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Glistening dark bodies roll in the water. Light-furred faces pop to the surface, then dive back in. And slinky forms trade places between pool and rocks.

For someone without a trainer’s eye, Shedd Aquarium’s sea otterslook an awful lot alike. And they seldom stay still long enough for you to try to discern differences. To help you figure out who is whom, here are each otter’s key characteristics, or field marks, as wildlife biologists say.

Let’s start with the easiest otters: largest and smallest.

Yaku the Sea Otter floats on his back in the otters' Oceanarium habitat

Yaku

Yaku, the only male in the group, is the largest. The 16-year-old otter, born at the Seattle Aquarium and a Shedd resident since 2001, is about 4 feet long and weighs 68 pounds. He has a white face. Trainers describe him as easygoing, and you have a good chance of seeing him taking a nap in the car-wash-strip “kelp” in the Regenstein Sea Otter Habitat.

Ellie the Sea Otter pulls herself onto the lip of her behind the scenes habitat to peer at the camera.

Ellie

The littlest one is Ellie, the rescued southern sea otter pup who joined the Shedd family almost a year ago. But she’s growing fast. At her last weigh-in, she tipped the digital scale at 34 pounds—more than three times what she weighed at her January arrival. She is also the darkest otter. Ellie is a typical high-energy pup, so she’s apt to be one of the two most kinetic otters in the habitat. Her newest (and cutest) activity is to slide down the rockwork slopes into the water headfirst on her back.

Sea otter Luna's dark coloring and light-colored whiskers make her easy to pick out among the other otters.

Luna

The second dynamo is another rescued southern sea otter, 2-year-old Luna, shown above on the right with Ellie. They often roughhouse together. Luna is a little longer and about 6 pounds heavier than Ellie but still noticeably smaller than the adult otters. Her muzzle has a hint of the lighter facial fur of the adults, compared to Ellie’s solid brown coloring, but Luna is still dark overall.

Kiana the Sea Otter lies curled in a bucket of ice, chewing on ice cubes.

Kiana

That leaves one more adult female sea otter, Kiana. Kiana came to us as a tiny rescue from Alaskan waters. She was pretty distinguishable by size and fur color.

Kiana is a real standout with an all-white head. You won’t confuse her with Yaku because she’s the smallest adult otter. She is also the most independent. While Kiana socializes some, she spends a lot of time on her own, grooming, napping and playing with her favorite toys, including balls. She also likes to hog the felt “kelp” strips when she sleeps. She’ll curl up in her cozy bed with the tip of her tail in her mouth.

Once you know what to look for, you should be able to identify at least a few of the sea otters, even if they don’t stay in one place long enough for easy comparisons. If you happen to catch a training session, you can decode who they are by the unique hand-held shapes used to call them to station, the same as the dolphins and belugas have: Yaku—blue square; Kiana—green triangle; Luna—yellow star; Ellie—red circle.

—Karen Furnweger, web editor