AKA Sea Stars

edited by: Meru Nangia, Omer Zaidi, Becca Levenson, Yasheka Sharma, NK, Jesse Carmen, Hanna Z., AR, Vonai Moyo, Brittany Marcus-Blank, Sam Blatchford, Sarah Vlach, Kevin Nayer, Josh Czik, Daisy Joo, SS, Sarah Schwarzschild, GR, DP, Ethan Richman

Underwater image of sea stars


  1. Asteroidea- Sea stars (see picture above)
  2. Ophiuroidea- Brittle Stars
    Red Ruby brittle star
  3. Echinoidea- Sea urchins and sand dollarssea-urchin.jpg
  4. Crinoidea- Sea lillies sea_lilies.jpg
  5. Holothuroidea- Sea cucumbers sea_cucumber.jpg

General Description

Echinoderms are in the Kingdom Animalia and the subkingdom Deuterostomia along with Phylum Chordata. Some shared characteristics are their radial cleavage, their development of coelom, which are fluid filled cavities formed during embryonic development, from the early stages of the digestive tract, and the formation of the mouth at the end of the embryo opposite the blastopore. (1)
Sea stars are categorized as coelomates, which means they have a fluid filled body cavity with a complete lining. This allows the organs to be attached to each other while still able to move freely within the cavity. Their symmetry is secondary radial anatomy, so the larvae have bilateral symmetry (mirror image when the animal is split down the middle) and the adults have radial symmetry (animal is symmetric along all axis).However, the adults are not symmetrically radial in respect to their anatomy (OZ). Echinoderms are either sessile (can not move at all) or slow moving. They have a thin skin that covers the endoskeleton, which is set of hard plates. Unlike the human skeleton, the echinoderm's skeleton is not at the center of the organism; instead the organs and water vascular system are inside the skeleton. (1)

Diagnostic characteristics

Echinoderms are known to move very slowly, which is not surprising considering they do not have feet like many other animals do. However, star fish do have a unique water vascular system, a group of hydraulic tubes that lead to the external environment. This systems aids in locomotion, respiration, and feeding(10 VM). Branching from the center are these tube feet which aid in movement, acquiring food, and exchanging gases. The majority of echinoderms has a spine except for the sea cucumbers, class Holothuroidea. Echinoderms are charactersied by radial symmetry and by a calcitic skeleton(10 VM). This skeleton is composed of calcium carbonate and several proteins, and acts as an exoskeleton(1O VM). Sea stars have bony, calcified skin that helps protect them from predators. Many sea stars display bright colors that camouflage them or scare off potential predators. Sea starts are purely marine animals, and they do not live in fresh water. They are famous for their ability to regenerate limbs, and in some cases they can regenerate their entire bodies. This is possible because all of their vital organs are located in their arms. (11) (BMB)

Acquiring and Digesting Food
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This starfish has trapped its prey, an entire crab, and is beginning to digest it. (15)(SS)

The digestive system of echinoderms consists of a mouth, a short digestive tract, and an anus. The mouth is on the bottom of the central disk and the anus is on the top of the central disk. Also digestive glands secrete digestive juices to aid in the breaking down of food and the absorption and storage of nutrients. Each of the classes acquires their food in different ways. (1)
Asteroidea use the tube feet to grab their prey which usually is a clam or oyster and brings the prey into the body of the star fish. Once it has a tight grip on its food, it turns its stomach inside out and at the same time breaks open the shell which contains the food. Then digestive juices are released and bring to break down the soft body of the mollusk. (1) Asteroids often have two stomachs called the cardiac stomach and the pyloric stomach. The cardiac stomach is the one that the asteroid pushes out of its body to release digestive juices. The pyloric stomach is used after the cardiac stomach. Its purpose is to further break down the tissues softened by the cardiac stomach. (JAC) (14)
Some asteroids are suspension feeders, which means that they catch food particles as they drift by. Asteroids have mucus on the surface of their bodies that plankton and detritus stick to. The food particles are then moved by the cilia (which are hair-like structures on the body surface) to the mouth (6)[HZ].

Asteriod eating a fish
(Asteriod eating an anchovy). This particular speces uses its cilia to move its food towards its mouth (13 SV).
Some asteroids have pedicellarae, which are structures found among the spines of a star fish. They are made up of a stalk with three valves at the end. The valves can open and close and function in getting rid of any debris that may cling to the asteroid's surface (7) [HZ]. They can also be used to capture prey (6) [HZ].

Ophiuroidea are mainly suspension feeders, acquiring food as it passes through the water around them, but some species are predators and some others are scavengers. (1)

Echinoidea have a mouth that is made for eating seaweed. It has a complex jar structure. (1)

Crinoidea are suspension feeders; their arms wave around circulating the water above the organism while the mouth, facing upward, catches any food that falls in. (1)
Holothuroidea are scavengers. (1)

Sensing the environment

Echinoderms have a nervous system that allows them to react to their environment. At the center of the organism is a central disk where the nervous system originates. From there are ring and nerve cords radiating from the central disk to the arms. (1)Also, Echinoderms have eyespots which can detect light, however, their eyespots are not as sharp as human eyes. (3. Sharma)
Although no species of echinoderms have anything fitting of the conventional definition of a brain, they do have a central nerve ring that surrounds the gut area. The central ring extends neurons into lesser nervous rings located throughout the echinoderm. Although echinoderms lack specialized sensory organs, they do have sensory neurons on their external surface leading to the internal neural rings (4, NK).


The classes Asterodiea, Echinoidea, and Holothurodiea all move in the same way. They use the tube feet that lay along the bottom of each of the five or more arms the branch from the central disk. The feet use a complex unit of hydraulic muscle action to create and release suction of the tube feet. This hydraulic muscle is know as an ampulla. Not only does the ampulla alow the echinoderm to move its feet, it gives the animal control of each individual foot separately. These muscles developed as the animal became free-living. (GR,16). By doing so the animal is able to move along the surface of a rock or commonly along the ground. Sea urchins, in the Echinoidea class, can also move by pivoting their long spines. (BL) (1)
The other two classes have their own ways of moving. Ophiuroidea are able to move by lashing their arms because their tube feet lack suckers to help them move (BL), and Crinoidea are permanently attached to the ocean floor. (1)


Echinoderm uses some of the bumps or spines on its surface to consume oxygen. (3. Sharma)Also, the skin of echinoderms is covered with small gills that are used to exchange gases with the environment. This is also know as a water vascular system. Meaning they use sea water to pump around their body instead of blood. (SJB)(12) This is how they perform respiration. (1)

Metabolic waste removal

Each cell releases its wastes into the water vascular system, which disposes of the wastes into the surrounding environment. (1)
After the organism has primarily digested its food, the metabolic waste passes into the pyloric stomach, the central stomach of the organism. From the pyloric stomach, the waste is forced out of the anus and into the external environment. (9)(RJS)


Circulation is done within the water vascular system of the echinoderm. The system consists of a ring canal in the central disk and five or more radial canals that go throughout the rest of the organism and then lead to the tube feet. It connects to the external environment by sieve like madreporite, semi-permeable holes. Gases, nutrients, and wastes flow through the water vascular system to and from the cells. (1)


According to National Geographic, sea stars utilize sea water rather than blood to pump nutrients through their bodies. (Jesse Carmen)

Self protection

Along with gills, the skin of echinoderms is covered with small spines that defend against predators. The only class of echinoderm that does not have spines for self-protection is Holothuroidia (BL) (1).

Osmotic balance

Each cell within the organism performs osmotic balance for itself. (1)


Echinoderms reproduce via sexual reproduction. Their gonads, organs that produce gametes, female eggs or male sperm, are located in one of the five or more arms, and the majority release their gametes into the ocean. Both sexes release copiuos amounts of gametes to ensure fertilization, in contrast to other organisms. However, there are some echinoderms that have do not release their gametes into the water. Instead fertilization occurs in specialized sacs or breeding bags, and some echinoderms keep these fertilized young within these sacs until the young reach a larvae stage. All echinoderms go through several larval stages before they reach there adult form and become sessile organisms. [2 Nangia]
To some extent, echinoderms also participate in asexual reproduction in that they are able to regenerate a lost limb or portion of their body. It is even possible for a new echinoderm to grow from the severed limb [2 Nangia]. For this to occur, a part of the body/disk needs to still be associated with that section. Asexual reproduction can occur as well through fission, the division of the body into two. Both successful fission and regeneration require that certain body parts be in each peice; for reproduction- in each half, and for regeneration, in the torn limb. In sea cucumbers, tissues are rearranged in both parts. (9) AR.

Temperature balance

Echinoderms do not actively regulate their body temperature. Instead they live in environments where the temperature of the environment is at a temperature that they can survive in. (1) Echinoderms are considered cold-"blooded" organisms, meaning that the temperature of their bodies corresponds to the temperature of the environment. (3 KN)

Role of Echinoderms

Echinoderms are important parts of their ecosystems. Some asteroids are keystone species (are abundant in the community and greatly influence the community structure by the role that they play in the community-their niche). Seas urchins can become too abundant and overgraze their habitat if their populations are not contolled by predators. Some echinoderms have commensal relationships with other species who feed on the leftovers of their prey. For humans, echinoderms are often used in research because they are easy to maintain in the lab and produce a lot of eggs. Humans also eat some echinoderms. For example sea urchin eggs are often used in sushi (8). (RK)

Review1. Suprisingly, the bumps and spines on echinoderms are not just for protection. What else are they used for? (DJ)
2. Explain the process of trapping food and digestion of asteroids. How is this different from the other classes of echinoderms? (SES)
3. Describe the ways that Echinoderms can reproduce. What is necessary for asexual reproduction? (DP)
4. Explain how echinoderms sense their environment. How do they process their sensory input? (ER)

Works Cited

(1) Campbell, Neil, and Jane Reece. Biology. Sixth ed. Ed. Beth Wilbur. Boston: Benjamin Cummings, 2002. 672-675.
[2] "Phylum Echinodermata - Enchinoderms." Tree of Life. Virtual Fossil Museum. 30 Nov. 2008 <>.

(4) Wray, Gregory. "Echinodermata." Duke University. 4 Dec. 2008 <>.

(5) "Red Starfish on Sea Bottom." National Geographic. 7 Dec. 2008. <>.
Mulcrone, R. "Asteroidea." Animal Diversity Web. 2005. 7 December, 2008 <>.
(7) "Pedicellarae." The Echinoid Directory. 7 Dec. 2008 <>.
(8) Mulcrone, Renee Sherman. "Phylum Echinodermata." Animal Diversity Web. 1995. 7 Dec. 2008 <>.
(9) "echinoderm." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 07 Dec. 2008 <****>.
(10) Wray, Gregory. "Echinodermata." Duke University. 7 Dec. 2008 <>
(11) "Starfish (Sea Star)." National Geographic. 7 Dec. 2008 <>.
(13)"Starfish eating an anchovy" 11 Dec. 2008.<>
(14) Stevenson, Jenny, comp. "Starfish - Sea Stars Features." Starfish. June 2006. 14 Dec. 2008 <>.
(15) Vermeer, Diana. "Starfish eating crab." TravelBlog. 17 Dec. 2008 <>.
(16) "Echinodermata." Invertebrates. 2008. 18 Dec. 2008 <>.