EPOW - Ecology Picture of the Week

Island Beach of Urchins

Pencil-spined Sea Urchin (Eucidaris thouarsii), Family Cidaridae
Floreana and Fernandina Islands, Galapagos Islands, Ecuador

Explanation:  These island beaches crunch underfoot!  This week we are on the remote shores of the Galapagos Islands in the eastern Pacific Ocean off of Ecuador, South America.  And some of the beach strands here consist of the broken exoskeletons of countless sea urchins ...

... more specifically, the aptly-named "pencil-spined sea urchin."

These urchins sport thick spines some 5 mm (0.2 in) thick and 5 cm (2 in) long.  In their marine environment, they feed on corals and algae.  As they get washed ashore, their remnants create these most unique beaches.  

More than a curiosity, sea urchin beaches such as these -- which are also found in other parts of the world -- can contribute to the coral reef carbonate cycle and signal great changes in the coral reef structure.  

The major 1982-1983 El Niño disturbance resulted in 95-99% coral mortality, but pencil-spined sea urchins were unaffected and eventually dominated the dead coral substrates; their feeding on the coral structures exceeded the rate by which the remaining live corals were able to grow and calcify.  

Stacked by the millions are the spine exoskeleton remnants
of these sea urchins along the islands' beaches.

Information:
     Bak, R. P. M.  1994.  Sea urchin bioerosion on coral reefs: place in the carbonate budget and relevant variables.  Coral Reefs 13:99-103.
     Glynn, P. W.  1994.  State of coral reefs in the Galapagos Islands: Natural vs anthropogenic impacts.  Marine Pollution Bulletin 29(1-3):131-140.
     Lawrence, J. M., and P. W. Glynn.  1984.  Absorption of nutrients from the coral Pocillopora damicornis (L.) by the echinoid Eucidaris thouarsii (val.).  Comparative Biochemistry and Physiology Part A: Physiology 77(1):111-112.