Sex among sea creatures is sexier than you might imagine

Living on Earth
Nassau grouper spawning

Nassau Grouper spawning. They are found throughout the Caribbean, but because they gather in large groups to mate, their numbers are greatly diminished from overfishing. 

Marah Hardt

As the saying goes in Hollywood, “Sex sells.” The same may be true for marine biology.

In a new book called "Sex in the Sea," Marah Hardt, a marine biologist and co-director of the non-profit Future of Fish, documents how life under the waves depends on the intricate, complex and mysterious mating rituals of its inhabitants — and she does it in a way designed to catch people’s attention.

Her skin shone in the moonlight, a flash of silver against a dark beach. She knew he wanted her. She could see him desperately fighting his way toward her from amongst the crowd. She positioned herself perfectly, knowing the sight of her bound body, restrained and prostrate in the sand, would make him — would make all of them — quiver with excitement. The first to reach her, he feverishly curled himself around her. Others soon joined him, forming their own half-circle embraces. This is what she came here for, what they had all come for.

This passage, while reading like a romance novel, actually describes the mating strategies of the grunion, a small silver fish that comes ashore every spring to engage in a mass orgy.

“If you talk about sex, people are curious,” Hardt says, with a laugh. “And maybe there's a way you can talk about sex, draw their attention and then subtly weave in the message of conservation — because ultimately successful sex is the heart of sustainability.”

Some of the reproduction strategies Hardt describes in the book are mysterious, fascinating and funny.

The Nassau Grouper, for example, normally live solitary lives until it’s time to mate. Then, right after the full moon, they will move to the edge of the reef and wait and watch for other groupers to start passing by.

“They gather at the same spot every year, at the same time. They will shift their coloration and change from a sort of desert camo pattern ... to a more ‘black-tie affair’ — dark black or black with a white belly,” Hardt says. “These colors signal that they're ready to be friendly and they're ready to engage more intimately.”

When the females become swollen with eggs, one will rocket out from the group and shoot towards the surface. The males will follow her and form a sort of funnel moving upwards in the water column. The female releases her eggs about 10 to 20 feet below the surface and then the males will streak through this cloud of eggs and release their sperm.

“Then all the fish tumble back down in this cascading sort of geyser that erupts and then falls to the sea floor,” Hardt says. “So you get these repeated geysers — it’s almost like Old Faithful, full of fish that just shoot up and explode like fish geysers. And the visibility will go from 100 feet to where you can't even see in front of your face.”

Coral reproduce in a somewhat similar fashion, even though they are stationary creatures, Hardt says. “They release their eggs and sperm into the water column, but the way that they do this is fantastic … [T]hey organize truly the biggest orgy on the planet and it's perfectly synchronized. You can set your watch to it. In fact, researchers do every year.”

The corals release bright pink bundles of eggs and sperm, which float up through the dark water column and then explode at the surface, creating a giant slick of coral gametes.

“It all mixes and mashes at the top,” Hardt explains, “and that’s how they get sperm from one individual to mix with an egg from another.”

But for sheer spectacle, perhaps nothing rivals the right whale.

“Looking at the anatomy of an animal can tell you a lot about the way that animal reproduces — in particular whether they tend to have many partners or not,” Hardt explains. “Right whales do not have the biggest penis, but they do have the biggest testicles on Earth. Each testis weighs half a ton.”

This indicates that males are having to compete with other males internally to the female, Hardt says. That is, they are competing to deposit more copious amounts of sperm during mating.

“The reason to do that is to try to flush out the sperm of other males that may be in there or to push their fellas as far forward as they can,” Hardt explains.

But the competition is even more fierce than that: Hardt describes an episode in which a female whale came to the surface and two males entered her at the same time.

“We always knew that Right whales were frisky and that there was some sperm competition going on, but if anything is going to encourage the production of gigantic testes it's having to ejaculate at the same time as another male is inside the female,” Hardt marvels.

But size may be no match for cleverness — and the female right whale has a trick of her own that may give her more control over who fertilizers her than previously known.

Dr. Sarah Mesnick, who works out of the Southwest Fisheries Science Center in San Diego, has found that the whale vagina is not a single tube leading into a uterus, but a kind of a maze, Hardt says.

“They found twists and turns and blind alleys and false flaps and trapdoors,” Hardt marvels. “And I remember Sarah saying to me, ‘When we first looked in this thing, all I could think was 'How the heck does a sperm find his way to the cervix? How do they even know where to go?’ That "last mile," is female territory. It’s their home turf, so they're controlling a lot more than we realized.”

This article is based on an interview that aired on PRI's Living on Earth with Steve Curwood