Starfish abound on most seacoasts, especially on rocky shores and wharf pilings. They are members of the Phylum Echinodermata and Class Asteroidea. The body consists of a central disk and tapering rays, or arms. At the end of each arm is a small tentacle and light-sensitive eye spot. On the aboral (upper) surface are many blunt calcareous spines which are part of the skeleton. The anus is a minute opening near the center of the aboral disk and nearby is a rounded madreporite, part of the vascular system through which sea water may enter. The mouth is in the middle of the oral or lower side. An ambulacral groove bordered by tube feet and spines extends down the oral surface of each ray. If one of the stiff arms is injured or broken, it can regenerate. Starfish move by the action of their arms on sand or smooth surfaces. On rough or upright surfaces, the starfish travels and holds on using its tube feet.
Starfish feed on mussels, barnacles, snails, limpets, tube worms, and other invertebrates. They also occasionally catch small active animals, even fish. Food is moved by the arms and tube feet toward the mouth. With bivalves, starfish lie over their prey until it opens its shells. The starfish can insert part of its stomach into a space only 1 mm between the shells. Alternately, a starfish can grip the opposite valves (shells) with its tube feet and gradually pull them apart. The starfish everts its stomach over the soft part of the bivalve or other food too big to be brought into the mouth, digests the food and then withdraws the stomach and its contents into its body.
Starfish are dioecious – have both male and female reproductive organs. The male sperm develop first and later the female eggs develop. During the transition, eggs and sperm are both produced. In early summer eggs and sperm are shed into the sea, where fertilization occurs. After releasing the gametes, there is no parental involvement. The several stages of larvae swim for 6 to 7 weeks then settle to the bottom where they metamorphose into the familiar starfish shape.
The pictured specimen is a purple star (Pisaster ochraceus). It was living in a tidepool at Endert’s Beach south of Crescent City CA (Del Norte County). An inertidal species, purple stars are found on wave-washed rocky shores from Prince William Sound in Alaska to Baja California and are very common. A colder water species, purple stars (also called purple starfish) are most common in the Pacific Northwest.
Purple stars have five rays. Their color can range from purple to ocher to brown or even reddish. The color is thought to be related to the diet. Generally, the purple stars in more sheltered waters are purple while those on exposed coasts tend toward the orange and brown shades. Purple stars are very resistant to desiccation and can loose around 30% of their body fluids and still survive.
Since 2013 purple stars (and about 20 other starfish species) are dying in large numbers from a wasting syndrome. The infected starfish exhibit lesions, necrosis, disintegration and death. There is some indication that starfish wasting syndrome may be caused by warming water. Research at Cornell University and University of California at Santa Cruz recently identified the cause as a virus.
Pisaster, the genus name, comes from the Latin “aster” meaning star and “pisium” meaning pea. “Pea” refers to the small bead or pea-like spines. The species designation refers to the color orange. The specimens that were first collected were of the orangish variety and the name was assigned before the full color range of purple stars was understood.