On Memorial Day Weekend, Lisa Ludwigsen went for an afternoon dip at Chicken Ranch Beach. A regular swimmer in the bay for two decades, she and a friend swam offshore as the tide ebbed to its lowest point. The water felt warm—no need for a wetsuit—but was cloudy and opaque. She treaded as they discussed where to swim next. That’s when she felt the bite.

“Something came up and clearly bit me. It felt like a big pinch. I just kind of calmly and swiftly went back to shore,” Ms. Ludwigsen, who works for Whole Foods in Petaluma, recalled. “It wasn’t an animal that wanted to take a chunk out of my leg. It was just a bite and then it let go.”

She never saw the creature that bit her, only the surface of the water rippling as it retreated. The wound on her thigh, which stung only mildly, was about five or six inches from end to end and three or four inches high. It looked more like road rash than a shark bite, except for the tiny “elegant pattern” of teeth marks in five to seven rows and columns Ms. Ludwigsen identified. “It looked like something was sticking out of each of the tooth marks, like little splinters, but it was just dark red blood,” she said.

When compared to the millions of people who frequent California beaches throughout the year, the number of unprovoked shark attacks seems low: the Shark Research Committee has documented 158 unprovoked shark attacks in California waters, only 13 of which proved fatal. Compare that to an average of roughly 400 unintentional drownings that occur each year in the state.

Ms. Ludwigsen’s bite has puzzled some of the nation’s foremost shark experts: the cuts are not deep enough to match the teeth of a vicious predator like a great white shark, but attacking a human would be unprecedented behavior from docile bottom-feeding sharks that breed in Tomales Bay. Many of the researchers had guesses, but no one was sure. So, who are the suspects?

Perhaps the most well known predator in the world, great white sharks and their rows of triangular, serrated teeth are responsible for almost all the unprovoked attacks, but they rarely devour the humans they chomp. Most people instead die from massive blood loss, said Carliane Johnson, an environmental consultant.

Up to about 21 feet long, great whites have “a voracious appetite,” explains David Ebert, the program manager at Pacific Shark Research Center at Moss Landing Marin Laboratories. One 18-foot female caught off Santa Cruz Island had two blue sharks and a shortfin mako shark in its stomach.

Another great white shark at the Farallones was observed killing and devouring three elephant seals in one bout.

They regularly congregate off the West Marin coastline to feed on abundant pinnipeds in an area known as the “Red Triangle,” stretching from Bodega Bay to the Farallon Islands to Monterey. For years, their population had dwindled from fisherman who wanted to display sharks or stuffed carcasses in aquariums, but a 1992 state law that banned their killing or capture seems to have restored their numbers at the top of the food chain. A new study this month suggested there could be 10 times as many great white sharks in the eastern Pacific Ocean as previously suspected, an estimated 2,418 sharks. Increased incidental captures of juvenile white sharks by fisheries and deaths of sea otters from shark bites also indicated a surging population, the state Department of Fish and Wildlife noted in response to a petition to reclassify great white sharks as endangered.

Juvenile great white sharks have been documented in Tomales Bay by incidental catches from fisherman, said Peter Klimley, a University of California, Davis, professor who focuses on tracking fish. “It’s hard to get in the mind of a great white shark,” said Heidi Dewar, a research biologist at NOAA’s Southwest Fisheries Science Center, but an attack would be “pretty much impossible,” because juveniles have not transitioned to feeding on marine mammals. After they are roughly five feet long, juveniles swim north past Point Conception as they feed on bony fish, cephalopods, crabs and maybe even other small sharks until they near full size, Mr. Klimley said.

The bite patterns also don’t match, scientists who document shark attacks said. An attack from a great white, even a juvenile, would have been much more severe. It could, however, match a Pacific angelshark or a leopard shark.

The angelsharks are a species that resemble rays with huge pectoral fins. They lie in wait on the bottom in mud or sand to snap at bass, mackerel, sardines and spawning squids. The sharks are much more common in Southern California and its 18 teeth on each jaw doesn’t seem to match Ms. Ludwigsen’s bite. But an angelshark would be most likely to leave a leg with a minor bite when provoked.

“It should have the name devil shark because it certainly doesn’t have the personality of an angel,” said Ralph Collier, who founded the Shark Research Committee and documents attacks. “I had divers who learned the hard way. In addition to being able to swing hard to the left of right, almost even with the tail, they can even bend over backwards and come back. It doesn’t take much to provoke them.”

Leopard sharks have small, oblique teeth—up to 55 on top and 45 on the bottom jaw—designed to grip rather than tear, explained Mark Marks, a field researcher at the Shark Research Institute based in New Jersey. The leopard sharks are “opportunistic feeders,” Mr. Ebert explains, eating crustaceans, mollusks, innkeeper worms, anchovies, herring, smelts, perch, rockfish, flatfish and occasionally bat rays and smoothhound sharks. The sharks move with the tides, entering the shallow mud flats as water flows into the bay and retreating to deeper waters as the tide ebbs.

Recognized by its black stripes and spots, leopard sharks give birth in bays and estuaries during April and May, releasing their young in beds of eel grass—over which Ms. Ludwigsen was swimming.

But if Ms. Ludwigsen’s bite came from a leopard shark, it could be the first recorded case in California history, Mr. Collier said. He noted two previous encounters, but neither qualified. In 1955 at Trinidad Bay in Humboldt County, a leopard shark bumped the side of the face of a man with a bloody nose. When the shark approached a second time, he waved it off without suffering any bite. In 1972 at Laguna Beach in Orange County, a man saw two small leopard sharks, about two to three feet, “cruising” toward him. “Before I could get out of the water they started attacking my fins and legs,” the skindiver recounted. “I speared one of them and made the I.D.” However, there was no confirmation that the man had actually suffered a bite by what he said were leopard sharks.

“This would be extremely rare. Leopard sharks are very tolerant,” Mr. Marks said. “They’re just not a very aggressive species.” He added that a small scrape visible just above the bite suggested a leopard shark’s snout rubbing against her skin.

So why would a leopard shark bite this time? “‘Why’ questions are really hard to answer,” Mr. Klimley said. “It’s up to your imagination what alternative hypothesis you consider.”

That same imagination is partly to blame for fear of sharks, continuing nightmares from Hollywood thrillers like Jaws, Open Water and Deep Blue Sea. Sharks and the possibility of falling victim to a toothy attack regularly instill unease in even the most experienced swimmers, a skulking paranoia that can send a jittery bather freewheeling back to land.

“Everyone is either so afraid of sharks in the bay or thinks that sharks don’t bite. It looks like that’s what [my injury] was, and it really wasn’t that bad,” she said. “We’re in their living room, swimming with those creatures all the time.”

For her part, Ms. Ludwigsen theorizes that a leopard shark “was just out looking for some lunch,” mistook her leg for a tasty fish in the murky water, realized it had bit onto something larger than it could chew and swam away. The bite wasn’t that painful, she added: brushes with poison oak had caused her far greater suffering over the years.

“My friend and I continue to swim there,” Ms. Ludwigsen said, “perhaps with a little more respect for the wild place we are sharing.”