Q. This past weekend we were down at Baby Makapu'u's, the side where you paddle out to surf. While my 9-year-old daughter was fishing, she noticed an odd-looking jellyfish. I've lived here all my life and this was new to me and the other fishing guys who were around. It was around 12 to 14 inches in diameter, its tentacles were around 8 to 12 inches long and very thick, not long and stringy. The really weird thing was it was a beautiful eggplant-purple color. What the heck is it? And does it sting? — Gail Garcia, Hawai'i Kai.

A. It's often impossible to identify marine life from a brief e-mailed description, but in this case, with the help of Waikiki Aquarium curator Jerry Crow and the book "Hawai'i's Sea Creatures" by John Hoover, we may have an identification for you.

It appears you saw a known but unusual visitor — one of the oddities that normally lives in the open ocean but sometimes comes close to shore in the Islands. The closest to what you describe is an open-ocean jellyfish in the genus Thysanostoma. The animals are rare, but "two or three come in every few years," Crow said.

Hoover describes them as "pelagic animals with firm, smooth, purplish domed bells and eight long trailing arms." You describe short arms, and it is possible that the trailing portions broke off as the jellyfish came across the reef, Crow said.

Those "tentacles" are actually what scientists call "oral arms," which originate from around the central mouth under the animal's bell. They are used to bring food up to the mouth. Like thinner tentacles — and some jellyfish have both — they can have stinging cells. With true jellyfish, not only the tentacles and arms but other parts, including the jelly-like "head" portion, can sting.

"All jellyfish have stinging cells, the strength of the sting depending on what they're feeding on," Crow said. If the primary food source has a hard shell, like a shrimp, the jelly might have a more powerful sting to overcome that. More delicate prey require less potent stings. Crow said this animal has a moderate sting, considerably less painful to humans than those of box jellyfish or Portuguese man-o-war.

And not everything that comes by is prey. Thysanostoma often swim with small papio flitting in and out among the tentacles — not getting stung themselves, but finding safety among toxic arms that might endanger their predators.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.