Why Sea Stars Huddle Together Beneath the Waves

Stars twinkle overhead, but under the sea, stars huddle together. Sea stars, that is! Sea stars (Asteroidea), commonly known as starfish, are invertebrates called echinoderms — creatures with hard, calcified skin — and are related to sea urchins, sea cucumbers, and sand dollars. Although these beautiful animals are typically considered solitary creatures, they will sometimes huddle together underwater. A video caught this gentle moment of connection, and it’s a cool glimpse into a sea star’s life.

Point Defiance Zoo & Aquarium Parks, in Tacoma, Washington, shared a video of sea stars on Instagram, taken by conservation engagement curator Wendy. “Chocolate chip sea star group hug!

,” the video was captioned. It showed several chocolate chip sea stars close together, with their tentacles intertwining with each other for a big hug. (By the way, chocolate chip sea stars are named after their black spikes that look like chocolate chips on their backs.)

Although they may appear to be simply stuck together, sea stars huddle for specific reasons. In the caption, the Port Defiance Zoo & Aquarium explained: “These spiky sweethearts aren’t just cuddling for fun; sea stars often gather together to feed, stay protected, or just take advantage of the best spot in our Tropical Shallows habitat!

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What Are Sea Stars?

There are nearly 2,000 species of sea stars living in coral reefs across the world’s oceans, which are identified by their bumpy skin, colorful design, and five-pointed arms that look like a star. They use seawater to carry nutrients and oxygen through their vascular system, so they can’t survive in freshwater or out of the ocean.

Starfish move slowly due to their fragile network of thin muscles and tiny ligaments. On each of their five arms, sea stars have up to 15,000 “tube feet” that help the sea star walk along the ocean bottom, according to Ocean Conservancy. These feet create suction through the sea star’s amazing water-driven hydraulic system. These feet also contain a glue-like substance to help the sea star stick to rocks and avoid being swept away to sea. When they are ready to move, they can secrete a solvent to dissolve the sticky substance and continue on their way. Another cool fact? Sea stars can regenerate their limbs and have even been known to regrow an entire new body from just an arm.

A Unique Digestive System

It might be a good thing we can’t see what the sea stars are doing underneath that group huddle, because it might be too unsettling to see. When a sea star eats, it’ll move its body over prey such as coral, sponges, shellfish, and algae to center its mouth over it. With clams, mussels, and oysters, sea stars will pry their bodies open with their suction cup feet.

This two-part stomach system first releases digestive enzymes onto the prey, then the second stomach engulfs the prey to complete digestion. The stomach extends outside of the sea star’s body to capture its food, then retracts back inside along with the digested material. We told you it was unsettling!

Many sea stars, including the chocolate chip sea star shown in the video above, will even eat other sea stars. Scientists believe this is usually accidental cannibalism rather than a deliberate attack. (But that doesn’t matter much if you’re the victim, does it?)

Sexual and asexual reproduction

Another reason sea stars may gather is during mating season. Starfish can reproduce both sexually and asexually, per the Denver Zoo Conservation Alliance. To reproduce sexually, sea stars will release spawn clouds into the water, and sperm will fertilize eggs outside the body. To increase the odds, sea stars gather in groups to spawn together, responding to environmental and chemical signals.

Once the eggs are fertilized, they turn into swimming larvae that will drift to the bottom of the ocean floor, where they will undergo metamorphosis. A single female starfish may produce over two million eggs per spawn, but only a few will make it to adulthood, as marine animals will feed on the eggs and larvae. Sea stars will move on after fertilization, not taking care of their young.

Asexual reproduction is a bit different; it happens when a starfish breaks into two parts and each half regenerates the missing portion of its body to form two complete sea stars. Recent research has found that starfish from different species are breeding and mixing their genetic material, which may help them survive threats like sea star wasting disease and climate change. Sea stars are cute, but they are also vital to our ecosystem as they break down organic matter on the ocean floor, keeping water clean and preventing the build-up of dead creatures.

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