Lucy, the narrator and antiheroine of Melissa Broder’s début novel, “The Pisces,” has a heart like a black hole: the force of her emotional need is so intense that it swallows everything. She breaks up with her boyfriend as an attempt to wring more attention out of him—after he starts dating a new woman, she punches him in the face, then asks a cop investigating the situation, “Would you say she’s better looking than me?” Later, she takes nine Ambien and goes out for a drive; when she wakes up, on the side of the road, the inside of her car is plastered with jelly doughnuts. Lucy lives in Arizona, and has spent nine years in a Ph.D. program, not writing her dissertation. It’s about the gaps in Sappho’s body of work, which, Lucy argues, should be read as intentional erasures. This is a “total garbage proposition,” she admits. But she has no other project. Like Broder, Lucy is fixated on attributing metaphysical meaning to nothingness and lack.

Lucy has a half sister who lives a glass box of a house in Venice Beach, with her dog. The half sister asks Lucy to dogsit, and so Lucy moves to California and tries to deal with her void. The dog, a foxhound named Dominic, helps a little. “It felt so intimate scooping his gigantic shits, the big hot bags of them,” she says. “I thought, This is the proper use of my love, this is the man for me, this is the way.” She goes to group therapy with fellow affection addicts, who inspire contempt rather than sympathy. (A “multi-headed hydra of desperation,” she calls them.) She buys expensive lingerie to go see a guy she found on Tinder. He gives her a UTI in a hotel-lobby bathroom.

Then, after a few nights of hanging out on a rock in the shallow ocean, Lucy meets a man named Theo, a young swimmer who seems ready to give her everything she’s ever wanted. There are some warning signs, more obvious to the reader than to Lucy: Sappho, according to legend, perished on such rocks, jumping off a cliff because she was heartbroken. Also, Theo is a merman: fish from the waist—well, not quite the waist—down. But when they kiss Lucy feels rooted in the universe. When he gives her an orgasm, the sound she makes is “the sound of the planet rotating.” (The tail begins lower down than we usually imagine.) He gives her a comfort like heroin, paralyzing and unconditional. “I wanted to create that feeling and live in that for as long as I could,” she thinks.

“The Pisces” completes an unofficial trilogy of love stories between humans and sea creatures that have lately cropped up in pop culture. Alissa Nutting’s “Made for Love,” published in 2017, featured a con artist who falls in love with a dolphin; Guillermo del Toro’s Oscar-winning “The Shape of Water” matched a mute woman named Elisa, played by Sally Hawkins, with a humanoid fish. (“Mrs. Caliban,” the 1982 novel by Rachel Ingalls about a similar woman-lizardman romance, was recently reissued, too.) I have been extra intrigued by these stories in part because, in 2015, for Jezebel, I interviewed Malcolm Brenner, a man who is most famous for once having sex with a dolphin. I found his story to be comic, disturbing, unexpectedly human, and very sad.

People don’t have sex with sea creatures unless the world has failed them. Brenner attributed his bestiality to a period of sexual abuse in his childhood. Elisa, in “The Shape of Water,” is an outcast because of her disability. Lucy is attracted to Theo because he, like her, brims with shame and helplessness. “Once in a while, the scent of his bottom half would waft up,” Broder writes. It smells “like blood, the ocean, shit, seaweed . . . a little like pussy, actually. I felt almost as though his bottom half were some sort of pussy . . . . Maybe because he was insecure about it.” She loves that Theo is embarrassed about his body. She is so happy to meet a man who is worried no one will ever love him, a creature whose mythological exhaustion and melancholy outweighs her own. Lucy is so thrilled about all of this that she forgets to consider what Theo’s insatiability might eventually demand of her.