On a cold April evening, a dozen biologists and conservationists headed into a wooded area of the Middlesex Fells Reservation, over 2,500 acres of forest, wetlands, and hills in portions of Malden, Medford, Winchester, Stoneham, and Melrose.

Wearing chest waders and using flashlights and headlamps, the group followed a dirt trail to a large vernal pool. Their goal: To search for the larvae of marbled salamanders, the rarest salamander in Massachusetts.

With a chorus of spring peepers calling, expedition leader Bryan Windmiller, director of field conservation at Zoo New England, gave instructions for the group to spread out and begin the search. Dip nets in hand, they waded into the chilly 2-foot-deep pool and carefully scanned the water with their lights.

After a couple of minutes, Windmiller exclaimed, “I found one!’’

He waded to shore and placed the inch-and-a-half long brownish-colored larva with feathery gills into a plastic tub of water.

A few minutes later, Windmiller found another larva. Soon, others in the group waded over to the collection tub and emptied their nets. Within about 30 minutes, they had half a dozen larvae.

In the spring of 2016, Windmiller coordinated the release of 62 marbled salamanders at several locations in the Fells. His goal was to reestablish breeding populations of the rare amphibians — listed as a threatened species in Massachusetts — in the Fells, where the salamanders hadn’t been seen since the early 1930s.

Windmiller and other biologists, with the help of teachers and students at Medford High School, raised the salamanders from aquatic larvae — captured in western Massachusetts — over the winter for release in the spring.

This process, called headstarting, gives the salamanders a chance to grow larger than they would in the wild before they’re released, and protects them from environmental hazards — such as vernal pools drying out or freezing to the bottom — and from predators, increasing their chances of survival.

Headstarted salamanders require a lot of care, said John Berkholtz, senior field conservationist at the Stone Zoo in Stoneham. The larvae require good water quality and eat live food, like baby brine shrimp, the tiny crustaceans daphnia, and blackworms, which the zoo staff raises. The larvae are kept in aquariums, then terrariums when they metamorphose to the land stage.

In 2018, Stoneham, Winchester, and Concord/Carlisle high schools joined Medford’s in raising salamanders. Then the pandemic hit.

“Since COVID, all salamanders have been headstarted at Stone Zoo,’’ said Berkholtz. “This spring Medford High School is helping to headstart 30 salamanders.’’

Berkholtz said 327 salamanders were released from 2016 to 2022, and his crew is hoping to release 107 more by June.

Claire O’Neill, president of the local conservation organization Earthwise Aware and part of the salamander search party, said that in February, she and fellow naturalists Joe MacIndewar and Ankush Kesri noticed a marbled salamander larva on a video they took while monitoring vernal pools in the Fells — the first evidence that the salamanders were successfully reproducing.

Windmiller and Berkholtz subsequently found a marbled salamander larva in the vernal pool, and organized the nighttime search to see if there were more.

“It’s like a needle in a haystack looking for larvae in big vernal pools,’’ said Windmiller. “I’m stoked that the folks from Earthwise Aware saw the larva.’’

Unlike spring-breeding salamanders, marbled salamanders lay eggs in dry vernal pools in late summer, according to MassWildlife. The female stays with the eggs until autumn rains arrive and refill the pools.

The eggs hatch in November and December. Marbled salamander larvae feed on zooplankton and aquatic insect larvae, and grow over the winter and early spring. By then, the larvae are big enough to eat newly hatched spring-breeding salamander larvae.

Windmiller said aquatic insect larvae, adult aquatic insects, and newts feed on marbled salamander larvae. The larvae metamorphose into juvenile salamanders about 2 inches long, and leave their natal pools in June and July. Juvenile salamanders disperse into the surrounding forest, where they feed on small invertebrates like earthworms and insects.

Marbled salamanders live underground, sometimes in small mammal burrows, Windmiller said.

Adult marbled salamanders are 3 to 5 inches long, according to MassWildlife, and are black with grayish to white cross-banding. Windmiller said that distinctive black-and-white pattern is called aposematic coloration — a warning to potential predators like raccoons and shrews they have toxic compounds in their skin.

Marbled salamanders occur from southern New England to northern Florida and west to Texas, MassWildlife said. Within Massachusetts, they are found primarily in Bristol, Franklin, Hampden, Hampshire, Norfolk, and Worcester counties.

Marbled salamanders may live more than 10 years, said MassWildlife.

After two hours of searching, the group gathered to weigh and measure the larvae, before releasing them. The final tally was 13 captured, and five more spotted.

“Our hope is that eventually there will be self-sustaining breeding populations of marbled salamanders in the Fells,’’ said Windmiller.

Don Lyman can be reached at donlymannature@gmail.com.