Echinoderms are my favorite intertidal animals. They belong to the phylum Echinodermata and live only in marine environments. You won’t find an echinoderm in a river or stream. Their name originates from the Greek word for spiny skin, and all echinoderms have either a hard spiny covering or spiny skin.
What is an echinoderm? While there are around 7,000 species of echinoderms, they fall into seven major classes. These are the sea lilies, sea urchins, sand dollars, sea cucumbers, brittle stars and basket stars, sea daisies, and sea stars – or starfish. Echinoderms live in every marine habitat from the intertidal zone to the deep ocean. They are often brightly colored, and their reds, greens, purples, and oranges make them stand out against the monochromatic background of clam and cockle shells.
Some species of echinoderms do not look as if they should belong to this phylum. A sea urchin does not resemble a sea star, and sea cucumbers look nothing like other echinoderms. Body types range from flowerlike sea lilies to slug-shaped sea cucumbers. However, in addition to the spines, all echinoderms have pentamerous (five-part) radial symmetry, an internal skeleton, and a water-vascular system derived from a central cavity. Look at a sand dollar the next time you pick up one and note the star in the center of the disk and the five-part symmetry. Examine a dead sea urchin once its spines have dropped, and you will see the five plates. If you dissect a sea cucumber, the pentamerous symmetry reveals itself. Some sea stars have many legs, but look closely, and you will see the five-part divisions.
An echinoderm has a simple digestive system with a mouth, stomach, intestines, and anus. In many species, the mouth is on the underneath side of the animal, and the anus is on the top. A sea stars can push its stomach through its mouth, allowing it to digest its prey externally. For example, it can insert its stomach into a clam shell once its powerful legs have pried open the shell. The ability to extrude its stomach allows a sea star to eat animals larger than its mouth.
While echinoderms do not have brains, they do have nervous systems. They have tiny eyespots that can detect only light and dark, and some of their tube feet are sensitive to chemicals, allowing them to find food. They do not have a heart, but they have a network of fluid-filled canals. To breathe, they use simple gills, and their tube feet take in oxygen and pass out carbon dioxide.
Echinoderms are either male or female, and they reach sexual maturity when they are two to three years old. Most species broadcast spawn by releasing their eggs and sperm into the water at the same time. A sperm meets up with an egg by chance and fertilizes it. This type of reproduction is hit or miss, but a female releases as many as one hundred million eggs at one time, improving the chances of some being fertilized. Larvae float free for a period and eventually settle to the bottom and develop into their adult form.
Lifestyles of the different groups of echinoderms vary greatly, and over the next few posts, I will take a closer look at some of the species found in Alaska.
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Kodiak Island Wildlife by Robin Barefield with Photos by Mike Munsey
Robin Barefield is the author of four Alaska wilderness mystery novels: Big Game, Murder Over Kodiak, The Fisherman’s Daughter, and Karluk Bones. Sign up below to subscribe to her free, monthly newsletter on true crime and mystery in Alaska, and listen to her podcast, Murder and Mystery in the Last Frontier.
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