An unusual deep-water shark took a bite out of a 58-year-old Pennsylvania man who was attempting to swim across the Kaiwi Channel between Molokai and Oahu early Saturday morning.

If verified, it would be only the second documented incident in which a human was attacked by the small, cigar-shaped cookiecutter shark, named for the perfectly round wound it leaves on its prey — and it happened 10 years ago to the day of the first one, which involved a Maui swimmer crossing the Alenuihaha Channel between Hawaii island and Maui.

Eric Schall, the swimmer involved in Saturday’s incident, took off from Molokai’s west shore around 6:30 p.m. Friday with another swimmer and escorts to begin their 27-mile channel crossing, according to retired Hono­lulu lifeguard Bill Goding.

Goding, a member of the Hawaii Swimming Hall of Fame who has completed 20 channel crossings throughout the islands, had been advising the mainland swimmers and their team in preparation for their channel attempt.

The second swimmer, Steven Gruenwald of Minnesota, became ill and had to abandon the effort early on, Goding said. About nine hours into the roughly 16-hour swim, Schall was forced to quit when a cookiecutter shark latched onto his stomach area about 2 inches below his navel.

The wounded swimmer was brought on board the escort boat and the Honolulu Fire Department was notified at 3:42 a.m. Firefighters and Emergency Medical Services ambulance met the vessel at the Maunalua Bay boat ramp in Hawaii Kai, according to Capt. Scot Seguirant.

Schall was in serious but stable condition when he was transported by ambulance to The Queen’s Medical Center, according to an EMS report.

Schall and Gruenwald are members of a group known as Great Lakes Open Water Adventures, or GLOW Adventures, according to co-founder Madhu Nagaraja. The members come from different states and are experienced long-distance swimmers, Nagaraja said.

Schall, who has completed 13 marathon swims, is expected to make a full recovery, Nagaraja said.

Gruenwald declined to comment Saturday, saying the group was still working through the “emotional” experience.

The rarely seen cookiecutter shark, which grows to a maximum length of only 16.5 to 22 inches, is known as a nuisance to fishermen but is not considered a threat to humans due to its small size and deep-water habits, according to the International Shark Attack File. The sharks linger in the depths during the day, swimming to the surface at night to feed on larger fishes such as marlin, wahoo, dolphin, tuna, sharks, stingrays and even marine mammals.

Its underside glows, attracting larger fish in search of a meal. When the larger fish moves in for a bite, it’s the cookiecutter shark that attacks, attaching to its prey with its sucking lips and sharp, pointy upper teeth, said the ISAF website. The small shark then spins its body, removing a cookie-shaped plug of flesh with its large, serrated bottom teeth.

The ISAF website cites only one other known case of a cookiecutter shark attack. The victim in that instance was Mike Spalding of Kula, Maui, who was bitten under similar circumstances on March 16, 2009, while swimming in darkness from Hawaii island to Maui.

Spalding, 71, said he was 10 miles out from Hawaii island at around 8:15 p.m. when he became surrounded by squid, drawn to the surface by the light on his escort boat.

“I got bumped by four cuttlefish and I got really nervous that something else was going to bump me,” he recalled Saturday. “I kept soldiering on then all of a sudden I felt the first prick in my sternum,” below his ribcage.

“It was a sharp, piercing pain. I screamed like a baby when it hit me,” said the real estate broker.

The bite didn’t penetrate deep, leaving only teeth marks. But as Spalding was climbing onto a kayak escort, he was bitten again, this time on his left calf as it dangled in the water. The shark bite was characteristic of the species, comprising a clean “melon scoop” of skin and tissue about 2.5 inches in diameter and almost an inch deep, he said.

“At the time, nobody in the whole world considered cookiecutter sharks to be a threat,” Spalding said.

News of Saturday’s incident “surprised me a little bit because I thought that maybe my attack was an event we were not going to see again.”

It took four months for his wound to heal, due to infection, but Spalding returned to complete his 19-hour swim of the Alenuihaha Channel about 18 months later. He has crossed all the ocean channels in Hawaii and continues to swim daily.

According to the Kaiwi Channel Association, five relay teams and approximately 65 solo swimmers have successfully crossed the channel between Molokai and Oahu.

There was one previous shark attack in Hawaii this year. On Feb. 4, a surfer was injured on the left leg by a tiger shark at Hanalei Bay on Kauai.