Tide Zones / Seaweed / Porifera / Coelenterates / Molluscs / Arthropods / Echinoderms

Echinoderm means spiny skin. Thus, echinoderm is the phylum that contains animals with "spiny skin". This phylum contains animals like the sea star, sea cucumber, sand dollar, sea urchin, and the brittle stars.

The body plans of echinoderms have similar characteristics. They all have radial symmetry. This means that there are equal units arranged in a circle around a central disc. Most of these animals have five of these equal parts, giving them pentamerous radial symmetry. Look closely at a sea star, sand dollar, or a sea urchin and you will see this pattern. It's easiest to see pentamerous radial symmetry on the "test" (skeleton) of a sand dollar or urchin. The sea star's equal units are its rays, or arms.

All of these echinoderms have tube feet for locomotion. They can be seen on the underside of a sea star, or between the spines of sea urchins and sand dollars. The cucumber has both tube feet on its underside and oral tube feet used for collecting food. Click here for a movie of a seastar moving.

The mouths of these creatures are located on the underside except in the case of the sea cucumber. Its mouth is located at the end of its tube-like body and in the center of its oral tube feet.

Sea Stars

The sea stars will be seen in abundance in the tide pools clinging to rocks or mussel colonies. They use the suction power of their tube feet to hold fast in the pounding surf or in the surge of the ocean. They can survive while the tide is out by storing water in their body and tube feet. However, they will become limp if pulled off rocks and kept out of water because their tube feet will leak. Therefore, it is a wise practice when observing them not to hold them out of water for long periods of time.

The sea stars breathe through small finger-like skin gills, which may be observed through a hand lens, sticking up through the bony "spines" of their bodies. Notice how their spines are actually little bony nubs.

Sea stars also have the ability to regenerate torn off portions of their bodies. In fact, this is one way they reproduce. The only requirement is that part of the central disc must be attached to the part that is to be regenerated. Fishermen used to make the mistake of cutting sea stars up when captured in their oyster scoops thinking this would kill the sea stars, which prey on molluscs. What they were actually doing was adding to the population. Each piece grew into a new and whole sea star!

Sea stars are carnivorous for the most part. They feed mostly on molluscs. They use their tube feet to pry open the shells. Then, they throw their pyloric stomach out to digest the meat before it is brought back into its body. They also feed on certain snails and barnacles.

We will see several kinds of sear stars at the tide pools. Some include the following: common purple stars, ochre stars, bat stars, sunflower stars, six rayed stars, and brittle stars. Brittle stars are so named because they will often cast off their rays to escape being handled or preyed upon. Brittle stars are mainly scavengers and can be found from the tide pools to the deepest parts of the ocean.

Sea Urchins

Sea urchins look like big pin cushions. They protect themselves with these large spines. However, both urchins and sea stars are more concerned about protecting themselves from smaller pests. They have small skin pinchers that protrude from their bodies, which keep their bodies free of pore-clogging algae and parasites. The spines also aid in locomotion acting as stilts to keep their bodies up off the ground so the tube feet can pull them around. It is wise to handle these creatures very carefully.

The urchins feed mostly on encrusting algae, which is scraped off the rocks with their five pointed teeth attached to their jaw inside their bodies. This jaw structure is also known as Aristotle's lantern. They also feed on the stem-like structures of brown algae known as the stipe.

The predators of urchins are certain crabs, gulls, seals, otters, humans and even sea stars. Certain parts of the urchin are used in oriental dishes and the collection of them has reached alarming proportions.

Sea urchins live, for the most part, attached to rocky crevices. Since they are round in shape, the crevices or ledges aid in protection from waves and surge.

Sand Dollars

Sand dollars are found on the sandy shore or muddy bottoms. They feed standing on edge with their tube feet and velvety-like spines acting as filter feeding devices. They have tube feet also for the same reasons as sea stars and urchins. However, movement is more a result of the wave-like motions of their bristle-like spines. The star pattern on the test is actually caused by the protrusion of specialized respiratory (breathing) tube feet.

The test, or shell, is often found on the sandy shore scrubbed clean by the sanding action of the waves and sand. The tests with holes in them belong to keyhole sand dollars and actually add strength to the test by fusing the upper and lower shell at more points. These holes also assist the sand dollar in allowing sand to sift through the test as an aid in locomotion and camouflage.

Shake a sand dollar test and you'll hear the bony structures of Aristotle's lantern (jawbones) rattling around.

Sea Cucumbers

Sea cucumbers are also echinoderms. They have tentacles at their oral opening used for feeding. They look like sea snails; however, they have the radial symmetry and other characteristics of other echinoderms. They are also found in the crevices of the rocky shore.

One interesting characteristic of the sea cucumber is that, when provoked, it will often cast out its intestines to entangle, frighten or confuse the predator. Sometimes, the predator will feed on these intestines allowing the cucumber to escape. The sea cucumber then, obviously, regenerates its intestines.

These creatures are active, filter feeders. They take microscopic, dead organic matter out of the water or from the muddy bottom with their tentacles. These tentacles are covered with a sticky slime to which particles adhere.

Fish, certain crabs and human beings prey upon sea cucumbers. They are considered a delicacy in Oriental dishes.