The lowly mussel is making a comeback lately, for many good reasons. Mussels are tasty, cheap, and virtually guilt free—an increasingly rare quality among animal proteins.

The environmental consequences of meat-eating are becoming difficult to ignore, which is enough to give pause to most any carnivore with a conscience. Cows fart and burp methane, which is 25 times more potent a greenhouse gas as carbon dioxide. Pig waste is polluting waterways. Nearly a third of the earth’s ice-free land is devoted to raising animals for eating and milking—or to the farmland on which animal feed is grown. Vegetarianism, many argue, is the only responsible way to eat. But we love our animal protein.

There are loopholes in this vegetarian guilt trip. If you live in a place where animals are sufficiently abundant that they can be hunted without pressuring the populations, you can eat wild game. You could eat insects, of which there appears no shortage, although that’s not the kind of extra protein most people have in mind. Someday soon you might be able to sink your teeth into lab-grown meat, but today that option remains prohibitively expensive.

What does that leave us? Mussels, that’s what.

Mussels are one of the cheapest and tastiest forms of animal protein available, and their environmental resume is impressive. They require no feed, as they filter plankton and other microscopic nutrients from the water. Diseases are few, making the use of antibiotics virtually nonexistent. And the fact that mussel shells are made of calcium carbonate means they absorb carbon from the environment. This is true of other shellfish as well, including clams, oysters, snails and scallops. But mussels contain the highest percentage of carbon per dry weight, in both their shells and soft tissues, of any shellfish, according to a 2011 paper published in the journal Marine Ecology Progress Series.

But while mussels might be part of the solution to high atmospheric carbon dioxide, they’re also victims of the problem. One consequence of high atmospheric CO2 is it lowers the pH of the oceans, making the water more acidic, which messes with the mollusks. Off the coast of Washington state, where ocean acidification has been observed, mussel beds have been replaced by acid-tolerant algae.

A Duke University study suggests that the increased acidity takes a toll on oyster and mussel shells, weakening them, and requiring more energy for the organisms to produce calcium carbonate. But while this may be happening, it’s also possible that mussels are trying to fight back. A study by the Woods Hole Oceanographic Institute found that in conditions of high ocean acidification, some shellfish, including mussels, grow thicker shells to absorb the excess carbon.

Meanwhile, a study published in Nature Climate Change and reviewed in Scientific American found that lower ocean pH saps the legendary strength of the mussel’s byssal threads, often referred to as “beards,” which connect them to the rock, ship, or whatever substrate they are attached to. “The shellfish industry, already adjusting to the fact that acidifying oceans hurt the abilities of sea creatures like oysters to make their shells, is also likely to experience losses when mussels lose their ability to cling,” notes Scientific American. “If the mussels' byssal attachments are weakened, they are more likely to fall off the rope when it gets pulled up.”

Mussels are commonly grown on ropes dangling in the water. Some mussels are grown on seafloor beds, a less desirable method because dredging is necessary to harvest them. This practice is extremely disruptive to the ocean floor, according to Seafood Watch, which rates farmed mussels a “Best Choice,” and recommends seeking suspended culture mussels, which are the most common.

An order of mussels served in a wine sauce can be the most affordable meal option at many a restaurant. But before you place that order, it’s worth heeding the cautionary words of Anthony Bourdain in Kitchen Confidential:

“I don't eat mussels in restaurants unless I know the chef, or have seen, with my own eyes, how they store and hold their mussels for service. I love mussels. But, in my experience, most cooks are less than scrupulous in their handling of them. More often than not, mussels are allowed to wallow in their own foul-smelling piss at the bottom of the reach-in.”

All it takes is one bad mussel to ruin a whole meal. When cooking mussels at home, you can take precautions to avoid that one bad mussel. These measures start at the store: if you notice a lot of open shells in the mussel pile behind the seafood counter, it might be an old batch, and perhaps mussels shouldn’t be in the cards that day. If you decide to go for it, make sure the fishmonger picks through them and removes any open ones, or mussels with broken shells. Smell them before you pay—they should smell like the ocean, not like fish. Keep them cool until cooking time.

Some people recommend purging mussels before cooking, to remove grit from their bellies. This is commonly done by soaking them in saltwater with cornmeal, which they supposedly eat, while expunging the grit from their bellies. There is conflicting evidence over whether the cornmeal works. In any case, cultivated mussels are purged before being shipped, so purging is only necessary with wild mussels.

For a simple preparation of the type most commonly served in restaurants, start by sautÃ©ing a minced shallot and a few cloves of garlic in olive oil in a pot or deep pan. Add a half-cup of dry white wine and let it boil for a minute to evaporate the alcohol. I like to add a few cherry tomatoes here, but it isn’t necessary. Add your mussels and a handful of chopped parsley, and cook for about five minutes, covered, until the mussels all open. (Any mussels that refuse to open should not be eaten.) Add another handful of parsley, mix it all around, and serve, jus and all, with bread.