Test of Urchin

Here's two white sea urchins seen this morning at Changi. Are they the same species? Are they different? Can we tell just by looking at a living sea urchin?

How do scientists generally go about determining the identity of a sea urchin?

Positively identifying the species of a sea urchin generally involves looking at the small features of the test of a sea urchin.

What is a sea urchin test?

The test is the spherical skeleton of the sea urchin.
The test has tiny bumps on its. The spines articulate on these bumps somewhat like the ball and socket joint of your arm at your shoulder. The test also has tiny holes where the tube feet of the sea urchin emerged. As well as other features.Sea urchins have skin and tissues that keep the spines on the test when it's alive. When a sea urchin dies, the skin and tissues decay and the spines fall off. The above sea urchin probably just recently died as some of its spines are still attached.

So what's the big deal about the bumps and holes on a test?

People who study sea urchins generally base identification on the bumps and holes and other small features of the test and other parts of the test.

Here is the identification key for the genus Salmacis on the Echonoid Directory of the London Natural History Museum.Tiny features distinguish one genus from another genus of sea urchins. And even more features distinguish the different species within a genus.

Here is the full list of diagnostic features for the genus Salmacis listed on the Echinoid Directory of the London Natural History Museum.In general, to positively identify a sea urchin as Salmacis, more than one diagnostic feature is used. And more than one diagnostic feature is typically used to identify further to species within a genus.

Here's more about the different parts of a sea urchin test from the Echinoid Directory of the London Natural History Museum.

Can we look at the bumps and holes on the test of a living sea urchin?

Compared to the test of dead sea urchin...It's certainly hard if not impossible to see the small features of the test on a living sea urchin with the spines still attached!The only way to be sure of a sea urchin's identity is to kill it, remove the spines and the tissues and look at the tiny features under a microscope.

Thus, field identification is generally tentative. Subject to detailed study of the specimen.

If we adopt a broader perspective, we realise that taxonomic distinctions can be refined or changed based on new findings and studies. There are not only variations within a population, but also geographical variations. This can be complicated by hybridisation. Other issues such as DNA may also provide different arguments for defining species.

"In science, all conclusions are provisional, subject to new evidence and better arguments".
Michael Shermer in Science Friction: Where the Known meets the Unknown

To the advance shore explorer, it's probably interesting to know more about how the different species might be identified and what these species might be. This is one reason why science is fascinating.

But, in my view, when introducing our marine life to ordinary people, it's meaningful enough to focus on general behaviour and the broad role of these interesting animals.