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‘Ina – The Rock Boring Urchins of Hawaii

Posted: January 25, 2008

I love rock hopping along the coastline to look for rock boring urchins or ’ina in Hawaiian. As their name implies ‘ina have an astonishing ability to bore holes into solid rock. It never ceases to amaze me how these creatures can honeycomb a rock with innumerable holes and channels over the course of time.

‘Ina are no more than 2-1/2 inches across and are the most common sea urchins in Hawaii. There are two species of rock boring urchins: (1) ‘ina’ula (Echinometra mathaei) which ranges in color from reddish-brown to tan to greenish-gray, and (2) ‘ina’ele’ele (Echinometra oblonga) which is black in color and slighty oval in shape. 

 

‘Ina’ula surround a much larger red slate pencil urchin in a tide pool at Miloli’i in South Kona on Hawaii Island. ‘Ina feed on seaweed film that grows on the surface of rocks.  It is said that they scrape a little bit of the rock with their “teeth” each time they graze over the rock surface.  The abrasive action of thousands of passess over the surface is said to create depressions in the rock. 

The surface of this basaltic rock is just riddled with holes about the size of silver-dollars an inch deep that were made by rock boring urchins.  A large wave or tsunami moved this previously submerged boulder where urchins made their home to a new location above the water line at Kaena Point, the western-most tip of Oahu.

 

‘Ina hide in holes in the rock and enlarge them by scraping the rock with their sharp strong spines as well as with their “teeth”.  In this photo taken at Milolii on the Big Island, rock boring urchins inhabit holes in the rocks as a much larger red slate pencil urchin clings to the rocks overhead.

One of the more unexpected sights is when ‘ina create living chambers for themselves. They enter holes and depressions in the rock when they are small and enlarge their holes over time.  As they enlarge the chamber in which they live, they can grow larger than the opening and can no longer leave. This is perhaps an effective strategy to protect themselves from predators.  In this photo taken Ahihi-Kinau Natural Area Reserve in Makena on Maui, several ‘ina are trapped in chambers they have excavated in the coral encrusted rock.