Echinoderms: Spiny-Skinned Creatures

Amazing Fact: Echinoderms use hundreds of tiny tube feet to walk, grip prey, and cling to rocks with incredible strength! Found only in the ocean, these spiny-skinned creatures include sea stars, sea urchins, and sea cucumbers. Despite their alien appearance, echinoderms are more closely related to humans than to most other invertebrates!

What Makes an Echinoderm?

The term echinoderm is from the Greek echinos (spiny) and derma (skin). Echinoderms are perhaps the most unusual of all animal phyla. At first glance, the sea urchin might not appear to have much in common with a sea star, but they are remarkably similar in construction.

Five-Part Radial Symmetry

All members of this phylum have a distinctive pentamerous radial symmetry (pentamerous meaning “five parts” and radial meaning radiating from a center point, like the spokes of a wheel).

Despite the fact that a sea star might have 6, 7 or even more arms, there are always five primary radii, and any extra arms are just multiples of five. For example, a sea star with 8 arms actually has two arms on one radius, and could be considered to have only 4 radii. Humans, like most other animals, have bilateral symmetry, meaning a right and left side.

The Water Vascular System

Echinoderms have a unique hydraulic system called the water vascular system. This system is a network of fluid-filled canals that extends throughout the body and operates the tube feet.

The tube feet are little suction cups which the animal can fill with water from the water vascular system, and then use for locomotion and grasping things (like food). The water enters the system through a structure called the madreporite (mad-ruh-PORE-ite), which is visible as a colored spot on the top surface of sea stars.

Endoskeleton

Unlike arthropods which have an external skeleton (exoskeleton), echinoderms have a hard internal skeleton (called an endoskeleton) made of calcium carbonate plates. While in life, the endoskeleton is covered by a layer of skin and tissue.

From Bilateral to Radial

Interestingly, although most mature echinoderms are benthic (meaning that they live on the bottom), the larvae are usually planktonic with bilateral symmetry. During the process of maturing, the echinoderm will change its body shape from bilaterally symmetrical to radially symmetrical, and in the process, settle down on the sea floor.

The Stelleroids: Sea Stars

Two sea star species resting on underwater rocks

Perhaps the most common echinoderm is the sea star. Although more well known as the “starfish,” sea stars are not actually fish. The scientific community prefers to reserve the term “fish” for vertebrates with fins.

The subphylum Stelleroidea contains two classes of sea stars:

Class Asteroidea (True Sea Stars)

Asteroids have arms which seem to connect together in such a way as to make it difficult to discern where the arms end and the central disk begins. Examples include cushion stars and sun stars.

The voracious Spiny Sun Star of the North Atlantic stalks and eats other sea stars! It’s an Asteroid with up to 12 arms!

Class Ophiuroidea (Brittle Stars)

Ophiuroids have a distinct central body part (called a central disk) with arms radiating out from the body. Adjacent arms do not connect with each other. Examples include brittle stars and basket stars.

Sea Star Anatomy

The sea star’s aboral (top) surface is spiny looking when closely examined. The rumpled skin contains several different types of formations:

Dermal Branchiae – Some of the bumps on the surface are used to absorb oxygen from the water.

Pedicellaria – These pincher-like pairs of organs can be used to pluck things off of the skin of the sea star. For example, they can remove a larval barnacle that might try to settle and grow on the sea star.

Madreporite – The single colored spot on the aboral surface is a calcareous piece of the water vascular system filled with tiny holes, like a strainer. It acts as a filter between the water vascular system and the ocean.

Eyespots – The sea star has a light sensitive organ at the tip of each ray. Although the star cannot see in the way we do, it can detect the presence and direction of light.

Regeneration Powers

Sea stars are capable of regenerating limbs in the event that one or more is severed or damaged. The wound first closes off, and in time, the new limb will begin to grow. It can take over a year, but the arms will be good as new!

In a few species, the severed limb can regenerate a new sea star, but in most species, the severed limb dies.

Feeding: The Ultimate Clam Opener

Sea stars eat a variety of different things, including barnacles, clams, mussels, snails, sea urchins, and in some cases, other sea stars!

Many sea stars, such as the Northern Sea Star, eat mussels and clams in a fascinating way. The sea star first surrounds its intended victim. Then it applies outward force (with its suction cup equipped tube feet) on the two mussel shells (called valves), to pull them apart.

Incredible Strength: Contrary to popular belief, the sea star does not need to apply force for a long time to tire out the mussel. The sea star can apply so much force to the mussel valves (7 or more pounds!) that it will actually bend the shell.

Seizing the moment, the sea star then everts its stomach out through its mouth, and into the mussel (only a 1/100th of an inch opening is required!). Once the sea star begins to digest its victim within the victim’s own body, the victim dies. When the star is finished with the mussel, nothing remains but a shell.

Echinoderm Classes

Asteroidea

1,900+ species – True sea stars, sun stars

Ophiuroidea

2,000+ species – Brittle stars, basket stars

Echinoidea

950+ species – Sea urchins, sand dollars

Holothuroidea

1,400+ species – Sea cucumbers

Crinoidea

600+ species – Sea lilies, feather stars

Echinoderm Gallery

The mouth of an Aristotles Lantern Sea Urchin

This basket star, photographed in the Caribbean, is a good example of a Ophiuroid.

Crinoid

Horse Star

The Orange-footed sea cucumber lives in the North Atlantic. It resembles a football with tentacles at one end for feeding.

A sea cucumber’s feathery tentacles. It catches plankton in its arms and then licks them off one at a time.

This sea cucumber is eviscerating a portion of its intestines to thwart an attack.

This Northern sea star, photographed in Massachusetts, is regenerating two lost arms. It will take over a year, but they will be good as new!

Two sea star species resting on underwater rocks

Sea Star tube feet

This Purple sea urchin from California has hundreds of pointy spines for protection.

The Tiger Tail sea cucumber feeds on particles found in the sand on the bottom.

Crinoid

Echinoids: Sea Urchins

The class Echinoidea includes the sea urchins, heart urchins, cake urchins and familiar sand dollars.

Close-up of a purple sea urchin showing sharp protective spines

Spiny Protection

Sea urchins are well known to most people, having a large number of sharp spines pointing out in all directions. These spines offer protection from many would-be predators. The spines are joined to the skeleton of the animal, called the test, in a form of ball-joint.

This coupled with the fact that there are muscles attached to each spine enables the urchin to swivel its spines in the direction of a predator. The test is an egg-like spherical structure constructed of rows of radially arranged plates fused together.

Close-up of Aristotle’s lantern, the mouthparts of a sea urchin

Aristotle’s Lantern

The creature has 5 paired rows of tube feet which, when extended, are long enough to reach past the length of the spines. They contain suckers. The anus is on the top of the creature, while the mouth is on the underside.

The mouth contains five teeth which are arranged pointing towards the center of the mouth. This structure looks and works like the jaws of a drill chuck and has been termed “Aristotle’s Lantern” because it was first described in detail in a book by Aristotle as looking like the top of an oil lamp.

The creature uses its tube feet to pull itself against the substrate so it can gnaw away at algae with its mouth.

Holothuroids: Sea Cucumbers

The class Holothuroidea is composed of creatures called sea cucumbers. A sea cucumber is so named due to the fact that many members of the group (but not all) resemble the garden variety of cucumber. The similarity ends there.

Body Structure

Sea cucumbers are usually somewhat football shaped and lay on their side on the bottom. They have 5 rows of tube feet running lengthwise, like the seams on a football. Three of the rows of tube feet are well developed and are in contact with the substrate. The other two rows are usually either underdeveloped (and not used) or missing completely.

At the oral end of the animal is a mouth surrounded by tentacles. These tentacles, usually branched, are actually special tube feet and are part of the water vascular system. The water vascular system is not filled with sea water in sea cucumbers, as in sea stars and urchins, but rather with a special body fluid. There is, therefore, no direct interface (madreporite) between the outside water and the internal organs of the animal.

Sea cucumber feeding with extended tentacles underwater

Feeding Behavior

The sea cucumber feeds in a fascinating way. It may position itself in a spot on the ocean floor where a current will bring a steady supply of food (plankton and other organic particles) its way. The tentacles are opened to collect the food.

Then, the cucumber sticks each tentacle in its mouth, one at a time, and licks them off. As soon as a tentacle has been licked, the cucumber takes it out of its mouth and holds it out to collect more food while it licks the next tentacle. The sea cucumber will do this for hours at a time.

Other kinds of sea cucumbers use their tentacles to sift through sand on the bottom for particles of food, rather than collect plankton from the water.

Sea cucumber ejecting Cuvierian tubules as a defense mechanism

Defense Mechanisms

Toxicity: Many sea cucumbers are quite poisonous. These cucumbers, if injured, can kill fish in the same aquarium. The poison of some sea cucumbers has shown promise as an inhibitor in the growth of cancer cells.

Self-Evisceration: The sea cucumber has another interesting (if not slightly disgusting) habit. When a sea cucumber is attacked, it may expel some of its internal organs. This could either satisfy a predator, or scare it off. The cucumber will then proceed to grow another set of organs.

Sticky Defense: Some sea cucumbers can secrete a sticky glue-like substance as a defense mechanism. This goo is so sticky that it absolutely cannot be removed from the skin without shaving off any hair with which it has come in contact. Historically, people have used this substance as a bandage to bind wounds.

Culinary Note: The Chinese eat certain sea cucumbers, and consider them quite delicious. Some species have edible muscular body walls with a somewhat “gelatinous” texture.

Crinoids: Feather Stars

The class Crinoidea includes sea lilies and feather stars, named for their superficial resemblance to flowers and drifting plumes. Despite their plant-like poise, these are distinctly animals with a crown of long, delicate arms branching from a cup-shaped central disk called the calyx.

Sea lilies anchor to the seafloor with a jointed stalk, while feather stars lack a permanent stalk as adults. Instead, they perch using small claw-like cirri or swim gracefully by rhythmically waving their arms, a sight that looks like an underwater ballet performed by ornate fans.

Crinoids feed in a way that is both efficient and mesmerizing. Each arm bears rows of tiny branches equipped with sticky tube feet that form a living net in the current. As water flows past, plankton and detritus become ensnared. The captured food is passed down grooves toward the mouth, which sits atop the calyx. A busy feather star can sit for hours, quietly harvesting whatever the ocean delivers.

If grabbed by a predator, crinoids shed their arms and regenerate new ones. Their fossil record stretches back over 450 million years. A living sea lily standing on its stalk today looks remarkably similar to its ancient ancestors.

Amazing Adaptability

No matter what you think about the looks of Echinoderms, they are very interesting and diverse animals with amazing adaptability. From the powerful sea star that can pry open clams, to the regenerating limbs, to the self-eviscerating sea cucumber, echinoderms have developed remarkable survival strategies over millions of years.

Echinoderm Classes

Asteroidea

1,900+ species – True sea stars, sun stars

Ophiuroidea

2,000+ species – Brittle stars, basket stars

Echinoidea

950+ species – Sea urchins, sand dollars

Holothuroidea

1,400+ species – Sea cucumbers

Crinoidea

600+ species – Sea lilies, feather stars

Echinoderm Gallery

The mouth of an Aristotles Lantern Sea Urchin

This basket star, photographed in the Caribbean, is a good example of a Ophiuroid.

Crinoid

Horse Star

The Orange-footed sea cucumber lives in the North Atlantic. It resembles a football with tentacles at one end for feeding.

A sea cucumber’s feathery tentacles. It catches plankton in its arms and then licks them off one at a time.

This sea cucumber is eviscerating a portion of its intestines to thwart an attack.

This Northern sea star, photographed in Massachusetts, is regenerating two lost arms. It will take over a year, but they will be good as new!

Two sea star species resting on underwater rocks

Sea Star tube feet

This Purple sea urchin from California has hundreds of pointy spines for protection.

The Tiger Tail sea cucumber feeds on particles found in the sand on the bottom.

Crinoid

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