammals are bilaterally symmetrical with nearly identical left and right sides and distinct front and backs.

Echinoderms “do not have a ‘heart’ or anything analogous to it,” Chris Mah, a marine invertebrate zoologist at the Smithsonian National Museum of Natural History in Washington, told National Geographic. They don’t have blood either. Instead, they have millions of tiny, hairlike structures called cilia that beat constantly, pumping seawater via “a system of internal pipes and bags,” Mah says. Their internal cavity also has “all the various cells needed for transporting nutrients, immune cells, and so forth.” [Source: Liz Langley, National Geographic, February 13, 2016]

Echinoderms live in temperate, tropical, polar, saltwater and marine environments in all the world’s oceans. Except for a few species which inhabit brackish waters, all echinoderms are benthic (bottom-dwelling) organisms found in marine environments. Echinoderms inhabit depths ranging from shallow waters at tide lines to the deep, deep sea.

Echinoderms are usually important and intricate parts of their ecosystems. They may be keystone species, meaning their presence or absence strongly affects populations of other species in area where they live. Their main known predators are otters, flounders and haddock 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. Sea cucumbers discharge sticky tubules, known as Cuvierian tubules , at a potential predator.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase; Encyclopedia of Life; Smithsonian Oceans Portal ; Monterey Bay Aquarium ; MarineBio; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Coral Reef Alliance ; Global Coral reef Alliance ; Global Coral Reef Monitoring Network

Echinoderm Species and Taxonomy


There are around 7,000 different species of echinoderm. Most echinoderms have tough skins and flexible spines and/or tube feet. They also have unique groups of hydraulic organs that serve several functions and work in conjunction with muscles to power the tube feet. The tube feet have three main functions: 1) they help they animal move with powerful suction that grasps onto things and muscles that expand and retract; 2) they can take in oxygen from the seawater and serve as gills; 3) they are packed with sensory neurons and help the animal sense its surroundings; and 4) help hold on to prey.

Echinoderms were once grouped with cnidarians (corals, sea anemones, and jellyfish) and ctenophores (comb jellies) because of their have radial symmetry (symmetry around a central axis), but it was later discovered that the radial symmetry of echinoderms is secondarily derived, both during embryological development and evolutionary history. Larval echinoderms are bilaterally symmetrical but not radial symmetrical. The oldest echinoderm fossils are neither radial nor bilateral. [Source: Cheryl Lewis, Animal Diversity Web (ADW) /=]

There are about 13,000 extinct species of Echinodermata known from the fossil record. The phylum is the largest without any freshwater or terrestrial forms. Echinoderms are osmoconformers (organisms that maintain their internal fluids iso-osmotic to their environment). This means they maintain an internal salinity similar to their ambient conditions with little ionic regulation. Most marine invertebrates including sea stars, crabs, jellyfish, lobsters, sea squirts, and scallops are osmoconformers. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]

Echinoderm Physical Characteristics

parts of a sea star (star fish)
1) stomach
2) Intestine
3) Rectal gland
4) Stone canal
5) Madreporite
6) Pyloric duct
7) Pyloric cecum
8) Cardiac stomach
9) Gonad
10) Ambulacral plates
11) Ampullae

Echinoderms are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (have a body temperature that fluctuates with the surrounding environment), have bilateral symmetry (both sides of the animal are the same) and have radial symmetry (symmetry around a central axis), [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web: Larvae range from a few millimeters to a few decimeters, while adults can range from less than one centimeter to two meters. While adult forms are radially symmetrical, larval forms are always bilateral. The have radial symmetry (symmetry around a central axis), 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. /=\

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.

Echinoderm’s Water Vascular System

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. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]

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. grams. AB)./=\

Echinoderm Behavior, Feeding Senses and Communication

Water vascular system of a starfish, showing the ring canal, the radial canals, ampullae (small bulbs), and tube feet

Echinoderms are diurnal (active during the daytime), nocturnal (active at night), motile (move around as opposed to being stationary) and sedentary (remain in the same area), Most radially symmetric animals are sessile (fixed in one place) but 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. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]

Echinoderms sense using touch and chemicals usually detected with smell and communicate with chemicals usually detected by smelling. They also employ pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species). /=\ The non-centralized nervous system allows echinoderms to sense their environment from all sides. Adult pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species) may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species).

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 (animals that primarily eat plants or plants parts), detritivores (mainly eat decomposed plants and/or animals), 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. Sea cucumbers may also eviserate their digestive and other organs in response to predation or seasonal events./=\

Echinoderm Reproduction and Development

tube feet on a sea star

Echinoderms are oviparous (young are hatched from eggs) and are iteroparous (offspring are produced in groups). Reproduction is external, meaning the male’s sperm fertilizes the female’s egg outside her body. Echinoderms engage in seasonal breeding and are sequential hermaphrodites in which individuals change their sex at some point in their lives and typically produces eggs and sperm at different stages their lives. They are also simultaneous hermaphrodites in which individuals have sex organs of both sexes and can produce both sperm and eggs even in the same breeding season. [Source: Renee Sherman Mulcrone, Animal Diversity Web (ADW) /=]

Echinoderms are polygynandrous (promiscuous), with both males and females having multiple partners. They are mainly gonochoristic (having separate sexes), with exceptions among the asteroids, sea cucumbers and ophuroids. Sea cucumbers 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 involvement 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. There is pre-fertilization provisioning. During the pre-birth stage provisioning and protecting is done by females.

The life cycle of echinoderms is characterized by metamorphosis — a process of development in which individuals change in shape or structure as they grow. 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 (chemicals released into air or water that are detected by and responded to by other animals of the same species) may attract larvae, which tend to settle near conspecific adults. Metamorphosis in some species is triggered by adult pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species).

Sand Dollars

living sand dollar

Sand dollars (Scientific name: Clypeaster subdepressus) are similar to sea urchins. Found in the Caribbean Sea and the western Atlantic Ocean from North Carolina to Rio de Janeiro, they typically live in coastal areas in sand fields or shelly sediments in the warm waters where there is no or little grass. They spend their lives buried in the sand and collect oxygen and food with one f kind of tube feet and excrete waste with another kind. Sand dollars have a five-pointed star pattern on their surface and very short spines that together have a velvety feel. They settle in the sand or mud and sifting out things like worms, algae and plankton for food. What you see on the beach is the hard material left when the sea creature dies.

Sand dollars are echinoderms. Fossils sand dollars have been reported from late Tertiary (25 million to 2.6 million years ago) deposits in Florida. Dried and bleached sand dollars are commonly sold at shell shops. They have not been evaluated for the International Union for Conservation of Nature (IUCN) Red List but it is believed their numbers are substantial. [Source: Catherine Chappell, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web (ADW): The burying behavior of these animals is interesting. Sand dollars push the thinner side of their bodies through the sandy substrate at a slight angle, and plows forward and upward beneath the upper surface using the spines on the aboral surface (bottom part of the animal). When excavated, they can rebury themselves in six to twelve minutes. When large individuals are turned upside down, they right themselves in approximately one hour. /=\

Sand dollars feed upon the organic material, including diatoms, found in the coarse biogenic sands in which they are buried. According to Animal Diversity Web: They select the larger grains to ingest using oral surface accessory podia endowed with sensory cells. It filters through the substrate and the food-bearing material is translocated to the ambulacral grooves where it is covered in mucus and conducted by the podia and spines toward the mouth. The helmet crab Cassis tuberosa is a major predator of Sand dollars.

Sand Dollars Characteristics and Reproduction

fossil sand dollar

Sand dollars are considered radially symmetrical (symmetry around a central axis). However, they show slight bilateral symmetry (both sides of the animal are the same), because they tend to be slightly elongated, an adaption to facilitate movement through sediment. According to Animal Diversity Web: The bony ossicles of this species fuse together, forming a calcareous, disk-shaped shell or test. The test is covered with short, movable, densely-distributed spines that aid in movement though sand while feeding. The aboral surface bears five conspicuous flower petal-like structures, called petalloids. These are actually the ambulacral grooves. Each petalliod is equal in size and consists of two double rows of paired pores used in respiration. Petalloids also contain tube feet that function in respiration. Unlike tube feet seen elsewhere, they lack suckers at the ends and do not function in locomotion. Sand dollars are large, up to 300 millimeters in length. The oral surface is quite flat, with a slight concavity at the mouth. The test color ranges yellow to dark tan. [Source: Catherine Chappell, Animal Diversity Web (ADW) /=]

The respiration of these animals is quite interesting. The respiratory podia are long low sheets that lie in the ambulacral groove (petalloid) extending from one pore to the other. Their long axes are parallel, and are close to, the surface of the test. The respiratory current generated by epidermal cilia flows over the surface of the podium from the center of the petalloid toward its periphery. The ciliated peritoneum of the water vascular system inside the podium moves a current in the opposite direction, from the periphery toward the center of the petalloid. Oxygen, following its gradient, moves into the podium from the sea water along the entire area of contact between tube food and sea water. Once inside the water vascular system, oxygen is then transferred to the fluid of the perivisceral coelom by another countercurrent mechanism, this one is between the water vascular system and the perivisceral coelom. /=\

The Aristotle's lantern is the name for the strong jaws of this animal that grind the sand particles for digestion. The five bird-shaped ossicles (sometimes called "doves") found in dry sand dollars are actually the five pyramids that bear their five slender teeth. This species has a complete digestive system, including a esophagus, stomach and intestine. The anus is located just posterior to the mouth on the oral surface of the test. Sand grains chosen for digestion are cleaned by the digestive system, then "clean" substrate (without any organic material) is passed out through the anus. There is a great quanitity of sand that passes through their digestive system each day. /=\

The sexes of this species are separate. The eggs and sperm are released into the water from five small gonopores located around the madreporite. (The madreporite is part of the water vascular system and is located on the center of aboral side of the sand dollar. Egg and sperm fuse to form a zygote, which mitotically divides to form a free-swimming bilaterally symmetrical larva. These larva are planktonic, and actively feed within the water column until they metamorphasize into their juvenile form and fall to the sea floor. In an average temperature of twenty-seven degrees Celsius, the larva completed metamorphoses in sixteen to twenty-eight days. The larva resembles an artist easel turned upside down. It has fragile arms formed by lobes of ciliated bands and is supported fragile rods of calcite. Sand dollars are capable of producing eggs and sperm throughout the year. At least sixty percent of individuals examined at any given time were capable of spawning. /=\

The Middle Cambrian was 509 million to 497 million years ago
The Early (Lower) Devonian lasted from 419 million to 393.3 million years ago

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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