Sandy Beach Intertidal Zonation
Among the beach organism there is also pattern of tidal zonation. Exposure gradients are found here, just as they are along rocky shores. The higher area of the beach gets only occasional wave wash, and most of the energy has left the water by the time it reaches here. At the low tide level the sand is kept moving by the wave action, and is rarely exposed to air.
The High-Tide Zone
Beach hoppers (Orchestoidea californiana) are found higher in the intertidal zone than any other sandy beach animals.
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They burrow about an inch beneath the surface of the damp sand, where they enjoy relatively constant salinity, temperatures, and moisture. In their burrows beneath the sand they are relatively free from the wave-associated turbulence and are hidden from the shorebirds view. They leave the safety of the burrow only at night to feed on beach wrack and detritus. |
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Slightly down the slope of the beach, but still above the wash of the waves, you find the Beach Pill Bug (Ligia spp.). Similar in appearance to the garden "sow bug", these are actually flattened crustaceans called isopods. Like the beach hopper, they also feed on wrack and detritus.
The Mid-Tide Zone
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Migrating with the tides up and down the sandy beach from the midtide to the low tide is the Mole Crab (Emerita analoga). This relative to the hermit crab burrows just beneath the surface of the sand. Facing up the beach they extend their feathery antennae to trap plankton and detritus from the wave backwash as it flows back down the wave slope. When a strong wave exposes the mole crab, it quickly re-buries itself. |
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The Low-Tide Zone
Below the zero tide level lives the Razor Clam (Siliqua patula). These clams are ideally suited for rapid digging, as their smooth shell give little resistance to the surrounding sand. In addition, they burrow in the wet sand at an angle away from the beach, which helps to reduce the chance of their being dislodged, by rough surf. Like all other clams, the Razor clam is a filter feeder.
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Sand dollars (Dendraster excentricus) occasionally live at low tide levels, but are usually in subtidal areas beyond the surf. Here they lie half-buried, burying themselves completely when the wave surge threatens to dislodge them. A bed of sand dollars may cover over an acre. |
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On protected beaches you may sometimes find the Purple Dwarf Olive (Oliverra pycna) at low tide. These attractive snails burrow beneath the sand during the day and emerge at night to feed as scavengers on algae, animal material, and detritus. |
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MacKerricher State Park Contact: Ranger Terry Kiser All rights reserved, California State Parks, Mendocino District |
