Hands-On Science: Dissolving Sea Urchin Spines

As homeschoolers, we try to be open to the “teachable moment.” My sixteen-year-old didn’t exactly volunteer for this experiment, but our latest teachable moment came courtesy of the ocean, a slippery rock, a sea urchin, and his over-sized foot.

When he came tromping in the house from the beach, asking if we had any vinegar, I knew exactly what had happened.

“You stepped on a wana?” I asked him. (Wana, pronounced “vana,” is the Hawaiian term for sea urchin.)

Indeed. He had at least nine spines in three of his toes, some as fine as a single strand of hair, but the most impressive was one about 1/8″ in diameter, embedded about 1/2″ deep. Yow.

Getting wana spines out of flesh isn’t like removing a standard splinter. They are composed primarily of calcium carbonate and have kind of a chalky texture. Tweezers just tend to break off bits at a time. The standard treatment is to soak the embedded spines in vinegar. (He declined offers from the boys on the beach who suggested that urine would solve the problem.)

Remember the old egg in vinegar experiment, in which the shell (made primarily of calcium carbonate) dissolves over the course of several days, leaving behind an egg with just a rubbery membrane? Same concept here.

[Soaking in vinegar] does three things.  It makes the urchin tissues inert to bacterial feasting, kills the bacteria themselves, and dissolves the spine skeleton, which is also made of the calcium carbonate stereom described above for the plates in the test.  Being limestony, this material fizzes and dissolves readily in any acid such as vinegar.  Vinegar adds to the hurtin’ at first, but trust me, it helps and greatly reduces future damage that can be caused by leaving the spine in there whole.

Instead of an egg in a jar, we had a foot in a pan. Over the course of several days (or in the case of that big spine, a week and a half) the spines dissolved and our human guinea pig returned to walking without a limp. Science lesson or no, I suspect that he’ll be a little more cautious in the future. Which is, of course, a lesson in and of itself.