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The lettuce sea slug (Elysia crispata) has enlarged fleshy appendages that are folded over one another, with colors ranging from blue to green, with purple and red lining. The green coloring is what gives this mollusk it's common name, resembling a head of leafy green lettuce. The sea slug eats green algae, but not all of the algae they eat is digested. Some of the green algae gets shuttled off to make a home in those fleshy appendages (called parapodia).

In this video Smithsonian research zoologist Dr. Martha Nizinski takes viewers with her as she searches for crustaceans in the deep sea. She's particularly interested in finding squat lobsters, which despite their name, are actually crabs. On this dive in the waters off Curaçao, she discovers some living on a sunken piece of wood.

An X-ray image of a Monterey skate (Raja montereyensis) reveals a spine that extends like a tail out from the pelvic fin. The skeletons of skates, rays, chimaeras, and sharks are made of cartilage, rather than bone. Scientists in the Division of Fishes at the Smithsonian's National Museum of Natural History use X-ray images, like the one shown, to study the complex bone structure and diversity of fish without having to dissect or damage the specimen.

The Laysan albatross (Phoebastria immutabilis) breeds mainly in Hawaii and other Pacific islands where male and female pairs will incubate their egg for nine weeks. The pair participates in an elaborate courtship dance where movements and noises bond them together for the rest of their lives. After breeding season is over the birds move north and west towards Japan and Alaska.

What happens to the waste coming out of a fish processing plant? Typically the fish guts and bones get sent to sea via a pipeline, and the Environmental Protection Agency regulates how much can be dumped. The sites can attract predators, such as these seagulls, fish, and even small sharks in Alaska, which feed on the scraps.

This beautiful bromeliad, also called an air plant because it gets its nutrients and water from the air, is a flowering plant in the pineapple family. All of them are epiphytes, meaning they get their support from and grow on other plants. Many are found in mangrove forests, such as this one making house on a mangrove root. It's not known whether they provide any benefit to the trees, but when the bromeliads lose their leaves and petals, they fall into the water and provide nutrients to the underwater ecosystem.

There are 47 different species of seahorses and 14 of those were discovered in the last eight years, including Pontoh's pygmy seahorse (Hippocampus pontohi), which was officially named in 2008. Seahorses’ ability to change their color and shape to blend in with their environment makes identification of individual species challenging. Because of this, some researchers previously thought there were as many as 200 seahorse species in the world, while others thought there were as few as 20.

These "elevator" rudists, an ancient bivalve, used one long heavy valve to anchor themselves in the sediment. They used their tentacles (shown here in pink) to filter food from the sea water. Discover more about the ancient ocean at our feature Ocean Over Time.

Like other sea snakes, the turtle-headed sea snake (Emydocephalus annulatus) has fangs and venom. But its venom is weak so, instead of defending with a bite, the species tends to react to danger by swimming back to a crack or crevice to hide. For food, the sea snake sneaks around coral reefs looking for fish eggs attached to coral or rocks.

A chambered nautilus shell.

How will changes in temperature affect glaciers and ice sheets? Dr. Sarah Das from the Woods Hole Oceanographic Institution explores this phenomenon first hand in Greenland, where she studies how the melted ice travels through glaciers and out to the sea. Learn more about climate change.

When it comes to many of our once-favorite seafoods, there aren't always plenty more fish in the sea. In fact, some studies estimate that up to 90 percent of large predatory fish (those that eat other animals—and usually end up on our dinner plates) have disappeared since humans began heavy fishing.