This story appears in the July 2017 issue of National Geographic magazine.

To a lobster couple, this is normal sex:

For days the female squirts urine into the den of her desired mate. Beguiled by the scent, he lets her move in. Foreplay—stroking each other with antennae and with feet covered in taste receptors—lasts several days.

Once she’s convinced that he’ll protect her, the female disrobes, slowly shedding both her hard shell and the pouch where she had banked sperm from a prior mate. Molting leaves her in a perilously soft new shell, so he stands guard for the half hour it takes to harden. Then, supported by his claw legs, he suspends himself above her and lifts her to face him, cradling her in his legs. Her new shell has a new sperm pouch; he thrusts a packet of sperm into it using appendages called gonopods. The deed is done.

As soon as one mate leaves, the male will welcome another. The female, meanwhile, will use the sperm packet to fertilize thousands of eggs, which she’ll carry under her tail for about a year until the larvae hatch.

But climate change is threatening this babymaking process, says Diane Cowan, founder of the Lobster Conservancy.