Last week I watched two fourspot butterflyfish glide around a coral head so close together they were almost touching. Was this a male and female? I wondered. Whenever I’ve seen two butterflyfish together, I’ve always assumed they were male and female, but to what end? Because butterflyfish release their eggs and sperm into open water, these parents don’t have to feed or protect eggs or offspring.

So with plenty of fish in the sea, as they say, and the kids drifting off before they’re even born, what’s the advantage of lifetime liaisons?

It’s clearly a benefit in some butterflyfish species. The world’s coral reefs host about 130 kinds of butterflyfish. Some hang together in schools; others live solitary lives. But at least 78, or about two-thirds, form pairs.

My idea that butterflyfish couples are male and female is mostly accurate. Researchers haven’t studied each species yet, but they have looked at the behavior of some that live in pairs.

One benefit to being a fish couple is that no romancing is required. Neither male nor female has to waste energy searching for a partner or performing elaborate courtship rituals. In one butterflyfish species, couples swim to the reef’s outer edge to spawn during periods of strong tidal currents. This sweeps the precious sex cells away from the reef’s many plankton eaters. And that’s that. The two go back to eating, while trying to avoid being eaten themselves. It’s here that monogamy has other perks. Butterflyfish have varied diets. Some species eat tiny shrimp and crabs, others prefer plankton and many graze on the soft bodies of live corals. One theory about butterflyfish pairing is that when it comes to finding shrimp and crabs hiding in sand and reef cracks, four eyes are clearly better than two.

Another notion is that because the minuscule animals and corals that certain butterflyfish eat live scattered across large areas of the reef, the fish have to travel far and wide to dine. Such roving prevents a male from creating and guarding a harem. But a monogamous male doesn’t have to. He simply keeps his female with him.

Researchers have also noted that some couples watch each other’s back. While one nibbles, the other functions as lookout, signaling when to flee from an approaching predator.

Of Hawaii’s 25 butterflyfish species, at least 12 live in pairs, giving us snorkelers and divers lots of chances to watch synchronized swimming in fish. It’s a marvelous sight to behold, two bright fish waltzing together in matching outfits of yellow, black and white, with the occasional blue or orange accessory. And that may be another reason some butterflyfish pair up. They love ballroom dancing.

Reach Susan Scott at susanscott.net.