Marine Life of California's Rocky Shores (Image 26)
An ochre sea star (Pisaster ochraceus), the 'keystone species' of California's rocky shores. While the ochre sea star does not technically live in the Mid Tide Zone, nor is it even commonly found there (at low tide), it still has a profound effect on the Mid Tide Zone due to its presence there at high tide (see "More About this Series" below, to learn more about the zones).
Each ochre sea star can eat up to 80 adult mussels each year and thousands of barnacles. It is a very important species because without its presence, the mussels would dominate and species like the aggregating anemone would be crowded out. Furthermore, there would not be the great diversity of species encountered in the Low Tide Zone (below sea level), as everything would be overgrown by mussels.
The sea star is one of the top predators in the ocean--few things prey on sea stars. It is the desperate shark and a few sea otters that are the main sea star predators. Even then, if the predator just bites off an arm or two, the sea star has amazing regenerative abilities, and can often regrow missing arms. They can sometimes even regrow an entirely new animal from just one leg. They have sexual reproduction mostly during spring and summer. This occurs when the separate-sexed adults release their eggs and sperm from five openings on their top surface. Often when one sea star spawns this causes those nearby to also spawn, creating a concentrated mass of eggs and sperm in nearby waters--increasing the chance for fertilization. [Image 26 in a series. See Image 27.]
More about this Series
This series of images examines the various marine life that can be found in the different sea levels, or zones, of the rocky shores of the California coast. The images were photographed by Genevieve (Genny) Anderson of the biological sciences department, Santa Barbara City College, as part of her ongoing research on the subject.
Rocky shores provide a stable substrate for plant and animal life or organisms, as opposed to sandy beaches where the substrate (sand) is constantly moving. When the tide goes out then the influences of the air and weather (sun, rain, snow) begin to play important roles--more with the higher zones.
At any tide level on a rocky shore, a pool of water--called tidepools--can be left with the receding tide. These pools provide welcome ocean water for marine life left high and dry with a receding tide. The pools highest in the intertidal may become very hot due to the sun which may not be comfortable for some species. The pools closest to the low tide have the least influence from the air and weather and thus the greatest variety of marine life. These tidepools often mirror what is actually subtidal (below the lowest low tide, as opposed to intertidal which is between the tides). As the water goes down, most of the ocean creatures go out with it, but some can't move and are left on rocks. These creatures must be adapted to withstand not only the dryness of their area, but waves, storms, wind and rain. It is their ability to withstand dryness, and their interactions with each other (eating, being eaten, competing for space and reproducing) that determine who dominates within the rocky intertidal areas.
In examining the marine life of the exposed rocky surfaces of California's shores, it is easiest to look at these rocky surfaces where they live in "zones." Above five feet, the surface is covered only by the highest high tide and thus dry three-quarters of the day. This is called the "Splash" Zone. Then, between five feet and two and a half feet, the surface is covered alternately by both high tides so it is dry between the high tides--about half a day. This band is called the "High Tide" Zone. Between sea level and two and a half feet the rocks are only left dry at the low, low tide. This area is thus dry only a quarter of each average day and called the "Mid Tide" Zone. Then there is what we call the "Low Tide" Zone, the area below sea level that is exposed for only a few hours every few weeks at special "minus" tides (remember, zero sea level is the average of the low, low tides).
To learn more about Genevieve (Genny) Anderson's research on marine life along California's rocky shores, visit her Web page, "California Tidepools (Rocky Shores)." Anderson, who has been a teacher of marine biology and biological oceanography at Santa Barbara City College for over 30 years, has other interesting lesson plans and lecture material available on her website, Here.
Credit: Genny Anderson, Santa Barbara City College
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