Breyden Parsen gingerly walked among the cobblestone rocks at San Onofre State Beach, searching for the slimy, shiny creatures lining the shore as if he were on a treasure hunt.

“There’s one!” the 4-year-old San Clemente resident shouted, the glistening waves breaking just a short distance away on a recent day. “And another one!”

Parsen had discovered what many beachgoers have accidentally stumbled across in recent months: big, black slimy sea slugs that feel squishy under the toes if accidentally stepped on.

Ocean Institute Marine Biologist Julianne Steers said the Black Sea Hares — the largest of the sea slug species — are experiencing a noticeable rebound. She called the Aplysia vaccaria species “one of the most spectacular marine creatures we have along our coast.”

While biologists have noticed a small increase in the Black Sea Hares during dives the past few years, they have seen a greater jump in their numbers along the shoreline in recent months — from San Clemente and Corona del Mar to the south, to near Redondo Beach and beyond to the north, Steers said.

“It’s definitely a sign of a healthier ocean. We still have a way to go, but it’s definitely a rebound,” she said.

Steers said this particular sea hare species — which can grow upward of a couple of feet in length and top 20 pounds — hasn’t been seen in big numbers for at least a decade. She credits the increased population to recent algae blooms that have helped feed the sea hares.

They’re even more visible this week, with the extreme low tides allowing for better viewing of creatures when the water recedes back and they rest on rocks or on the sand.

Steers said the sea hares look like squishy rocks.

“It can blend in with all the other boulders and stones, as well as tar,” she said. “It’s the same color.”

Breyden’s mom, Savannah, encountered some sea hares lazily lingering after she took a dip into the shallow water at San Onofre.

“They’re everywhere,” she said. “You can see them floating all over the place.”

The slugs seen at San Onofre and Doheny State Beach are about the size of a hand, and during low tide they lay on or wedge between rocks or in small pools of water.

“Most often, they are somewhere in the 6- to 10-pound range,” Steers said. “They get that big by consuming a buffet of algae. It’s definitely not your garden variety snail.”

She said their cousin, the smaller California sea hares, are likewise rebounding after a population decline.

These slugs squirt purple ink if they feel threatened. “That way they can get away or they can deter or distract their potential predators,” she said.

Steers said though it might be tempting to pick them up, they are better left alone to thrive in their environment.

“The more we observe and learn with their lives, the more the species can grow and proliferate and be a strong member of our ecosystem,” she said.

They have short life spans, she said, and will die if they are on shore and exposed for too long. The ones left on dry land after low tide will likely be washed back when the tide rises again.

“It always puts a smile on my face — it’s nice to see a species we haven’t seen in a number of years make a rebound,” she said. “For me as a biologist, the opportunity to observe and learn more than you can from a book, observing and having that experience of seeing a seasonal or annual cycle, definitely aides in the continuing of my education as well.”

Ali Huerta, a 22-year-old surfer from Mission Viejo, felt the slimy creatures under her toes after a San Onofre surf session Thursday, Oct. 5, when she accidentally stepped on a few while making her way back to shore.

“They are really, seriously slimy,” she said. “I like when there’s animals in the water, it’s kind of cool. I also don’t want to squish them, I feel bad.”