SOUTH KINGSTOWN — Prentice K. Stout never forgot the first time he saw prehistoric-looking horseshoe crabs. It was about 70 years ago. He was a small boy, birding with his father near Sandy Hook, N.J., when thousands of crabs crawled ashore to lay their eggs.

“That just blew my socks off,” says Stout, sitting on the gunwale of his boat in Point Judith Pond. He was amazed by their ancient appearance, and their sheer numbers.

Horseshoe crabs, to many people, are ugly. And Stout has seen some ugly things done to them — from children impaling them on spikes to workers at a crab factory in Leonardo, N.J., shoving still-living animals into grinding machines to render them into chicken food.

Those memories of Limulus polyphemous still cause Stout pain. So he has spent a lifetime serving as the crabs’ defender while teaching people about the natural world.

The vanity plate on his car is “Limuli.” His boat bears the same name. He devoted 12 pages to the crabs in his recently published book: A Place of Quiet Waters: The History and Natural History of Rhode Island’s Point Judith Pond and the Harbor of Refuge.

This summer, Stout, enjoying his 50th year as an educator, oversees a marine-studies program he created for youngsters at YMCA Camp Fuller, on the pond’s western shore.

With his gentle teasing, he spreads his wonder and appreciation of the pond. The kids and the counselors call him “Prentice” or “P.K.” as they ask him to identify creatures they tug from the pond’s muddy bottom, or seine from its waters.

Stout says this is his last year of teaching. He’s taught at all levels. For nearly 20 years, he was the face of the University of Rhode Island’s Graduate School of Oceanography. Now he no longer has the energy to be on the pond all day.

He’s pleased with the young people who are leading his program, Cathy Brandt, Jack Moore, and Varvara Nikulina from the Ukraine, and feels certain they will carry on with his work.

But he worries about his crabs.

“Through my work, I want people to see this creature and realize its spike won’t hurt them,” says Stout. He said the crabs are used to make an important medical product — enzymes in the crabs’ blue blood are used to detect impurities in medicines. Other critical uses may yet be discovered. But in the end, he says, horseshoe crabs, like all other forms of life, deserve peoples’ respect. And he adds:

“They are, after all, one of the oldest creatures on earth. And it’s just a beautiful animal.”

SEVEN YEARS ago, horseshoe crabs didn’t look to have much of a future in Rhode Island. Fishermen were harvesting them for bait or for sale to a Cape Cod firm that used their blood for medical processes.

At Conimicut in Warwick, where locals once saw thousands of crabs come ashore during the new-moon and full-moon mating periods, Save the Bay’s Wenley Ferguson and some volunteers found just 20 crabs at the height of the spawning period in 2000. They no sooner made their count when a man approached and threw every crab into a skiff. Not one had laid its eggs.

Horseshoe-crab populations were plummeting all along the East Coast, so a regional regulatory body, the Atlantic States Marine Fisheries Commission, approved plans to lower catch quotas by 25 percent in seven states.

But the problem in Rhode Island was that there were no quotas and no regulations. Anyone could catch as many horseshoe crabs as he wanted, when he wanted. Many focused on the mating periods when the crabs came ashore and were easy to grab — a practice guaranteed to reduce populations because the crabs were taken before they laid their eggs.

The state Department of Environmental Management proposed new rules, but many people called for an outright moratorium.

Jan Reitsma, the DEM director at the time, surprised environmentalists and fishermen by shutting down the fishery just before the spawning period was to begin in June 2000. The regulations wouldn’t be ready in time, Reitsma explained, and the DEM’s own biologists painted a worrisome picture.

Later, the DEM enacted rules that ban horseshoe-crab fishing 48 hours before and after the new and full moons in May, June and July — the prime spawning periods. To fish commercially, you need a multipurpose commercial license. Recreational fishing also requires a license.

Ferguson, who continues to monitor crabs for Save the Bay, now says it appears that the state stepped in just in time . . . maybe.

“It’s one of the few good news stories, but I’m not sure yet if it really is,” Ferguson said.

SCOTT OLSZEWSKI, a biologist for the DEM, has been monitoring local horseshoe crabs since the controversy seven years ago and he believes their numbers are on the way back up.

Using a variety of sources, including work by Stout, DEM biologists believe there was a high point in the mid-1970s, when an average of about 150 crabs would be picked up with each trawl tow by biologists.

In 2003, there was another peak. Retired DEM biologist Arthur Ganz was there when the crabs came ashore in Quonochontaug Pond, on the Westerly-Charlestown line. There were so many they piled on top of each other and rose out of the pond, like a small island. Ganz said he was sure he’d never see such a scene again.

The populations dropped after that. The surveys show the catch is now down to a handful of crabs in each tow.

During the spawning periods, Olszewski walks the beaches at Napatree Point, in Westerly and Conimicut Point, in Warwick, and counts crabs. Olszewski counted 409 crabs on Napatree Point during the full moon in May.

Efforts to reduce the catch began only seven years ago, Olszewski says, while it takes 9 to 11 years for the crabs to become sexually mature. So while the spawning may still be off, he said, there are increasing sightings of young crabs.

STOUT SPECULATES that there have never been a lot of horseshoe crabs in Point Judith Pond. They would have trouble finding their way in through the breakwater.

But he motored his boat over a sand flat recently and ordered three young YMCA campers to jump in and start looking. Stout explains, “The crabs come out here to feed on baby clams.”

Sonia Moura, of Hollister, Mass., Lucy Tillman, of Narragansett, and Brianna Staziante, of Katonah, N.Y., are each 13 years old and prone to giggles and shrieks as they jump into the water.

When Stout has trouble with his cell phone, he gives it to them and they quickly figure out who was trying to call him.

And when someone hauls in a horseshoe crab with a basketball-size shell and a lethal looking spike tail, they quietly turn it over and begin inspecting its array of appendages.

They tell Stout the crab’s sex. And when he asks them to search for the yellow worm that lives only in the armpits of horseshoe crabs, they look carefully.

The crab carries a cluster of thumb-size slipper shells on its shell, and the girls worried that one was covering its eye.

They hand the crab to Stout, and he briskly knocks off the offending slipper shell.

He hands the crab back, and the girls continue their inspection, much as they had been studying Stout’s cell phone minutes earlier.

They knew there was nothing to be scared about.

Copies of Stout’s book on Point Judith Pond are available at Saywell’s store, 344 Main St., Wakefield.

plord@projo.com