Ochre Sea Star
|[Note Guidelines] Photographer's Note|
|Pisaster ochraceus, the ochre sea star - some are purple, some are orange, and I have found a few that were very dark, almost black. . .|
Each ochre sea star can eat up to 80 adult mussels each year and thousands of barnacles. This is the 'keystone species' in our rocky intertidal. 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) that I like to refer to as 'treasures.' Everything would be overgrown by mussels.
The sea star's preference for shellfish is because they are uniquely adapted to hold onto solid shell with hundreds of sucker-tipped tube feet that are found under each leg. These are run by water pressure that enters the sea star through a special sieve plate on the upper surface of its body. This sieve plate (also called a madrepore) can usually be seen if you look closely at the back of a sea star just off-center.
The tube feet can pull two pieces of shell apart for hours (or days, if needed) until their prey tires. Sea stars never tire of pulling open a shellfish because they have hundreds of tube feet - always resting a few. Once there is the tiniest crack (a tenth of a millimeter is all that is needed) the stomach of the sea star can emerge, ooze into the crack and digest the prey. In general, it takes more than six hours to consume a mussel. The upper limit of where sea stars prey on their favorite food, the mussel, is 2.5 feet above sea level because the rocky shore dries out every six hours when you get higher than this (the High Tide Zone) and sea stars do not tolerate that much dryness.
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. This broadcast spawning is well known by aquariums that quickly remove any spawning sea stars so as not to cloud the water for their visitors.
PaulH has marked this note useful
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- [2006-11-21 11:25]
All i can say is it's a good thing they don't grow very big! Great note about these amazing creatures, the shot itself could be a bit sharper i guess but i like the composition.