Marine Science Chapters

3.2.5

The Low Tide Zone

(sometimes called Zone 4)




Low Tide Zone

Sea stars prefer the lower reaches of the rocky shore and are most common below sea level in the Low Tide Zone. The ochre sea stars, found in rich shades of orange, brown and rose, venture through the Mid Tide Zone, clearing shellfish and thus leaving room for the aggregating anemone clones.

Ochre Sea Star color variations
Ochre Sea Star color variations (above)


A closely related species, the giant sea star, Pisaster giganteus, can also be found in the lower pools and comes in shades of blue and purple. It is also a mussel predator, but cannot withstand the desiccation of the Mid Tide Zone as well as the ochre sea star. Several other sea star species are commonly found in the Low Tide Zone like the bat star, leather star and sunflower star.

Devereux tidepools at a minus tide
Devereux tidepools at a minus tide (above)


Surfgrass

One of the few flowering plants in the ocean is surfgrass, Phyllospadix torreyi. It is on almost every Santa Barbara shoreline that has rocks and waves at sea level.

Looking seaward at a minus tide with surgrass exposed
Looking seaward at a minus tide with surfgrass exposed (above)


Not able to withstand much desiccation, this plant grows much like garden grass, sending out lateral runners along the surface and establishing new plants, creating masses of vibrant green at sea level.

Surfgrass bed exposed at a minus tide
Surfgrass bed exposed at a minus tide (above)

During minus tides the surfgrass is left dry for a short time, but is a wonderful visual cue to where sea level is located. As a photosynthetic plant it creates oxygen as a waste product of photosynthesis. Normally this is washed away by the ocean currents, but at a minus tide, surfgrass in still tidepools on sunny days is covered with bubbles of pure oxygen.

Oxygen bubbles produced by surfgrass on a sunny day at low tide
Oxygen bubbles produced by surfgrass on a sunny day at low tide (above)


It is fun to look through the strands of surfgrass for the 'treasures' hiding in the low tide pools.

Starburst (Sunburst) Anemones

Before we get to the 'treasures', you should be introduced to the starburst (or sunburst) anemone, Anthopleura sola. Closely related to the aggregating anemone, this species was given its own name only two years ago. Up until then it was known as a form of aggregating anemone that lived below sea level in Southern California and did not clone, remaining solitary. There is another solitary sea anemone that looks similar, called the giant green anemone (Anthopleura xanthogrammica) that lives intertidally in the cooler waters of Central and Northern California but it lacks the radiating lines on the oral disk and is generally not found in Santa Barbara tidepools.

Solitary Starburst Anemone
Solitary Starburst Anemone (above) showing radiating lines on the oral disk.


Open Starburst Anemone showing feeding tentacles Closed Starburst Anemone

Open Starburst Anemone showing feeding tentacles (above left), Closed Starburst Anemone (above right)

Starburst Anemone Regurgitating
Starburst Anemone regurgitating (turning inside out to remove undigested material in its stomach).


The starburst anemone also has fighting tentacles, called acrorhagi, just like the aggregating anemone.

Starburst Anemone with a few acrorhagi inflated on left
Starburst Anemone with a few acrorhagi inflated on left (above)


It fights with its neighbors, using these acrorhagi, to remain a little more than tentacle distance apart. If two starburst anemones touch their feeding tentacles they inflate their acrorhagi (generally deflated and hidden between the feeding tentacles and the side of the anemone) and fight until one of them moves.

Acrorhagi fully inflated on Starburst Anemone
Acrorhagi fully inflated on Starburst Anemone (above)


Thus they maintain even spacing in the Low Tide Zone. When tidepooling it is the starburst anemone that may often be caught in the middle of a fight, with its white, blunt acrorhagi inflated. As they fight (touching each other with their acrorhagi) the white areas of the acrorhagi become tattered. The white is a concentration of stinging cells and when touched, to an enemy, it will slough off - to keep on stinging the enemy again and again. Eventually one anemone moves away from the tentacle-reach of its neighbor to stop the fight.

Starburst Anemone war (just beginning) Starburst anemone war after a few hours (tattered acrorhagi)

Starburst Anemone war (just beginning) above left, Starburst anemone war after a few hours (tattered acrorhagi) above right


Tattered acrorhagi of Starburst Anemone
Tattered acrorhagi of Starburst Anemone (above)


The starburst anemone is rarely found above sea level. At low tide you often must walk on top of the starburst anemones while exploring the tidepools of the Low Tide Zone. Remember that each day all of these species are battered by the waves so your gentle foot is not much when compared to crashing waves. These are a hardy species.

Exposed Starburst Anemones (closed) at zero sea level
Exposed Starburst Anemones (closed) at zero sea level (above)


It is interesting to look at the different color patterns on the tentacles and oral disks of these starburst anemones. The various shades of green come from a combination of the natural color of the anemone and from green-colored symbiotic algae that grow in their tissues. Anemones found under rocks or in the shade have little symbiotic algae so are generally very pale. The various striping on their tentacles is genetic and serves to show how each is unique (unlike the clones of aggregating anemones where each clone member is identical).


Green Starburst Anemone (from symbiotic algae) White Starburst Anemone (in the shade)

Green Starburst Anemone (from symbiotic algae) above left, White Starburst Anemone (in the shade) above right


Starburst Anemone with yellow striped tentacles Starburst Anemone with white spotted tentacles

Starburst Anemone with yellow striped tentacles (above left), Starburst Anemone with white spotted tentacles (above right)




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(Revised 26 June 2007)
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