EPOW - Ecology Picture of the Week

A Marine Primary Cavity Excavator

Cavities from Purple Sea Urchin (Strongylocentrotus purpuratus), Family Strongylocentrotidae
left: Bodega Bay, California; right: Waldport, Oregon

Explanation:  In the realm of birds, some species are "primary cavity excavators," which means that they peck out their own cavities and holes in dead trees, dead parts of live trees, termite mounds, soil banks, and other surfaces.  Primary cavity-excavating birds include woodpeckers, barbets, and other species. 

But it is not commonly considered that other kinds of animals can play this same "ecosystem engineering" function of primary cavity excavation.  

This week we are exploring rock cavities made by the humble purple sea urchin, a very common echinoderm of coastal intertidal and subtidal environments.  Urchins route out their cavities using their spines and their gnawing teeth.  Recent studies have revealed that the teeth are made of calcite, the same stuff as the limestone rocks into which they commonly burrow.  

Also, just as tree cavities that are hollowed out by woodpeckers and other birds become later occupied by other species, such as small owls, so too are the rock cavities made by urchins used by a suite of other marine creatures.  

These are "secondary cavity-using species," and again, little attention has been paid to the marine environment to study this ecological relationship.  

Among the creatures that can inhabit urchin rock cavities are fish, mollusks, crabs, and much else.  

 
Here we are peering into a flooded tidepool at low tide along
the Pacific Coast, U.S.  The various cavities dug by urchins
have been filled with sand by the tide and are now
occupied by various other organisms, including mussels,
algae, sea anenomes, and other life forms,
just as some bird species occupy tree cavities
excavated by woodpeckers.