Sea Stars, Sea Urchins [Echinodermata]

Instead of having a shell for protection and support, an echinoderm (spiny-skinned animal) has calcareous structures just under its skin. Sea stars have a meshwork of tiny spines. Sea urchins and sand dollars have fused plates forming rigid skeletons, called tests.

Each class of echinoderm has pentaradial symmetry: their bodies have five similar sections arranged around a central point. The sea star has five arms, and the fused plates of sea urchin and sand dollar tests are also arranged into five identical sections.

An internal water vascular system allows echinoderms to move, breathe and gather food. Each draws water through a pore and circulates it through a network of canals extending to the animal’s five sections. By contracting and relaxing its muscles, the animal forces water into and out of hundreds of tube feet, the end points of this vascular system. As the feet fill with water, the increased pressure lengthens them. As the water recedes, the lower pressure shortens the feet, creating suction holds on a surface. The pressure of the water vascular system is what makes a sea star so rigid when you pick it up. A floppy star is sick or dead.

Most of these animals do not tolerate variations in salinity, and so are only found in marine conditions of the lower estuary and outer Atlantic shoreline.

Northern Sea Star
Asterias rubens
The northern sea star can be found in the estuary since it is part of the genus Astrias, made up of species able to tolerate salinities lower than ocean water. You’ll know you have found this particular species if it has tube feet arranged into four rows on its underside, and a yellow disk in the center of its oral (upper) surface. The disk is the madreporite, through which the sea star absorbs water. Otherwise, their skins vary in color from yellow, orange or red, to olive or purple. A similar species south of Cape Cod, the common sea star (Asterias forbesi), has an orange madreporite.

They usually move across sandy or stony bottoms in subtidal areas, preying on sessile creatures like hydroids, barnacles, snails, oysters, mussels and clams. The sea star engulfs a bivalve with its arms, slowly prying the shells apart with its tube feet. Hours of continued pressure eventually weaken the bivalve’s strong adductor muscles, forcing it to open slightly. Into this crevice, the star secretes a digestive enzyme to break down the soft body inside. Then the star everts its stomach through the crevice to consume its prey. Once in a while you can spot a slightly opened, but otherwise intact, bivalve shell in the intertidal zone.

Bottom-feeding fish prey on sea stars. If a star loses an arm, it can grow it back as long as one-third of the central disk remains.

Green Sea Urchin
Strongylocentrotus droebachiensis
Look for these round, spiny creatures in rocky tidal pools and subtidal areas of the lower Piscataqua. You may find only their remains, in the form of a round skeleton with fused plates arranged into ten sections, called a test.

On the surface of a test, five rows of bumps radiate from the bottom center. These are what the long, moveable spines of living urchins attach to, in ball and socket fashion. Between these rows of spines alternate five bands of tube feet, which carry food into the mouths living urchins.

They move along gravel, shell or rock bottoms scraping up kelp, algae and, if algae are scarce, dead fish and small animals. The lower side of an urchin's body has a mouth with five jaws and continually growing teeth arranged in a symmetrical pattern. The mouth is called “Aristotle’s lantern,” say researchers at http://askabiologist.asu.edu/, because Aristotle himself first compared its pattern to a type of lantern of his day having five thin pieces of horn to protect and illumine a flame.

Urchin predators include sea stars, lobsters, crabs, cod and other large fish, as well as humans. Green sea urchins are harvested from coastal waters for their gonads, called roe. Roe is exported for sale in Japanese markets where it is called uni.

Sand Dollar
Echinarachinius parma
These feed on the microorganisms growing in sandy intertidal and subtidal areas of bays and outer beaches.

The surfaces of these flat echinoderms are covered in spines, smaller and flatter than those of the urchin. This spiny layer, which Kenneth Gosner (author of the "A Field Guide to the Atlantic Seashore" Peterson Field Guide) likens to felt, falls off when the animal dies. The remaining white test is what is commonly found along beaches.

Copyright © 2006 Barbara Driscoll.