Butterfly Shells or Wedge Shells (Donax sp.)
Donax shells, also known as wedge shells, butterfly shells or (in Australia) Pipis, are small, sometimes brightly coloured shells that can often be found along the tide-line of temperate and tropical sandy beaches. I personally like the name ‘butterfly shells’; when you find empty shells on the beach, the two halves of the shell still attached at the hinge, it’s easy to see why they were given this name.

Butterfly shells are bivalve molluscs. Like all bivalves they have soft bodies enclosed within a hard, calcareous, exoskeleton that forms a protective shell. This shell is divided into two halves, or valves (hence the bi-valve name) joined at the hinge by ligaments. Butterfly shells live buried just beneath the surface of the sand along sandy beaches and in shallow water. They can be especially abundant on wide sandy beaches just above the surf zone, in the strip of sand the waves wash after they break. here they feed on phytoplankton. A number of butterfly shell species, including Donax incarnatus, migrate up and down the beach with the tides in order to stay in the rich feeding area just above the surf zone. How do they do that? Well essentially they surf. The precise technique seems to vary a little between species, but essentially, on a rising tide they retract their fleshy foot and siphons just as a wave washes over them, thus allowing themselves to be washed up with the wave. As the water runs back they dig their foot into the sand like an anchor. Thus on a rising tide they ratchet themselves up the beach with the tide. Once the tide turns, and there is a danger they may be left stranded, they retract their foot and siphons as the water runs back so they are dragged and roled seaward. They will then dig in to prevent themselves being pushed back up the beach. Occasionally a wave will bury them quite deep under the sand. If that happens they will rapidly dig themselves back to the surface to make sure they don’t lose contact with the waves and their source of transport.
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