Ah, summertime! Lazy days reading a book on the beach, looking at the waves breaking upon the shore, certainly not daring to dip one toe into the ocean. For we remember “Jaws” and Steven Spielberg’s warning: “Just when you thought it was safe to go back into the water…”

There’s something deadly out there, folks, a-swim in the wine-dark sea.

In Nicholas Belardes debut novel, other mysteries lurk, but the menace is no great white shark. No, the monsters in The Deading are snails. Killer snails. Millions of those delicious gastropods rising up like a plague to take their revenge on the small town of Baywood, California, in reckoning for humanity’s abuse of the environment.

Truth is, Belardes somehow conjures a big monster out of a wee snail. There’s a scene early on, told lovingly and with grim detail, in which the snails consume a person or two. The owner of an oyster farm, having discovered the snails eating one of his workers, pours gallons of molluscicide into the bay. The oozy horde then attacks and makes him one with the slimy mass contagion.

Just when you thought it was safe to go back into the estuary…

The snails decided on a peculiar place to launch their invasion. Baywood is a quiet burg on the California coast, most noteworthy for being the epicenter of the social-media phenomenon known as “deading.” Not dying or deadening or playing dead, but deading as a thing, a noun. (You’ll get used to it.)

Thirteen months before snailmageddon, teens in Baywood began pretending to die and then posting pictures of their faux corpses online. The trick was to stage the scene in the most interesting way possible as a kind of wry statement about life or the end of the world as we know it or just plain boredom. First, a few show-offs faked their demise and “group deaded as a joke”; the trend went viral before fizzling out.

After the contagion is unleashed, the fun starts again, only this time, adults are the ones deading, perhaps as a way of dealing with the harrowing contamination coming in from the sea. Add to this the arrival of the Feds to quarantine the population, and the paranoia intensifies. Drones keep watch to make sure nobody leaves Baywood. Neighbor turns against neighbor as sides are drawn between those who join in the deading and those who refuse.

Belardes ratchets up the tension by giving the deading cult a sect called the Risers that embraces unique rituals, including burying people alive and festooning their costumes with feathers and bones. Just like nature itself, the Risers have run amok and are threatening all. Not only is the climate going to hell, so is society.

Our proxies through this nightmare are a teenager named Blas, who loves to birdwatch, his older brother, Chango, and various enthusiastic and gentle birders. (Much avian lore flits through the pages, as exotics and plain old backyard twitterers chirp for narrative attention.) There’s a fantastic sequence around Blas and Chango’s attempted escape from Baywood and a long epilogue that may offer a glimmer of hope. (But don’t count on it.)