The Singles Bars of the Sea

Where tiny aquatic creatures congregate to find their mates

Just two copepods looking for love (Visuals Unlimited / Corbis)

Dating on land may be unpleasant, but for microscopic sea animals, searching for a mate in the sea is like looking for a needle in a haystack, where the haystack is the size of Mount Everest.

Consider copepods, distant relatives of shrimp that fuel the ocean’s food web. Tiny and packed with fat, they are the baby food of the sea, feeding countless larval crabs, fish, and squid. Crunchy on the outside, full of gooey oils on the inside, they’re also the go-to meal for enormous, swirling schools of bait fish—sardines, anchovies, herring—that in turn feed the tuna, snapper, and cod we like to eat. Attaining numbers large enough to satiate all those appetites requires copious amounts of copepod copulation, which requires some close physical contact between males and females—but the copepods’ size doesn’t make that easy. Some species are as small as a sesame seed, others about as long as your thumbnail. Even within the confines of the average home aquarium, a male copepod swimming around randomly is likely to bump into a female copepod about once per year—yet individuals may live only a few months and some only a few weeks. Not great odds for finding a partner.

So with an entire ocean to contend with, how in the world does an animal smaller than a grain of rice find an equally tiny (and transparent) mate in all that blue? Peter Franks, a professor at Scripps Institution of Oceanography, has a simple answer: Copepod singles’ bars, of course. And, as is most often the case for our own dating rituals, the males at these hotspots always make the first move.

To close the gap between himself and a swimming female, a male must have excellent detective skills, homing in on subtle clues she leaves in her wake. Jeannette Yen of Georgia Tech is an expert in the way minute animals move through water, and she explains that when you are as small as a copepod, water behaves differently; it’s thicker and stickier.  As copepods swim, they must dig their way along, pushing the water out in front of them and leaving temporary tunnels of disturbed water flow behind.

Fine, feathery hairs on copepods can detect subtle differences in water movement. A surge of motion from one direction may indicate a predator; the rippling waves of a swimming female create a different pattern, which males then trace by feeling their way along. In some species, females may also infuse their personal corridor with pheromones, making their signal even stronger.

Whether by feel or scent, when a male crosses a female’s track, he literally flips. He rapidly spins his body, cartwheeling into the middle of the trail as he begins a frenetic high-frequency zigzag—in three dimensions—across the trail. Once he’s locked on, the male’s pirouetting pursuit is remarkably tight, and he successfully narrows the distance from up to one hundred body lengths away. That’s the equivalent of a guy standing on top of a 60-story building and picking out his girl down on street level by the smell of her perfume.

These trails last but a few seconds, however, which is where the importance of singles bars—spots of particularly still water in an otherwise swirling sea—come in. Copepods congregate in spots where the footprints of females tend to last a little longer: the quiet, thin section of water where two different pieces of ocean meet.

Far from a uniform pool of blue, the ocean is much more like a layer cake. Different water strata of varying temperature or salinity stack up on top of one another throughout the water column, and where two water masses meet, a distinct boundary layer appears. Boundaries such as thermoclines are created by differences in temperature, but such an interface can also be created by differences in salinity or by currents and gyres when swift, spiraling eddies move through one piece of the water column but not another. These carry sections of the sea along at different rates, similar to high-and low-altitude clouds riding different winds.

For copepods swimming through the open sea, the thin boundaries between warm and cold, saltier and fresher, faster and slower water provide distinct “landmarks” within an otherwise featureless blue. These boundaries also remain relatively stable, with little mixing of water across two layers. This means the water within the boundary layer—a thin bar—remains relatively still. It is here that the females can pour their eau de copepod into their freshly carved tunnels. The quieter the water, the longer the message remains.

We may only detect the most extreme contrasts between warm and cold or fresh and salty, but copepods experience water as we would feel the texture of different fabrics. For them, the contrast between the still, Zen-like vibe of the boundary layer stands out against the other parts of the sea like silk versus corduroy, allowing them to easily detect and cluster within a much narrower patch of ocean. So in addition to increasing the shelf life of the female’s footprint, these sections of sea do what all good singles bars do: They concentrate the horny hordes. The more copepods that arrive within the boundary layer, the thicker the crowd, and the more likely each male is to find a trail and begin his sprinting pursuit.

But down the road, copepods may have trouble finding these trusted singles bars. As climate change progresses, ocean surface temperatures warm, and it’s still unclear how that will affect copepods’ go-to mating spots. Warmer water on top could strengthen the boundary layers, but it also may shift where those layers occur, how much oxygen exists (warmer water holds less oxygen), and the availability of food within each zone. Warmer oceans also fuel fiercer storms, which churn the surface and can disturb or completely erase the previously reliable layered landmarks of the open blue. It remains to be seen, in other words, whether future generations of male copepods will have as much success with the chase.

This article has been adapted from Marah J. Hardt’s book, Sex in the Sea: Our Intimate Connection With Sex-Changing Fish, Romantic Lobsters, Kinky Squid, and Other Salty Erotica of the Deep.