The animals and plants underwater interact in many different and often complex ways. We are only starting to understand them.


Benefits of living together

When two different species live together it is called Symbiosis. Through cooperative relationships within the ecosystem organisms profit from strength of other organisms providing easier access to food and shelter. By avoiding competition they also waste less energy.

The term symbiosis is used to describe any kind of association between two different species. Marine animals can live in more than one symbiotic relationship. For example a fish carrying parasites which are then eaten by another animal living in a symbiosis with the fish.


Mutualism, commensalism and parasitism

Symbiosis is divided in three different groups – mutualism, commensalism and parasitism, but what does all that mean?

Mutualism is like a friendship, both profit from the interaction,

in commensalism one party profits while the other is not affected by the relationship

and in parasitism one party profits on cost of the other.


Let’s look at some examples. It is now always easy to figure out in which type of symbiosis two animals live. Often we lack knowledge about one or the other animal which makes it hard to know if they benefit from a relationship or might be harmed.


Clownfish and anemone

This is one of the most famous symbiosis through the movie “Finding Nemo” and one of the divers’ favorite. It was long believed that this was a commensal relationship where only the clownfish profits. The fish gets a safe home with stingy tentacles to hide from predators.

clownfish and anemone

However, later on researchers realized that the anemone profits as well from its inhabitant. It gets nutrients from the clownfish waste, stays healthy due to the aeration by the constant movements and the quite aggressive fish scares off predators.


Decorator crabs and decoration

decorator crab

The decorator crabs use material from their surrounding to camouflage from predators and prey or might even become toxic for predators. Some of the crabs also carry their favorite food with them.

If the decoration profits from being carried around is not entirely clear. Some animals used as decoration will stay intact like sponges or anemones. These might profit from being mobile and access to more food.


Emperor shrimp and sea cucumber or nudibranch

emperor shrimp on spanish dancer

Emperor shrimp use sea cucumbers or nudibranchs to get around. Like this they have access to more different food sources. It is not entirely know if this is mutualism or commensalism as the shrimp might keep its vehicle free from ectoparasites.

solar powered nudibranch wakatobi
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Pearlfish and sea cucumber

This is a quite odd symbiosis. The pearlfish hides in the intestines of the cucumber during the day and leaves at night to feed.

Most of the pearlfish are just entering the sea cucumbers’ bum to find shelter most probably in a commensal relationship as the cucumber doesn’t profit.

Other pearlfish, however, are parasitic and eat the sea cucumber inside out (NatGeo).


Sea urchin and shrimp or crabs

coleman shrimp and fire urchin

One very pretty example and a favorite of underwater photographers are the coleman shrimp living on fire urchins. The shrimp live exclusively on the urchin, normally in pairs.

They are protected from predators through the poisonous spikes of their host remaining unharmed themselves. While the shrimp have a protected home they feed among other foods on parasites from the surface of the urgin. Therefore this could be regarded as mutualism.


Parasitic Isopod and different species of fish

Different types of parasitic isopods exist. Some of them enter the fish through the gills and attach themselves to the tongue. The parasite causes the tongue of the fish to fall off and then serves as tongue-replacement. Gross!

It is thought that the fish can use the isopod like a normal tongue. The isopods feed on blood or mucus. This is definitely bad-ass parasitism.


Remora with sharks, turtles or mantas

manta socorro

The remora has modified dorsal fin building a sucker organ on its flat head. With this they are able to attach themselves to large animals. The shark, manta or turtle provides shelter on one side and readily available food on the other side. The remora removes ectoparacites and loose skin from its symbiotic partner. Therefore they both profit.remora on leg

Did it ever happen to you that a remora tried to attach itself to you? It happened a couple of times to me. It’s actually quite hard to get rid of them again when they start to follow around. I never let one actually attach itself to me.


Shrimp and goby

A very cute symbiosis can often be observed on the sandy bottom. A shrimp works relentlessly digging a hole to live in by pushing sand out. In front of the hole sits a goby like a guard dog.

When a predator or even a scuba diver gets close to the hole the little fish starts to flick its tail several times. This causes the almost blind shrimp to stay inside the hole. When the situation gets even more dangerous the goby hides in the hole as well. A perfect pair one can see and the other one takes care that the house is in good shape.


There are numerous examples of different types of symbiosis in the ocean. Many of these offer amazing opportunities to observe marine behavior, to take stunning images and in many cases to get closer to animals.


What types of symbiosis have you observed during your underwater explorations? I would love to hear about it in the comment section below.