In the hierarchy of the animal kingdom, clams and oysters rest in the bottom half of the complexity spectrum.

Overseeing the state's shellfish beds, though, is far from a simple undertaking.

"It's very complicated," Kristen DeRosia-Banick, environmental analyst for the state Bureau of Aquaculture, said Thursday, showing a color-coded map of Norwalk harbor crisscrossed with lines marking the recreational shellfish areas and leased commercial beds.

The Bureau of Aquaculture, based in a small waterfront building in Milford, is the 12-person division of the state Department of Agriculture charged with leasing 30,670 acres of state-owned shellfish beds and regulating the commercial harvest from them and from beds that towns lease to growers. It also tests samples from recreational and commercial beds in 24 shoreline towns, orders closures when dangerous levels of pollution are detected and works with volunteer shellfish commissions and commercial shellfishermen, among other tasks.

The central feature of the building is the laboratory where 8,000 water and meat samples per year are tested for harmful levels of bacteria including vibrio, a warm-water pathogen that first appeared in Connecticut waters in 2013. There are also a few administrative offices and a dock where the bureau keeps the five boats it uses for water sampling and tending natural oyster beds in western Long Island Sound, where the majority of seed oysters for commercial growers originate.

"Our main responsibility is to protect public health and support shellfishing," said David Carey, director of the bureau. "We take the appropriate actions, whether it's short-term or long-term. The last thing we want to do is have an illness outbreak."

The agency dates to the 1800s, when the state began issuing franchises to commercial shellfishermen. That changed to a leasing system in 1915. Over the years a multi tiered system of shellfish licenses and classifications of areas developed, with different amounts of rainfall triggering closures and testing requirements based on the location and its sensitivity to the effects of polluting runoff, septic waste and sewage treatment plant discharges. Some areas also are affected by heavy metals left in tidal soils from long-shuttered factories.

Western Long Island Sound, Carey said, supports the majority of the state's clam and oyster beds, because the surface of the sea floor there is more suitable for attaching and growing colonies of shellfish.

"Ninety-nine percent of all the shellfish harvested in Connecticut is between Branford and Greenwich," he said.

Along with regular testing of water and meat samples, the bureau also conducts ground surveys of shorelines in each of the towns with shellfish beds, checking for stormwater outfalls, marinas, waterfowl colonies and other potential pollution sources, about which they alert commercial growers and recreational commissions. Keeping beds open, Carey said, is no easy matter in a state with such a heavily developed shoreline.

"All of our areas are conditional growing areas, and a large number of our samples will show various levels" of harmful bacteria, Carey said. "We do a lot of partial openings and partial closures. Some of our recreational areas are very vulnerable."

One example, he said, is the Niantic River in East Lyme and Waterford.

"It's under threat all the time," he said. "It's a challenge to keep that area open. We're working with the shellfish commission there to try to improve the sampling."

The bureau also tests what's going in and coming out of sewage treatment plants that empty into the Sound and its major tributaries. And it's begun testing water samples from areas where shellfishing currently is restricted to learn whether recent upgrades to treatment plants and other changes have improved water quality enough to open those areas. But maintaining vigilance to ensure that clams and oysters from Connecticut waters are safe to eat remains the agency's main mission, DeRosia-Banick said.

"Our shellfish goes all over the world," she said. "If people get sick from it, it's a big deal."

j.benson@theday.com

Twitter: @BensonJudy

CONNECTICUT SHELLFISH BY THE NUMBERS:

• Annual sales: $30 million

• Employees: 300

• Bushels of hard clams harvested commercially: 450,000

• Bushels of oysters harvested commercially: 200,000

• Acres of state and town-leased shellfish beds under cultivation: 70,000

• Licensed oystermen: 21

• Licensed clam harvesters: 20

• Revenue generated by state shellfish leases: $827,856.

Source: State Department of Agriculture, Bureau of Aquaculture