Finding several brittle stars while snorkeling this week reminded me of an invertebrate zoology class I took long ago at University of Hawaii at Manoa. For our lab studies, our professor took us to Diamond Head reef flats where we turned over rocks.

The students who grew up in Hawaii weren’t impressed with what we found, but I sure was. To this former Wisconsinite, discovering brittle stars was as exciting as finding Tribbles under the stairwell.

Brittle stars are starfish relatives, but unlike most starfish, brittle stars have skinny, jointed arms and one central disc that contains all the animal’s organs.

Regular starfish carry their sex organs in their thickset arms, which is why you can’t get rid of the coral-eating crown-of-thorns starfish by cutting them up. Because starfish can grow an entire new body from one arm, cutting one up just makes more starfish.

Brittle stars get their name from their remarkable line of defense. When a predator attacks, the brittle star drops the captured arm, or part of it, and regrows the missing limb.

Although they have no eyes, brittle stars see just fine. Tiny, clear calcite crystals cover the brittle star’s arms and body, focusing light to light-sensitive cells beneath. The crystals act together as a single eye, guiding the creature to food, shelter and maybe cute brittle stars of the opposite sex.

Brittle stars run for cover when exposed. Unlike their plodding starfish cousins, which use suction-cup tube feet to walk, brittle stars scurry about using a rowing motion of the arms. They have tube feet under those waving arms, but those feet aren’t made for walking. The animal secretes a sticky mucus from each foot that collects food from the ocean floor.

The tube feet transfer any plants or animals they find, dead or alive, to the mouth on the underside of the disc. Having no anus, the brittle star passes undigested food out of its mouth.

The world hosts about 2,000 brittle star species, 57 known in Hawaii. Some prefer shallow water; others live at great depths.

Moving rocks around a reef isn’t usually an ecologically sound thing to do, because it exposes creatures, such as brittle stars, living beneath. I moved some rocks last weekend because last winter’s high surf scattered a pile that sheltered reef fish that nibble algae and parasites off turtles.

Several of us snorkelers decided that because it was for a good cause — that is, rebuilding a turtle spa — piling the stones back up was OK.

I know my invertebrate zoology professor had similar reasoning during our morning of rock turning on the Diamond Head reef in 1983. In that case the good cause was teaching students to appreciate Hawaii’s marine invertebrates.

Some lessons last a lifetime. Thank you, Mike Hadfield.

Reach Susan Scott at susanscott.net.