Who Helps Whom in the Sea?

Partnerships, cheating and parasites: the strange relationships that connect marine animals.

A grouper stops beside a little cleaner wrasse and opens its mouth. The wrasse may take a parasite, or it may bite off some of the slippery layer that protects the grouper’s skin.

By DIVE.EXPLO360.FR · 10/11/2026

At a cleaning station, a grouper tolerates the approach of a cleaner wrasse. Its pose alone does not reveal whether the wrasse takes a parasite or mucus. © dive.explo360.fr — All rights reserved. Reproduction requires permission.

What does each partner gain?

Look at the three cards. For each pair of animals, ask what each one gains and what the encounter costs it.

Start with the same question for every encounter: what changes for each animal?

Three outcomes to compare

Mutualism

Both partners benefit.

Commensalism

One benefits and the other is largely unaffected.

Parasitism

One benefits by taking resources from a host that bears a cost.

When the cleaner takes mucus

The client can interrupt the visit. The option to leave helps explain why taking mucus can cost the cleaner a meal.

A cleaner wrasse eats small parasites off the fish that visit its station. Experiments show it can prefer the fish’s mucus to some parasites. If the wrasse bites into that protective layer, the visitor can break off the cleaning and may go elsewhere.

The fish that keeps watch for two

Watch the goby at the burrow entrance. It spots danger while the shrimp moves sand. An antenna touching the fish lets the shrimp feel its movements. The goby gains access to the shelter the shrimp digs.