The phylum Echinodermata, which contains about 6000 species, gets its name from the Greek, literally meaning “spiny skin.” This phylum exists exclusively in the sea and includes sea stars, urchins, brittle stars, and sea cucumbers. They are simple animals, lacking a brain and complex sensing organs. Echinoderms are characterized by their radial symmetry, water vascular system and internal skeleton.
The most striking feature of all echinoderms is their pentamerous radial symmetry. That is, the body can be divided into five parts (or appendages) which point outward from the center of the body. Interestingly, although most mature echinoderms are radially symmetrical, the larvae usually have bilateral symmetry. During the process of maturing, the echinoderm will change its body shape and settle down on the sea floor.
Another trait common to all echinoderms is their unique internal plumbing system known as the water vascular system. A hydraulic network of canals runs throughout the body, usually ending in a series of tube feet. By examining the ventral side of a sea star, for example, one will be able to see hundreds of tiny feet usually arranged into several rows on each appendage of the star. These are called tube feet. By varying the internal water pressure, the echinoderm can extend and contract its tube feet for locomotion, food collection and respiration.
Characteristic of all echinoderms is the presence of an internal skeleton covered with spines and skin. The skeleton varies with the type of echinoderms. In sea stars and brittle stars, the skeleton consists of multitudes of small calcareous plates called ‘ossicles’ that move with one another, forming flexible joints. Sea urchins and sand dollars have ossicles that are fused, forming a rigid skeletal shell, known as the ‘test’. In sea cucumbers, the calcareous plates have degenerated and are buried in the fleshy body.
Some echinoderms are carnivorous and scavenge the ocean floor. Certain species of sea stars actually extend their stomachs into their unwary victims in order to digest them.
The feather stars and sea cucumbers are mainly filter feeders, and catch whatever they can find floating in the ocean currents.
Reproduction and life cycle
Echinoderms are fairly advanced invertebrates. This is evident in their embryology, which is similar to that of the vertebrates. Most species of echinoderms are diecious, meaning there are separate male and female individuals. Although reproduction is usually sexual, involving fertilization of eggs by spermatozoa, several species of echinoderms, such as sea stars and sea cucumbers, can also reproduce asexually.
Asexual reproduction in echinoderms usually involves the division of the body into two or more parts (known as fission) and the reproduction of missing body parts. Successful fission and regeneration require a body wall that can be torn and an ability to seal resultant wounds. Successful regeneration also requires that certain body parts be present in the lost pieces. For example, many sea stars can regenerate a lost portion only if some part of the central disk is present.
Sexual reproduction involves the external fertilization of eggs by spermatozoa. The fertilized eggs develop into planktonic larvae. The larvae typically go through two stages, called bipinnaria and brachiolaria. They are bilaterally symmetrical and have bands of cilia used in swimming and feeding. As the larvae gradually metamorphose into adults, a complex reorganization and degeneration of internal organs occurs. The left side of the larva becomes the oral surface of the adult, which faces down, and the right side becomes the aboral surface, which faces up. The larvae settle to the sea floor and adopt their distinctive adult radial symmetry.
Sea stars, also known as “starfish”, are probably the most well known echinoderm of the sea. The common name ‘starfish’ is quickly being replaced by the more accurate term ‘sea star’. Since starfish have no relation to fish, the term ‘sea star’ is preferable.
Sea stars typically possess five arms that grade into the central disc. The mouth is located in the center of the underside of the disc and the entire undersurface of the disc and arms is called the oral surface. The opposite, or upper side of the body, is the aboral surface that bears the anus and the ‘madreporite’, which is a small perforated plate through which water is taken into the water vascular system. The water is then directed into canals in the arms and then into the tube feet, which extend with the water pressure. The tube feet are very important to sea stars; they are used for locomotion, food collection and respiration. Sea stars move by adhering their tube feet to the substrate and pulling themselves along. They also use their tube feet to catch their prey.
The skeleton of a sea star is made of embedded ossicles that form an internal framework to support connective tissue. The skin of sea stars is covered with mucus glands and cilia that get rid of the dirt and debris that lands on the sea star.
All sea stars are predators that prey on worms, crustaceans and bivalves. Many sea stars eat with their stomach outside their body. Their tube feet pull the two shells of a bivalve apart. While still attached to their prey’s body, they extend their stomach out through their mouths into the bivalve shell. Digestive juices liquidate the victim and cilia transport the bivalve into the sea star’s body.
Brittle stars are the most mobile, yet the most fragile and inconspicuous group of echinoderms. They have a skeleton of calcareous plates
(or ossicles) embedded beneath their skin. These ossicles are arranged in a line down the arm, each covered by four small shields. Spines and a pair of tube feet extend from tiny pores between the shields.
Brittle stars are rapid crawlers and can usually escape from enemies. Should a predator manage to lunge for it, however, the brittle star will cast off an arm, leaving the predator with a snack as the echinoderm flees. Arms can be regenerated as long as most of the central disc is intact.
Most brittle stars are scavengers and detritus feeders, although they also prey on small live animals such as small crustaceans and worms. Some filter-feed on plankton with their arms.
The skeleton of a sea urchin, known as the ‘test’, is a rigid shell made of flat and fused calcareous ossicles. The test is divided into ten sections, extending from the mouth on the bottom to the anus on top. Five of the sections, known as the ‘ambulacral’ plates, are pierced with holes through which the tube feet protrude. The five plates without the holes are known as the ‘interambulacral’ areas.
Urchins eat using a structure known as Aristotle’s lantern, located in the mouth. At the center of Aristotle’s lantern are five strong teeth that allow the urchin to scrape algae off the rocks. The teeth continue to grow even as they wear down. Their mouth is located on the underside of their body, while any wastes are excreted through the anus at the top of the animal.
Sea urchins mainly congregate in colder, offshore waters, but sometimes travel into shallower waters looking for food. The green sea urchin of New England is often found in tide pools and below the low-tide line. This particular specie of sea urchin has one of the longest scientific names – Stongylocentrotus droebachiensis.
These echinoderms are generally long and wormlike and don’t look much like sea stars or sea urchins. However, they retain five part symmetry, with five rows of tube feet running from the mouth along the body. Sea cucumbers maintain the skeleton of echinoderms, but in most species the skeletal plates are reduced to microscopic ossicles embedded in the thick body.
Surrounding a sea cucumber’s mouth are ten to thirty modified tube feet. These sweep the surrounding water and capture bits of food that are then transferred to its mouth. Some sea cucumbers burrow in the sediment, and digest what is edible, excreting the rest.
Sea cucumbers breathe as water is pumped through two respiratory trees located on either side of their digestive tract. Some sea cucumbers can eject their digestive system and associated organs when disturbed or overcrowded and grow a new set within a few weeks.
The most common species in New England is the Orange-footed sea cucumber. It can grow to almost a foot in length, although individuals found in tide pools are usually smaller.
Once upon a time there was a wise man
who used to go to the ocean to do his writing.
He had a habit of walking on the beach
before he began his work.
One day he was walking along the shore.
As he looked down the beach,
he saw a human figure moving like a dancer.
He smiled to himself to think of someone
who would dance to the day.
So he began to walk faster to catch up.
As he got closer, he saw that it was a young man
and the young man wasn’t dancing,
but instead he was reaching down to the shore,
picking up something and very gently
throwing it into the ocean.
As he got closer he called out,
“Good morning! What are you doing?”
The young man paused, looked up and replied,
”Throwing starfish in the ocean.”
“I guess I should have asked, why are you throwing starfish in the ocean?”
“The sun is up and the tide is going out.
And if I don’t throw them in they’ll die.”
“But, young man, don’t you realize that there are miles and miles
of beach and starfish all along it.
You can’t possibly make a difference!”
The young man listened politely.
Then bent down, picked up another starfish and threw
it into the sea, past the breaking waves and said-
“It made a difference for that one.”