EPOW - Ecology Picture of the Week

Tidepool "Acorns"

Acorn Barnacle (Balanus sp.), Family Balanidae
Oregon, USA

Explanation:  Adhering to a piece of driftwood along the west coast of North America is this wonderful colony of crustaceans.  These are acorn barnacles, filter-feeders found widespread in tidepools and on hard surfaces along rocky shorelines of the northern hemisphere.  

  
Perhaps surprisingly, acorn barnacles belong to the same subphylum (Crustacea) as crabs, shrimps, lobsters, and other crustaceans ... but in their own order called, appropriately enough, Sessilia, as they are sessile and adhere to surfaces.  

There is actually a tiny arthropod living inside the shell!  

  
When out of the water, as in low tide, acorn barnacles close up their six-sided hard shells to prevent desiccation.