I found a frilly red heart on the beach and thought it so unusual, I brought it home. But my valentine from the sea didn’t stay bright for long. Its crimson color leached into the white paper towel I laid it on, and in an hour or so, the fringes turned brown.

Well, that’s blood for you. The heart-shaped organ I was toting around was a gill, freshly liberated from its fish.

Most fish have four gills on each side of the head, supported by frames of cartilage. Apparently, a fisherman had cleaned a catch nearby, and the cartilage supporting the gills floated to shore.

Because we humans are so air-oriented, it comes as a surprise to some that fish breathe oxygen. But like us land animals, fish need oxygen to transform the food they eat into energy. And because they live in water, most aquatic animals must extract this crucial life gas from their surrounding water.

At best, however, water is a lousy carrier of oxygen, and far worse when it’s warm and salty. For instance, fresh water at 40 degrees Fahrenheit holds about one-twenty-third as much oxygen as air. Heat that water to 95 degrees Fahrenheit and it holds only one-forty-third as much oxygen as air. Add salt to that warm water and you get only one- fifty-second as much oxygen as air.

This is why offshore tropical waters, including those surrounding Hawaii, are the deserts of the oceans. It’s also why gills must be super efficient at getting oxygen out of water.

Not moving water back and forth, as we do with the air we breathe, saves fish considerable energy. The average fish uses muscles to pump water into its mouth and past its gills, organs that contain numerous tiny blood vessels, the frilly edges of the piece I found. As the water passes the vessels, the oxygen dissolved in it seeps into the blood. At the same time, the water carries away the waste gas, carbon dioxide.

It’s easy to see some fish breathe because they open their mouths wide when sucking in water. In some kinds of moray eels, this gaping exposes sharp teeth, making the fish look as if it’s threatening you. Usually, though, the moray is simply breathing.

Sharks take breathing efficiency a step further. Most species don’t waste energy pumping water over their gills. Rather, they swim nonstop with their mouths open, the water bathing the gills and leaving through slits. In sharks the forward movement gets two jobs — breathing and foraging — done at the same time.

In the water gill projections float, but on land they collapse, blocking their surface area and smothering the fish.

This year I’ll be celebrating Valentine’s Day with Craig and several friends as we make our way to Zanzibar for my first snorkeling experience in the Indian Ocean. But even on this dream trip with the love of my life, I’ll be one of the few people in the world thinking of fish gills.

To reach Susan Scott, go to susanscott.net and click on “Contact” at the top of her home page.