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Photo of Sea Star's 'Butt' Goes Viral. Spoiler: That's No Butt.

Vermilion sea stars (Mediaster aequalis) are known for their vivid red-orange color and the symmetry of their five arms. But one individual quickly gained internet fame for a body part that isn't usually associated with starfish: a shapely rear end.

Twitter user @Babyshoujo recently photographed and tweeted an image of the "thicc" starfish as the animal clung to a rock in an exhibit at the Aquarium of the Pacific in Long Beach, California.

The sea star's prominent derrière has captivated hundreds of thousands since the photo was shared on June 30. Quite a few commenters drew comparisons to the "SpongeBob SquarePants" starfish character Patrick Star, who often proudly displays his prominent, rounded bottom. However, experts were quick to point out that the "butt" isn't quite what it seems. [In Photos: The Wonders of the Deep Sea]

Vermillion sea stars are found near low-tide lines and on the rocky sea bottom in the eastern Pacific Ocean, with their range extending from Baja California north to Alaska, according to the Georgia Aquarium. Though sea stars are often referred to as starfish, they are not fish. They are echinoderms, the group that also includes sea cucumbers, sand dollars and sea urchins.

And no, sea stars don't have "butts"; they have a centrally located anus, but they don't have human-like buttocks, as the cartoon Patrick Star does. What we're seeing in the photo are the contracted muscles of the starfish's arms as they grip the rock, Nate Jaros, curator of fish and invertebrates at the Aquarium of the Pacific, told USA Today.

Because the sea star's body is vertically aligned, gravity causes the creature's internal structures to "slump," and that produced the shapes resembling the human buttocks in the picture, Jaros explained.

Previously, in March, social media users were intrigued by a similar illusion of thiccness in a white-faced saki monkey (Pithecia pithecia) in Finland's Korkeasaari Zoo. The monkey, named Bea, appeared to be heavily muscled, her biceps as rounded as the sea star's butt at the Pacific Aquarium.

However, the so-called buff monkey's bulk was really just fluffy fur, zoo representatives told Live Science.

Originally published on Live Science.