Three marine creatures historically found in Long Island Sound have been added to the state’s list of “special concern” species, and the status of a fourth has changed from “threatened” to “endangered” on the recently updated list of the state’s rarest plants and animals.

Connecticut’s 2015 list of Endangered, Threatened and Special Concern Species, which took effect Aug. 5, includes two small, rarely seen aquatic animals new to the list, the radiated shanny and the Atlantic sea snail. Peter Auster, senior research scientist at Mystic Aquarium, successfully petitioned the state Department of Energy and Environmental Protection committee that worked to update the list, a process which takes place every five years, to add them.

“If they disappeared from Long Island Sound, probably nobody would notice, but they are part of our natural heritage and the state and federal government are stewards of our natural resources,” said Auster, who is also marine sciences professor emeritus at the Avery Point campus of the University of Connecticut in Groton. “Every cog in the wheel has a level of importance.”

Both animals grow to a maximum size of about six inches, feed on invertebrates and live in rocky areas. Most people — even those who regularly spend time on the sound — have probably never seen them, he said.

“They might come up in a lobster trap, or kids might see them in early spring near shore while they’re turning over rocks,” Auster said.

A member of a committee of marinelife experts who work with DEEP on updating the list, Auster said he petitioned for the shanny and seasnail to be added after reviewing more than a decade of DEEP trawl surveys, data collected from Niantic Bay by the environmental lab at the Millstone Power Station in Waterford, and surveys by volunteer divers.

Both species “have always been rare in Long Island Sound,” he said, but the surveys and data show they have been becoming rarer.

He surmises that since both are cold-water-adapted species at the southernmost extent of their range in the sound, warming waters in the sound due to climate change are driving their numbers down further, similar to what’s been happening to lobsters.

Jacqueline Benway, DEEP fisheries biologist, said Monday that in addition to the shanny and seasnail, DEEP staff petitioned to have the sand tiger shark also added to the list of “special concern” species due to evidence of declining numbers. The sand tiger shark, which grows to a maximum of 10.4 feet, had been fished commercially in areas outside of the sound, she said, and reproduces slowly.

The status does not impose any new restrictions, she said, but signifies that there are indications of species decline and that more research is needed.

“It gives awareness,” she said.

A fourth marine species, the Atlantic sturgeon, was moved from “threatened” to the “endangered” status to concur with the species’s federal listing, she said.

Auster said the new listings could provide further justification for creating protected refuge areas in colder, deep-water areas of the sound, as well as support efforts to reduce greenhouse gas emissions contributing to climate change.

“This could encourage state agencies to manage for ecological resilience, not just for single species,” he said.

j.benson@theday.com

Twitter: @BensonJudy