"I've never seen so many people stung so badly," said paddler Catherine Fuller of Nui Nalu Canoe Club. "The two guys in our club that are the worst seem to have them concentrated at chest level — about where the surface of the water would hit them when treading water."

The state Health Department has ordered a plankton net and plans to sweep the waters at the canoe beach to find samples, so the organism responsible can be identified, said Watson Okubo, supervisor of monitoring and analysis section in the department's Clean Water Branch.

The situation is not normal at the beach, although there were some reports of stinging during the state's high school paddling season earlier this year. O'ahu Hawaiian Canoe Racing Association president Hannie Anderson said her association's Ke'ehi activities are on hold until further notice.

Okubo said he heard that one paddler who rubbed petroleum jelly on his skin seemed to have been protected from stings. He said he was also going to look into a commercial product, a sunscreen called Safe Sea that was developed in Florida and is marketed as protection against stings by jellyfish, Portuguese man-of-war and sea lice. It is sold at Waikiki Beach Activities on the beach near the Hilton Hawaiian Village.

Waikiki Surf Club paddler Luana Froiseth said she has "millions" of stings after holding canoes last weekend.

"I quit after four races. That stinging was too painful. They aren't just hitting you once. They keep stinging and stinging," Froiseth said.

Okubo said he has been told that tiny stinging sponges might be the problem. Alton Miyasaka, an aquatic biologist with the state Division of Aquatic Resources, said that "it could be a number of different things."

Among his suspects are tiny jellyfish so small that they might not be noticed, but still bearing powerful nematocysts or stinging cells.

"In calm inshore areas, they just kind of sit until the next tide takes them out, and they can be pretty hard to see. I've been in situations like that, and they will sting you," Miyasaka said.

He said another culprit could be marine creatures called bryozoans, a group of small marine animals that can look like either corals or algae. Some of them have stinging cells, and stinging bits can be knocked into the water column when they are disturbed by people walking on them or doing construction work along the coastline.

Both University of Hawai'i botany professor Celia Smith and zoology professor Julie Brock said a number of small, stinging animals — or parts of stinging animals — could be responsible. They could include forms of cnidarians, a vast group of marine animals that can include jellyfish; Portuguese man-of-war; reef-building and other corals; sea anemones; black corals; and hydroids.

Smith said another possibility — although it does not seem to exactly fit the Ke'ehi symptoms — is a blue-green alga called Lyngbya majuscula, a photosynthetic bacterium that can cause something commonly called "swimmer's itch." It can be particularly annoying in places on a swimmer's body where it has rubbed between the skin and swimming attire.

Fuller said that some paddlers wondered if mangrove clearing near the canoe beach might have released stinging animals into the water. Okubo said he has been told by scientists that the clearing could be linked to the stinging.