Sex in the Sea

by Marah J. Hardt

April 27, 2016

It smells like sex. I couldn’t shake that somewhat disturbing thought as I floated at the surface, watching the moonlight glisten off the ever-widening slick—the residue of the night’s intimacy. The distinct, musty odor was undeniable. I guess I shouldn’t have been so surprised. I had just spent the past two hours watching corals spawn. But that event looked so unlike sex that I never expected the aftermath to smell like it. As I pulled strands of mucous-y coral goo out of my hair, I caught the eye of the other researchers similarly engaged in de-spawning themselves. We smiled at one another with a knowing look. There is a certain level of bonding that occurs when floating amid the leftovers of one of Nature’s biggest orgies.

That’s the thing about sex in the sea. It is at once utterly foreign, yet there are hints of the familiar—but only just. For the most part, sex beneath the waves looks nothing like what we think of as intercourse. That’s what happens after several hundred million years of intense battles over who can reproduce the most—evolution gets a little funky.

From the highest reef crests to the deepest trench, nearly every animal on the planet is focused on one of two things: reproducing and not being eaten. Thus Nature invests heavily in the art of both sex and survival. Life’s great purpose—to successfully pass along to future generations one’s good looks and all the genes that go with it—relies on both skills. But not equally. A deft survivor that lives a long but celibate life loses the evolutionary game; a great lover, adept at attracting and securing a mate, needs only to survive long enough to get the deed done.

In the end, it all comes down to sex.

Thus the mind-boggling array of ways to seal the deal in Nature. And I’m not talking Kama Sutra–style creative contortionism here. That’s just a bunch of minor postural adjustments. Real sexual innovation occurs in the wilderness, and nowhere are things more wet and wild than in the ocean.

Under the sea, the missionary position is in the minority. Instead, sex may look like a handshake; an interlocking ring of multiple individuals forming a closed loop of lovers; or it may be a microscopic male squeezing out sperm while living his days inside his giantess mate’s kidney. Peek below the shimmering surface and witness worm penis jousting matches, full-moon sex parties, sunset spawning blitzes, and likely the biggest threesome in the world (with the lovers holding their breath the entire time).

Every version has been honed to maximize the chance of successful reproduction. It’s a salty symphony of sex that ensures the big blue sea stays bountiful year after year, age after age, right up to the present day. Almost.

Over the last century or so, those myriad instruments of sex have started to go a bit off-key. This is bad news not just for fish, but for us as well. The way marine life gets busy in the deep matters—it matters for food security, human health, coastal development, climate change, and other global issues. Take food security. Nearly three billion people rely on fish as a major source of protein. Half of that fish comes from the sea. To feed that many people requires a lot of fish successfully making a lot of baby fish every year. But that’s not the only reason the sex lives of sardines are of interest.

Beachgoers and coastal homeowners take note: like the great walls that defended medieval cities, the mighty underwater reefs built by millions of oysters or corals protect and stabilize the shore. These natural barriers break up wave energy, helping to guard the coastline from storms and heavy surges. And because they are living walls, they can grow—rather than erode—over time, rising even as sea levels do. Lose the reef builders and it is not long before the sand starts slipping back into the sea.

For animals to build and maintain those giant blocks of reef takes enormous effort. Add to that the energy they now have to spend fighting off ocean acidification, pollution, and invasive species, and there is just not much left to invest in sex. It’s like the exhaustion that follows long days at the office—sex tends to take a backseat to sleep.

We rely on the rapid reproduction of tiny crustaceans to feed the fish that feed us. We depend on the mass spawning of corals to create the reefs that provide habitat for the millions of species that we use for medicine, food, and simple enjoyment. We rely on the prolific procreation of oysters, clams, and other shellfish to filter and clean our coastal waters. Whether it is finding the next cancer-fighting compound, feeding a growing population, or fueling economies, we depend upon the extraordinary abundance of ocean life to sustain us—and that abundance depends upon lots and lots of sex.

Without successful sex in the sea, we’re sunk.

That’s why knowing what actually goes on “down there” matters to us up here. Sex is the heart of sustainability. So, this Earth Day, journey below the surface to witness the salty rituals of seduction and sex that ensure the future abundance and diversity of life within our planet’s largest habitat. Understanding these strategies, and how we effect them, is a first step on the path to forging a more productive relationship between people and ocean life.

Don’t be shy. Just dive on in.

Seahorse Seduction: Jane Austin Meets Magic Mike

We’ve seen it happen before. An attractive man, well dressed and polite, draws the attention of some single ladies; an attractive man, well dressed and polite and rocking a baby in his arms melts the heart of nearly every female in sight—single or otherwise. Some female fish feel the same way.

Especially seahorses, which are a group of sexual extremists like no other. First, they are relatively monogamous. These days, the power of paternity tests has shown that very few species are really monogamous. But a few species of seahorses appear impressively faithful to their partners, at least throughout a breeding season.

In addition to this unusual behavior, male seahorses take the dutiful dad thing to an extreme as well, going so far as to become pregnant. You read that correctly. In seahorses, the male, not the female, fertilizes the eggs and then nourishes the developing embryos inside a warm pouch that bathes the eggs in fluid and protects them until they hatch. The male then births live young that spill forth into the sea as perfect mini seahorses.

Pregnant. Males.

But before that can happen, a male must convince a female to entrust him with her costly eggs. On the other hand, a female needs to ensure she can rely on the male to turn up when her eggs are ready. In order to coordinate their love affair, males and females engage in a little foreplay each and every day over the breeding season—a morning courtship ritual that looks a bit like a ballroom dance scene out of a Jane Austen novel.

A male might approach a potential mate with his head bowed down, ever so courteously, and flutter his fins rapidly. He also dilates the opening on his belly where the eggs are stored, inflating it at much as he can to show off his wares. He may also lighten the color of his stomach to accentuate the pouch. If the female likes what she sees, she’ll reciprocate by similarly lightening in color and bowing her own head in return. The male and female, once met, then separate for the night, like good, chaste Victorian singles should.

For the next several months, at dawn each day, the female slowly swims over to the male’s territory, where he patiently awaits her in their usual greeting spot. Upon her approach they both brighten in color, as if blushing. Then they each wrap their tails around a blade of seagrass and begin to circle their holdfast, the male swimming the larger outer sweep with the female on the inside. After a few moments, they release the stalk and line up side by side, then drift in parallel over to another blade of sea grass. As they do so, the male often wraps his tail around the female’s, like two lovers holding hands on a morning stroll. This flirtatious dance repeats a few times, with the entire greeting lasting several minutes.

After a few weeks, the male gives birth, usually at night. The very next morning, he indicates to the female that he’s ready to move beyond dancing by displaying some energetic thrusting. He alternates between folding in half, contorting his tail up toward his head, and lengthening out. This forces water in and out of his belly pouch, helping it swell in size. Females find a big bulging abdomen rather hot, and after watching this display, she’ll respond by pointing her snout upward. It’s the equivalent of the subtle head nod toward the bedroom that says, hey, how about we go up there?

This thrusting courtship can last a long time—up to nine hours in one seahorse species. Then, after weeks of daily dancing and hours of foreplay, the pair swim upward in the water column and have sex for about five seconds. In this regard, female seahorses really are just like the majority of males in the animal kingdom. Where seahorses differ, however, has major implications for how their populations respond to fishing pressure—which is not well.

Dried seahorses are still sold as popular curios, ground up for traditional medicine, and consumed as an aphrodisiac—ironic, given that their greetings are so innocent and their sex bouts so fleeting. Additional harvests also occur for the live aquarium trade. Over the past several decades, fishing to satisfy these markets has taken a toll on seahorse populations. The damage is more than may have been anticipated in part because of the way they reproduce.

Unlike fish that spawn directly into the water or a nest, seahorses are limited by how many eggs the male can carry at any one time. Where species such as cod may release millions of sperm and eggs into the water, seahorses release only one or two thousand over the course of the entire breeding season. This is the first reason seahorse populations decline quickly when fished: they simply don’t produce enough babies to replace the number harvested by fishers.

Second, while romantic, the strong allegiance to a mate in some species means that, should a seahorse be scooped up, the mate left behind will spend days waiting for their lover to return. And, even when ready to move on, they will require several days of courtship with a new lover to synch their cycles. This means missed breeding opportunities.

Fewer babies and fewer mating opportunities add up to rapid declines under even low fishing pressure, something we couldn’t have accounted for two or three decades ago. But we can now. As research provides more details on the courtship and reproduction of seahorses, we can set more realistic limits to harvest. In addition, the science on captive breeding of seahorses continues to improve every year, with major programs now successfully producing itty-bitty baby seahorses with which to supply the aquarium and medicinal trade demands and alleviate pressure on wild populations. These are steps in the right direction for ensuring the survival of the planet’s only pregnant males.

Fifty Shades of Grunion Run

If fish could write novels, the romance section would be filled with titillating tales of grunion runs. Come Spring, thousands of the slender, silver fish hurl themselves ashore for a momentary mass beaching. Spanish for “grunters,” the cigar-length fish arrive beginning in March and continue through summer along California’s southern shores.

Though a male typically leads the charge, the female directs the show (some behaviors really are universal). Deftly arching her iridescent blue-green back, she digs a hole with her tail in the slushy sand, sinking down until only her head sticks out. In this awkward position she waits for the males to come a-flapping over.

As she deposits her eggs into the sandy underground, as many as eight males wrap themselves around her protruding upper half. They assume this “spooning” position not to help haul her out, but to use her side as a slide for their sperm. By releasing their milt in this manner, the males concentrate their sperm right on top of the female’s eggs, boosting fertilization rates.

The males then bolt, leaving the female buried up to her arm pits, so to speak. Chivalry long dead, she twists herself out of the sand and catches the next wave back to the sea. The entire event takes less than a minute—since a fish out of water cannot breathe, the entire amorous act must happen on a single breathe hold. A typical spawning run lasts a few hours with thousands of fish flopping onshore over the course of the evening.

Like any experienced surfer, grunions know that a good ride depends on perfect timing with the waves. Up and down the coast, different populations of grunion adjust their erotic exodus to match—exactly—the peak tides of their local beach. They coordinate sex so precisely with the physical cycles of the sea that their arrival can be predicted a year in advance. It’s a successful strategy . . . as long as there is a sandy beach for them to get to.

Today, these animals compete with approximately half of the world’s seven billion people who now live along the coast. From fancy seaside resorts to urban slums, the space where land meets sea is increasingly crowded. In the United States, there are more than four hundred people per square mile living in coastal counties. Farther from shore, the average density is one-quarter of that.

Not only are more people moving in—the beaches are also moving out. Erosion has increased significantly, due in part to rising seas and bigger storms—effects of global climate change. Additionally, seawalls and jetties change the natural ebb and flow of sand along the coast, altering beach replenishment. Unfortunately, our efforts to bulk up beaches often result in damaging the terrain, with beach slopes too steep for marine life to climb or unsuitable for nesting.

There are initiatives underway to keep the grunions grunting, but success for all species that delight in sex on the edge—from sea turtles to elephant seals to horseshoe crabs—will depend upon global action on climate change and more comprehensive coastal management.

Oceanic Orgies

Beneath the waves, group sex is often the preferred practice for prolific procreation. Even solitary fish embrace some intimate company at least once a year. The Nassau grouper is one of them. Found on Caribbean coral reefs, Nassau grouper grow to about three feet long and live for an average of sixteen years. Aggressive and highly territorial, they are not often seen together. Until the winter moons rise.

As the days shorten and temperatures drop, something awakens within these usually “homebody fish.” They turn from hermit to hedonist: an uncontrollable itch drives them from their home reefs to travel up to a hundred miles or more to engage in a New Year’s celebration of sex. But how do they find the party, and find it in time? For Nassau grouper, the bacchanalia only last for two or three days each year.

Around Little Cayman Island, south of Cuba, researchers with the Grouper Moon Project have begun to unravel the mystery. Fish tagged with small acoustic transmitters can be heard by an array of underwater listening stations that encircle the island. The recorded “pings” of passing grouper paint a picture of how these lone fish seek out fellow spawners.

For many, the journey starts one or two days after the full moon, when they venture towards the outer boundary of the reef. There they hover, watching and waiting. And while they wait, some begin to slip into something a little more inviting.

Fish have a remarkable ability to change their coloration. In Nassau grouper, they switch from daily desert camouflage of mottled browns to “black tie”: a sexy two-tone get-up that contrasts a bright white belly with a dark back. Others go for an all-black look. Displaying these colors helps advertise their readiness to spawn. The boldly contrasting shades seem to signal “friend with benefits” to other Nassau groupers—an important gesture by a normally highly territorial fish. Last thing anyone at an orgy wants is a swift fin-kick to the face for getting too friendly.

With sexual intentions clearly on display, the fish join up with other Nassau grouper, many similarly dressed, as they pass by on their way to the final destination. By the fourth day after the full moon, every single adult Nassau from the waters surrounding Little Cayman has caravanned down to the southwestern tip of the reef, an aggregation of four thousand fish that represents the entire breeding population of the island.

And that’s where the problems lie. Migrating to the same spot year after year on a highly predictable schedule certainly helps fish find mates, but it also helps fishers find fish. There, in one small location, swim all the biggest fish from an otherwise scattered population. On Little Cayman, when fishers first discovered a spawning aggregation in 2001, there were approximately seven thousand fish. Two years later, about three thousand Nassau grouper remained. A few fishers with simple hook-and-line gear removed more than half the total population.

“When we insert fishing into a spawning aggregation, it disrupts all of this behavior,” notes Dr. Brad Erisman, assistant professor at University of Texas at Austin. “We know from other species, such as birds, that when we disturb these events, it can break down the system.” The problem, notes Erisman, is not just the physical removal of fish. Fishing activity is likely stressing fish out, too, which can affect the hormone levels in the fish left behind. These are the hormones that control color changes and behavior associated with courtship. “When you’ve got a bunch of fishing gear going through a spawning aggregation, it may be knocking back these courtship cues, and in effect resetting the clock, which would reduce the ability for the fish to spawn.”

Around the world, where there used to be tens of thousands of fish spawning, there are now only dozens. About half of all known Nassau grouper spawning aggregations have been lost entirely. They are so rare, they have been proposed for the US Endangered Species List.

The good news is that better science is helping to inform better management.

The data collected by the Grouper Moon Project shows when the fish start to travel to the spawning site, where they come from, how long they stay, and where they go. Extensive monitoring at the site has provided records of numbers of fish, and estimated size and age structures of the population. Based in large part upon these efforts, in 2011 the Cayman Islands government extended the seasonal ban on fishing on Nassau grouper spawning sites—current and historical locations—for another eight years. Thus far, the ban seems to be working, with signs of recovery in the size of individual fish, new “teenage” fish showing up for their first spawns, and approximately double the number of fish since 2003 when the ban first went into effect.

Alleviating Performance Anxiety

We can help take pressure off overtaxed populations by avoiding vulnerable species and becoming a bit more adventurous with our own seafood selections. Especially in America, we tend to turn to the same species—such as tuna, shrimp, and salmon—all the time. Our limited preferences put untold pressure on these wild species to perform and tend to support the least environmentally friendly aquaculture. Meanwhile, we miss some excellent opportunities for an extremely satisfying, and perhaps more exciting, culinary experience. The ability to diversify our palate has never been easier, as chefs increasingly embrace the opportunity to be both more creative and more sustainable with their menus.

One of the visionaries behind this effort is Barton Seaver, a chef and National Geographic fellow who has proven that using lesser-known varieties of seafood in creative ways can work—in terms of taste, sustainability, and economics. One night a few years ago, Seaver’s local supplier unexpectedly delivered a box of flying fish—the day’s bait—rather than the day’s anticipated catch to his popular DC restaurant, Hook. When pressed for an explanation, the supplier simply said it was a “bad day” for fishing. So, Seaver whipped up a delicious sauce, instructed his wait staff to tell the story about the fish, and promptly sold out of the dish by seven p.m. that night—at $26 per plate.

For Seaver, when choosing seafood we should ask what the oceans can sustainably provide, rather than demanding of them only what we want to eat. It’s a paradigm shift that could serve to redistribute fishing efforts to more responsible (and sex-friendly) harvests—spreading the demand of millions of hungry human mouths across more diverse and robust types of marine protein. This includes underutilized species that are often tossed away as bycatch (such as plentiful redfish or pollock in New England), invasive species that need culling (lionfish, anyone?), and what some consider “ugly fish” that may not look that appetizing but taste good (geoducks, a clam that looks like a giant phallus, apparently are all the rage in Asia).

Diversifying which species we demand helps give depleted stocks more chance to recover while still supporting fishers who can make a living targeting other species. As these species rebuild, fishers’ incomes can increase and menu selections can broaden even further.

Copyright © 2016 by Marah J. Hardt

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Dr. Marah J. Hardt is the Research Director for the non-profit Future of Fish. A coral reef ecologist by training, she is a former research fellow at Blue Ocean Institute. Her articles have appeared in academic and popular media, including Scientific American and The American. Her book, SEX IN THE SEA: Our Intimate Connection with Sex Changing Fish, Romantic Lobsters, Kinky Squid, and Other Salty Erotica of the Deep, is now available from St. Martin’s Press.