W is for Wash Zone and Wrack Zone

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As a part of the A-Z Photographic Glossary of Biological Terms challenge, today we have two Ws for Wash Zone and Wrack Zone as illustrated by these Mangrove Periwinkles (Littoraria angulifera; Littorinidae) on the shores of the Sittee River Wildlife Reserve, Stann Creek District, Belize.

Both the wash zone and the wrack zones are adjacent to each other, the former the last zone of the foreshore, and the latter right at the delineation between foreshore and backshore, both found where the waves meet the beach.

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The wash zone is often also called the swash and is the turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach materials up and down the beach, which results in the cross-shore sediment exchange.

The wrack zone part of the shore is just above the mean high tide line where seaweed is deposited on the sand. This area is identified by the piles of seaweed and other debris like driftwood, and is often located on a slight shelf above the moist sand that slopes down toward the water.

The Mangrove Periwinkle a small marine gastropod that lives mainly above sea level on the branches and prop roots of the red mangrove, both in the wash and wrack zones of the Caribbean. It is ovoviviparous, where fertilized eggs are brooded inside the periwinkle and the veliger larvae are then released into the outgoing offshore flow (backwash) and become plankton; after about 9 weeks these develop into pediveliger larvae (with a foot) when they then undergo metamorphosis and settle.

Another marine gastropod that shares a similar habitat is the Four-toothed Nerite snail (Nerita versicolor; Neritidae), these ones found at Sanctuary Caye, Belize. This snail grazes on hard wet surfaces such as those in rocky intertidal zones, feeding on films composed of organic material and mineral particles, its only predator apparently various species of octopus; after prying the snail off of the rocks, the octopus runs the tip of an arm over the shell opening and operculum to determine whether the shell is inhabited and then will drill into the shell or pull the snail out through the aperture.

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