The Shrimp That Lives Inside a Starfish’s Butt

The Most Unexpected Roommate

Deep under the waves, where coral reefs bloom with color and life, lives a shrimp so sneaky, it makes its home in one of the strangest places in the ocean — inside a starfish’s butt.

Yes, you read that right.

This tiny shrimp doesn’t live under rocks or in anemones.

It lives inside the body cavity of a living starfish, entering through the anal opening and setting up camp inside.

Say hello to Periclimenes soror and its relatives — a group of commensal shrimps known for their bizarre (and very personal) choice of residence.

Inside the Starfish

Unlike parasites, which harm their hosts, these shrimps are considered commensal — they benefit from the relationship, while the host is mostly unaffected.

But still… living inside another animal’s body, especially through the rear end, raises a lot of questions.

How do they breathe?

What do they eat?

And most importantly — how do they get in there?

Let’s dive in.

Meet Periclimenes soror

This species of shrimp belongs to a genus known for symbiotic relationships with other marine animals — like sea cucumbers, sea urchins, and even jellyfish.

But P. soror has a favorite: starfish.

These tiny, semi-transparent shrimps — usually no more than 1–2 cm long — sneak into the starfish’s body through the cloacal opening, a shared passage used for waste and water circulation.

Once inside, they hide in the internal body cavity, safe from predators and strong ocean currents.

It’s dark, warm, and well-protected — the perfect little studio apartment for a shrimp.

What’s in It for the Shrimp?

The starfish offers:

• Protection from predators

• A stable environment with controlled water flow

• A constant supply of organic particles, mucus, and micro-waste that the shrimp can feed on

Think of it like living in a fully automated bunker, complete with snacks and air conditioning.

The shrimp doesn’t seem to harm the starfish. In fact, many starfish continue living normally, unaware (or unbothered) by their little guest.

How Do They Enter?

It’s believed that juvenile shrimps use chemical cues to locate their preferred host. Once found, they make their move — sliding through the cloacal opening at the center of the starfish’s underside.

This entrance might sound invasive, but in marine biology, it’s actually a common strategy among symbiotic invertebrates.

For the shrimp, it’s a one-way ticket to safety.

Some Are Exclusive Tenants

Interestingly, some Periclimenes species show host specificity — meaning they only live with certain types of starfish.

Others are more relaxed and may hop between starfish or even other echinoderms like sea cucumbers.

In some rare cases, more than one shrimp lives inside the same starfish — usually a male and female pair. Talk about underwater real estate sharing.

A World of Hidden Relationships

This isn’t the only strange living arrangement in the ocean:

• Some fish live inside sea cucumbers

• Certain crabs live in the gills of oysters

• And a few worms live in the stomachs of sea urchins

The marine world is full of weird, hidden homes — and Periclimenes soror is a master of the art.

Is It Gross… or Genius?

At first glance, the idea of living inside a starfish’s butt sounds absurd — maybe even disgusting.

But when you think about it, it’s a clever survival strategy.

• The shrimp doesn’t need to build a burrow

• It avoids almost all predators

• It feeds passively without hunting

• It’s protected from waves, sandstorms, and cold water

In a harsh environment like the ocean, where danger lurks everywhere, this little shrimp has found a perfect hideout — even if it comes with some awkward plumbing.

More Than a Punchline

Sure, it might sound funny.

“The shrimp that lives in a starfish’s butt” makes for a great headline.

But behind the humor is a real marvel of adaptation and survival.

This shrimp doesn’t just live somewhere weird — it thrives there.

It makes a home out of the unexpected, and in doing so, reminds us just how wild and inventive nature can be.