Geography 316:  Biogeography     In progress 12/16/2003

The Biogeography of  the Purple Ochre Sea Star (Pisaster ochraceus)
 

by Virginia Humphreys, student in Geography 316  Fall 2003

Thank you for visiting our site. This web page was written by a student in Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD.  All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided and sources and references are cited at the end of each page.

Species Name:  Pisaster ochraceus

 

Kingdom  Animalia Phylum:  Echinodermata Class:  Asteroidea Order:  Forcipulata Family:  Asteriidae Genus:  Pisaster Species:  Pisaster ochraceus

Description of Species:

    The Pisaster ochraceus, generally known as the purple ochre star or ochre star, is a common sea star found among the waters of the Pacific Ocean and is distinctively different to the sea stars found in the waters of the Atlantic, according to Ricketts (1985).  Although they are called the purple ochre star, they can be found in a variety of shades from purple to orange and brown depending on its location and water temperatures.  In more protected waters, they are more purple in color whereas on exposed coasts, their colors tend to be more orange or brown.   This is especially notable in juvenile sea stars, whereas with adults it's not as much of a factor.  The purple ochre star has five rays that range in length from 10 to 25cm long.  When submerged beneath the water, the arms of this sea star are flexible allowing them to move about freely, however when exposed during low tides, the tissue is usually very stiff. 

The stiffening of the tissue allows it to anchor itself to rocks or under rock crevasses during periods of exposure to air, a mechanism that is most likely used to protect itself from predators (Lambert, 2000).       Its aboral surface is rough with many white spines that form a web-like shape.  These spines are actually teeth, which the sea star uses to keep itself free of parasites (Ricketts et al., 1985).  Below the aboral surface and extending through each of its rays, is the water vascular system and rows of tube feet (photo 1).  The water vascular system acts as a “hydraulic system” that operates on the fluid chleom.

Photo 1. Pisaster ochraceus signes (Hotchkiss,2000)

 This fluid is generated through their tube feet, which are used to propel the sea star.  In addition, the tube feet act as little suction cups that control the ochre stars respiratory system and allow it to attach itself to hard surfaces to prevent them from being swept away by the pounding surf.  Sea stars are slow moving bottom dwellers moving on average at a rate of three inches per minute.  Outside of the water, they are unable to use their feet and lay there exposed and stranded until the tide moves in again (Guberlet, 1962).                                                                                                                                                                     

Natural History:

    The purple ochre star are carnivores and are considered to be the “principle predator of the Pacific Coast intertidal (Johnson 1976).”  It is able to overcome its prey by wrapping its arms around it and prying its shell open.  Once the shell is slightly opened, it expels its stomach and inserts into the shell to begin digesting its prey (Ricketts et al., 1985).  The preferred food source of the ochre star is the California Mussel (Mytilus californianus) and typically, they can be found in great abundance in and around these mussel beds however, where M. californianus cannot be found, the ochre star will feed on a variety of crustaceans, such as barnacles and small crabs.  Because of its predation, the ochre star is considered a keystone species in the regulation of the M. californianus.  During high tides, the ochre star will move to the upper limits of the subtidal zone where they will feed on the mussel.  By doing so, it limits both their size and distribution.  If not for its predation, M. californianus would quickly move in and take over the lower intertidal zone keeping other sessile organisms from colonizing and inhabiting the area (Niesen, 1994).  For the most part, the ochre star faces very little adversity within their habitat.  Typically, there is a great abundance of food with little competition and the ochre star has very few predators.  Its only known predators are sea gulls, which eat very few sea stars, and sea otters.  However, otters do not eat the entire sea star.  They eat only the tips of the rays leaving it alive and able to regenerate back its arms.  The largest threat to the sea star population may be humans and particularly young school children that come to explore the tide pools and take sea stars home as souvenirs (Ricketts et al., 1985).

            The sex of the ochre star is separate and reproduction occurs externally.  Both the egg and sperm are released into the open waters where they meet purely by chance.   The process from larvae to sea star occurs over a period of about two months.  Once the sperm and egg have met, the gametes go through a series of changes (see diagram 1), which begin with it dividing several times to form its first larval stage called the bipinnaria.  During this stage the larva is free swimming.  During the second larval stage, which is called the brachiolaria, the development of “arms” becomes noticeable.  The larva then starts to settle to the sea floor where it will attach itself until it completes it metamorphism into a juvenile sea star (Carefoot, 1977).   Spawning generally begins during the month of April and May in Monterey, Ca. and during the month of June in Alaska.  The lifespan of a sea star from its larval stage thru adulthood can be up to 40 years (Johnson, 1976). 

Evolution:

   The existence of echinoderms can be traced back through fossil records during the Cambrian period about 540-500 million years ago. Some time shortly after that during the Ordovician period about 500 million years ago, echinoderms then evolved into the five classes (fig. 2) that are known today as sea lilies (Crinoidea), which are oldest, sea stars (Asteroidea), brittle stars (ophiuroidea), sea cucumbers (Echinoidea), and sea urchins and sand dollars (Holothuroidea).  All echinoderms are strictly marine invertebrates and have a skeleton of calcium carbonate, a free-swimming bilateral symmetric larval stage, radial symmetry during adulthood, water vascular system, and tube feet.  Early forms of echinoderms were sedentary or sessile suspension feeders that later evolving into free moving organisms (fig. 1).  The first asteroids appeared in the lower Ordovician period as somateroids, which were thought to have been extinct until 1961.  Sea stars are direct descendents of somateroids and are considered to be the true asteroidea (Clark, 1962).

        (Madison, 2001)

Figure 1: Echinoderms evolved from a group of animals characterized by bilateral symmetry, having a right and left side to their bodies (Madison, 2001).

(Wray, 1999)

Figure 2. Asterozoa (asteroidean) are descendants of eleutheroza, characterized by no longer being sessile, and development of arms (Wray, 1999).

Distribution:

Pisaster ochraceus is a member of the family Asteriidae, which also include the flat bottom star (Pycnopodia helianthoids), the sunflower star (Asterias amerensis), and the fish-eating star (Styiaserias forreri), to name a few (Wray, 1999).  While sea stars in general can be found throughout the shallow to deep waters of all oceans bodies, the distribution of Pisaster ochraceus seems to be restricted to the cold waters of the North Pacific Ocean, and are continuous from Prince William sound Alaska to Point Sal (Santa Barbara County), California (see map).  Pisaster found within the warmer waters within parts of Santa Barbara County and beyond to Baja, California is a subspecies of Pisaster ochraceus known as Pisaster ochraceus segnis (Ricketts et al., 1985).

                                                                      Map. Distribution of Pisaster ochraceus ( Mack, Katzmann,2003).            

Photo 2.  Rocky Intertidal Zone ( Evans, 2003)

       Within their distribution, ochre stars occur within the mid to low areas of the rocky intertidal zone to about 97 meters in depth (Lambert, 2000).  The substrate within this zone is described as “rough” ranging from very large boulders to rocks, gravel, and pebbles (photo 2), which are often exposed due to the conditions of the tides (Dando, Burchett, Waller, 1996).  During the low tide, the ochre star are one of the most common organisms that can   be viewed among tide pools and can usually be seen   attached to boulders by way of their tube feet, which keeps them firmly in place and prevents them from being washed away by the brutal wave action that are common within the intertidal zone (Russin, 1996).                                                                                                                                                                                                            

Other interesting issues:
   

• The purple ochre star actually has 5 eyes, each located at the tip of its rays.  However, they have no vision.  The eyes act as sensors and are sensitive to light (Lambert, 2000).

• Sea stars which are also are known by most of the general public as "starfish," are not actually fish, thus referred to by the scientific community as sea stars.  Because of their appetite for shell fish, they are perceived as a threat by commercial fisheries, particularly those that harvest clams, mussels, and oysters.  Sea stars are known to get into these commercial shellfish beds and devour them entirely.  In the past, as a way to rid the beds of sea stars, fisherman removed them from the beds and cut them in half, which doubled the population because of the sea stars ability to regenerate.  Today, they are swept from the beds with a mop-like tool and then ground them up and sold as fertilizer and poultry feed (Litrico, 2000).

Bibliography
 

Carefoot, Thomas. Pacific Seashores. Seattle: University of Washington Press, 1977.

Dando, Marc, Michael Burchett, and Geoffrey Waller.  Sea Life: A complete Guide to The Marine Environment.  Washington D.C.: Smithsonian IP, 1996.

Evans, Kip. Photo. Rocky intertidal. [Online] Internet. 8 Nov. 2003. Available:    http://www.sanctuaries.nos.noaa.gov/pgallery/pgmonterey/habitats/rockyshore_100.jpg

Guberlet, Muriel Lewin. Animals of the Seashore. Portland: Binfords & Mort Publishers, 1962

Hotchkiss, Frederick H C. Inferring the developmental basis of the sea star abnormality "double ambulacral groove" (Echinodermata: Asteroidea).  Rev. chil. hist. nat.. [online]. Dec. 2000, vol.73, no.4 [cited 10 November 2003], p.579-583. Available from World Wide Web: http://www.scielo.cl/scielo.php?script=sci_arttext&pid=S0716-078X2000000400003&lng=en&nrm=iso&tlng=en. ISSN 0716-078X

Johnson, Brian Paul.  “Daily Activity and Migration of the Starfish Pisaster Ochraceus.” Thesis. San Francisco State University, 1976.

Kozloff, Eugene N. Marine Invertebrates of the Pacific Northwest. Seattle: University of Washington Press, 1987.

Lambert, Philip.  Sea Stars of British Columbia, Southeast Alaska, and Puget Sound. Canada: UBC Press, 2000.

Mack, Joseph and Michael Katzmann.  “Map of North America.” San Francisco State University Geography Map Lib. [Online] Internet. 25 Oct. 2003.  Available: http://jan.ucc.nau.edu/~alew/maps/na-base.gif.

Madison, David R, Wayne Madison. "Tree of Life." [Online] Internet. 2001. Available: http://tolweb.org/tree?group=Deuterostomia&contgroup=Bilateria

Niesen, Thomas M.  Beachcomber’s Guide to California Marine Life.  Houston: Gulf Publishing Company, 1994.

Ricketts, Edward F., Jack Calvin, Joel W. Hedgpeth, David W. Phillips. Between Pacific Tides.  California: Stanford UP, 1985.

Russin, Joe. “Marine Biology.”  [Online] Internet.  1996. Available: http://www.lanecc.edu/science/zonation/ochrearm.htm.

Sefton, Nancy. Photo. Pisaster ochraceus. [Online] Internet. 8 Nov. 2003. Available: www.sanctuaries.nos.noaa.gov/pgallery/pgolympic/living/Ochreseastars_100.jpg

Wray, Gregory A. "Tree of Life." [Online] Internet. 1999. Available: http://tolweb.org/tree?group=Echinodermata&contgroup=Deuterostomia

Litrico, Mary Beth. "Star of the Sea." [Online] Internet. 20. Nov. 2003. Available: http://www.amelianow.com/spring00-star.htm.

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