I love snorkeling in the Great Barrier Reef Marine Park, but not more than here at home, especially in summer, when the water is warm and the days are sunny. Hawaii might not have as many kinds of coral (50 here, 411 there) or fish (600 here, 1,500 there), but even so, my swims off Oahu almost always reveal some plant or animal to add to my list of discoveries.

Last weekend, for instance, fighting jet lag from my Australia flight, I jumped into the water at low tide and found a nudibranch I had never seen, nor did I know its name.

When I located the creature in my books, its common name, Jolly Green Giant, made me smile. It was hardly a giant. The snail-like animal I held in my hand had grown to its maximum length of 1-1/2 inches. And unlike the green giant of veggie ads, it wasn’t all green. Blue spots and yellow bumps broke up the nudibranch’s lime-colored body.

But jolly works. Discovering that lovely, rare animal in a patch of brown seaweed would make anyone feel jolly.

Nudibranchs are marine snails that have shells in their larval stage but lose them when the youngsters mature into adults. These marine animals are commonly called sea slugs, but the word “slug” conjures up the slimy creatures that eat our garden plants and leave trails of mucus on our lanai windows.

Although they’re related, land slugs are another world from sea slugs, a difference reflected in some of the marine creatures’ common names: gold lace, snow goddess, Spanish dancer and more. Hawaii hosts at least 500 sea slug species. Although a few are large, such as the Spanish dancer at 12 inches long, most species are only 1 to 2 inches long.

The scientific name, nudibranch (NOO-dee-brank) means naked gills, so-called because some, such as my Jolly Green Giant, have circlets of frilly gills on their backs.

All nudibranchs are carnivores, most species eating immobile animals such as sponges, corals, anemones, barnacles and fish eggs.

Bright colored nudibranchs are easy to spot, but fish predators don’t eat them because sea slugs either taste bad or are toxic. Most species don’t make their own toxins, but rather recycle them from prey. Sponges, a food favorite, contain irritants or poisons that the nudibranchs stockpile. One offshore nudibranch called the blue dragon eats Portuguese men-of-war and somehow keeps that creature’s stinging tentacle cells intact for its own defense.

Most nudibranchs are hermaphrodites, bearing both eggs and sperm. When two nudibranchs get together, each receives sperm from the other to fertilize the eggs it will lay. For such sparsely populated animals, this system increases the chance to reproduce.

Finding a Jolly Green Giant would make the day of any snorkeler or diver. For me it was a jolly welcome home.

To reach Susan Scott, go to susanscott.net and click on “Contact” at the top of her home page.