Hermit Crab and Sea Anemone Relationship


by Simon Griffiths


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Many living organisms out in the wild depend on each other for survival.

Have you seen cattle grazing with birds on their backs and wondered why the cattle do not seem to mind? This is because they both depend on each other for benefits. The birds will pick off and eat the annoying pests on their backs and in return, as the cattle disturb the insects on the grass, securing an easy meal for the birds.

In this article, I will focus on a marine relationship. Symbiosis is actually very common amongst tropical marine creatures. An example is the hermit crab and the sea anemone.

First, let’s discuss a bit about these relationships.


Types of marine relationships


Symbiosis is a long-term relationship between different species of either plants, animals, or microorganisms that benefits either one or all the members of the relationship.

Most symbiotic relationships are mainly for protection from predators, food, and shelter. Symbiosis takes many different forms such as


In mutualism, both the organisms in the relationship stand to benefit from one another. A good example is the one mentioned earlier, between birds and cattle.


In this form of symbiotic relationship, only one of the organisms stands to benefit while the other has nothing to gain or lose. The other organism is not harmed in the process.


In parasitism, one organism benefits from the other while simultaneously harming it in the process. A good example is ticks sucking cattle blood.

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Neither of the organisms benefit from the relationship and one of them is harmed. An example is penicillium killing bacteria without gaining anything in the process. Competition is also said to be a form of amensalism. This is because in the process of fighting for scarce resources all the involved organisms face a lot of setbacks and challenges, some more than others.

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This is a symbiotic relationship whereby one organism will alter its characteristics to match those of another organism, for its own benefit. It can either be exploitative to one party or mutually beneficial to both. An example of mutually beneficial mimicry is seen in bumblebees.

Now, the relationship between sea anemones and hermit crabs has been widely studied. While some people say it is mostly mutualistic, other researchers have stated that their relationship ranges between different forms of symbiosis. This means that they can be parasitic, commensalistic as well as mutualistic depending on the conditions.

How Does Hermit crabs benefit from sea anemones?


Sometimes hermit crabs will find a sea anemone in the early stages of its life and form a life long partnership. The hermit crab will attach the sea anemone on its gastropod shell for protection. Sometimes they will even bring the sea anemone along when they switch shells.

The sea anemone has stinging tentacles that keep predators, especially octopuses and large fish at bay. The sea anemone will spread its stinging tentacles over the hermit crab whenever predators are closeby. These predators are less likely to approach a hermit crab when there is a sea anemone attached to it.

Strengthened shell

The sea anemone that is attached to the hermit crab will form what is known as a ‘living cloak’ for the hermit. It will form a rigid chitin structure known as a carcinoecium which strengthens the shell and gives the hermit crab extra protection for its body.

This lessens the need to switch shells frequently other than for sizing purposes.

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Sadly, in this case, the nature of their relationship is parasitic. In extreme cases of starvation, the hermit crab will turn on its sea anemone friend and eat it for nourishment.

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How Does Sea Anemones Benefit From Hermit Crabs?


When the sea anemone finds a hermit crab to attach itself to, it finds a new home or what is known as a substratum. A substratum in simple terms is a surface on which a living organism attaches itself to and grows.


From being attached to the hermit crab, the sea anemone gets to eat whatever is left from the hermit crabs foraging. Since the hermit crab covers more ground than the sea anemone would on its own, the sea anemone has access to more food. The sea anemone never has to worry about looking for food.


The hermit crab provides protection to the sea anemones as well. Sometimes some of the predators that try to prey on sea anemones will be fought off by the hermit crab.


While the hermit crab provides a comfortable substratum, the sea anemone expands its movement opportunities and uses this time to find plankton and small fish. A sea anemone not attached to a hermit crab will only cover a fraction of the sea bed in movement compared to one that is attached.

Disadvantages arising from the hermit crab and sea anemone relationship


As mentioned earlier, if in the course of their relationship, the hermit crab is too starved, he can easily decide to eat the sea anemone. This means the sea anemone is presented with a level of risk attached to having the hermit crab as its host.

Heavier shell

The hermit crab gains protection from its predators, but at the cost of its energy. Hosting the sea anemone is extra weight on its shell and extra energy used to move around and look for food.

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Hermit crabs do not only fight over shells, but have also been said to fight over sea anemones for their protective value. Some bigger hermit crabs will also gather an increased number of anemones or attempt to steal them from their original hosts. They are also fought by other sea creatures that try to feed on the sea anemones.

Hermit crabs will thus face both intraspecific competition and interspecific competition. Intraspecific competition is competition for resources between organisms of the same species while interspecific competition is competition from different species.


In conclusion, hermit crabs and sea anemones have a symbiotic relationship.

Sometimes their symbiotic relationship can be described as mutualism, with other situations calling for it to be referred to as commensalistic or parasitic.

They depend on each other for a few things. The hermit crab gets extra protection on their shells while the sea anemone mainly increases its range when it comes to food. While their relationship is beneficial to both of them, sometimes it can present some challenges that affect their survival.


Simon Griffiths

Hi guys, my name is Simon, a fellow pet lover. I love everything about traditional and exotic pets so I am here to help you create a better home for your pets.

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