column spacer Symbioses
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of anemone with symbiotic shimp taken from a video

"If you look closely at anemones you often see small shrimps and crabs living within or under the tentacles. This one is a type of cleaner shrimp. Different coral-reef organisms living together are said to be symbionts." - Bonaire 2003

NOTE this one is the giant anemone Condylactis gigantea

NOTE Periclimenes sp.


There are 3 main types of symbiotic relationships amongst reef dwellers, including ones that involve photosynthetic microorganisms. The relationships are often hard to define and definitions tend to overlap. For example, if you find one organism living on or with another, the first question to ask is, "Is one participant gaining benefit, but harming the other?". If so, the relationship is a parasitism. The next question is, "Is one participant gaining benefit with no harm to the other?". If so, the relationship is a commensalism. Finally, "Are both participants benefitting?". If so, the relationship is a mutualism.

This sounds simple, but in practise it is often difficult to determine benefit and harm. A barnacle on a crab shell benefits from being provided a place to stay, but what about the crab? If it truly is not harmed by the barnacle's presence, then it is a commensalism. However, if there are costs, no matter how seemingly small, such as for the crab to support the mass of the barnacle and to counteract the increased drag forces it creates in a current, then it is parasitism. If you are a reductionist (a "lumper"), then you can consider them all parasitisms. If, however, you are more of a "splitter"-type person, then you may enjoy the challenge of trying to separate them out.

NOTE refers to organisms living together in close physical association. In North America the term does not usually indicate benefit or harm; in the U.K. and Europe, however, the term usually refers to associations of mutual benefit, or what we in North America call mutualism

NOTE there is a fourth type of symbiosis known as inquilinism (lit. "lodger" or "tenant"), referring to a species that also lives in association with another without harming it, but specifically within the latter's nest, burrow, or dwelling. A oft-quoted instance of inquilinism is the occupation by certain aquatic insect larvae of water contained at the bases of pitcher-plant leaves. The larvae paddle about and feed on algae within. Other examples, however, tend to resemble parasitism and/or are hard to separate from what we think of as commensalism. For this reason, possible inquilines are lumped in with commensals in BCCR

Commensalism hot buttons for symbiosis topics in BCCR hot button for commensalism part of BCCR hot button for symbiosis quizzes part of BCCR hot button for mutualism part of BCCR hot button for parasitism part of BCCR
The section on symbioses is divided into 3 main parts: commensalism, considered here, and parasitism and mutualism, accessible via the icons. After reading these, you can test your knowledge by taking the symbiosis quizzes.
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a sponge with a brittle star on top taken from a video

"A symbiotic relationship where one animal benefits and the other is not affected, is known as commensalism. Here, brittle stars benefit from an elevated spot where they are protected and can feed. In theory, the sponge is not harmed, but is this true?." - Key Largo 2004

NOTE sponge brittle star Ophiothrix sp.

  photographs of heart urchin with simulated crab commensal
As noted in the preamble above, good examples of commensalism are hard to find as usually some harm or other befalls the passive partner, even though it may not be evident on casual view. For example, heart urchins, which lay partially or wholly buried in the sand, often host small crabs that are thought to be commensal. However, if the crab inflicts damage to the urchin as it crawls about, then it is behaving more like a parasite.

photograph of a scaleworm crawling on a sea cucumberA scaleworm that lives on the skin of a sea cucumber presumably benefits from a place to live. Its presence seems innocuous and it is usually considered to be a commensal. However, if it irritates the skin of the sea cucumber, takes food from it or, perhaps even feeds on bits of skin, then it could also be termed a parasite.



Arctonoid-type polychaete scaleworms live symbiotically
with sea cucumbers and other echinoderms 0.75X


brittle star with its sponge hostThe relationship between brittle stars and their hosts is usually thought to be commensal but, as their presence may contaminate the outer layer of cells of the sponge and increase frictional drag in currents, possibly leading to breakage, then once again they may be thought of as parasites. Photograph courtesy Anne Dupont, Florida.





Sponge brittle-star Ophiothrix suensonii
crawls on a vase sponge Niphates digitalis 1X

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a crinoid on a gorgonian taken from a video

"When one animal shelters in or on another, we call it inquilinism. The term is meant to describe fishes or other animals resting on corals and sponges, where they have no effect on their host, neither benefit nor harm." - Turks & Caicos 2006

NOTE shown here a crinoid Nemaster grandis on a gorgonian

  But, is this true about having no effect on their hosts? Brittle stars and feather stars crawling on or attached to a gorgonian are likely to interfere with feeding and respiratory activities of their hosts. Also, by increasing the effective surface area exposed to currents, they may result in additional energy being diverted by the host to strengthen its supporting skeleton.

photo collage of feather stars and brittle stars on their supportivehosts

  photo collage showing arrow crabs on different types of substrata
Yellowline arrow-crabs Stenorhynchus seticornis are often found with sponges, sea urchins, and other invertebrates but, while they may gain protection by the associations, the interactions are only transient, as the crabs tend to wander widely throughout the reef. So, they cannot be called commensals or iquilinines.
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of decorator crab in a gorgonian taken from a video

"What have we here? Oh, it's a little decorator crab in a gorgonian. Not too well camouflaged, is it? This seems like a good example of organism hanging out on another. without causing evident damage." - Cayman Brac 2001

NOTE possibly the cryptic teardrop crab Pelia mutica


Well, the dive leader seems to think the decorator crab in the gorgonian as an example of inquilinism, but this is not at all certain. In fact, a crustacean living on or in a sea anemone could be a commensal, a mutual, or a parasite. photograph of a banded clinging-crab in a sea anemone, with the crab's position made obviousphotograph of a banded clinging-crab in a sea anemoneWhether commensalisms/inquilinisms exist may ultimately depend upon one's definition of benefit and harm, and on the extent to which we understand what is going on in a symbiotic relationship.

How do anemone-inhabiting crabs and shrimps deal with their host's stinging cells? Either they fail to elicit discharge in the stinging cells or their tough, impervious exoskeletons provide adequate protection.


Banded clinging-crabs Mithrax cinctimanus
commonly associate with sea anemones
Condylactis gigantea 1X


The same view with the
crab's position indicated



photograph of a sea anemone hosting several symbiotic shrimps









Squat-anemone shrimps Thor amboinensis in a giant anemone
Condylactis gigantea
could be termed a commensalism, but if
the shrimps were to attract their own predators into the
host's stinging tentacles, then this would clearly benefit the
anemone. If so, the relationship would be a mutualism 2X


photographs of hermit crab Calcinus tibicen associating with fire coralIt is not uncommon to see hermit crabs Calcinus tibicen sheltering among the extended fingers of fire corals Millepora spp. Observation of tagged individuals in St. John, US Virgin Islands show that most crabs return to their original hosts following night-time forays or experimental displacements, and will follow the scent of upstream Millepora colonies to seek out other hosts. These findings and the fact that individuals removed and/or tethered away from their hosts are more likely to be consumed by predators than ones tethered to their hosts suggest to the researchers that protection from predators may be the chief benefit to the crabs. The authors describe the relationship as a commensalism but, as this definition implies that one participant (the crab) benefits while the other (the fire coral) is not harmed, it is hard to accept. A dozen or more hermit crabs walking and climbing on the surface of a single colony is likely not harmless, so parasitism might be a more apt descriptor. Brown & Edmunds 2013 Coral Reefs 32: 127.

NOTE to be fair, the authors discuss the limitations of applying the term commensalism to this and other types of coral-reef relationships

Hermit crabs Calcinus tibicen
sheltering among the branches
of a fire coral Millepora sp.
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of bicolour damselfish in a sponge taken from a video

"Well, hello! What's a bicolour damselfish doing defending its territory inside a sponge? You should be in your garden. Or, if you want to help, why don't you clean out some of the sediment in here?" - Little Cayman 2001

NOTE Stegastes partitus

  photo collage of fishes in sponges
Small fishes such as blennies often use sponges to perch on or to peer out of. This type of behaviour might be considered a kind of inquilinism, were it not for possible deleterious effects to the host. These effects could include interference with water flow, abrasion of tissues, and fouling with feces, urine, and decaying food residue if the fish brings food back to the sponge to eat.

hot button for parasitism part of Biology of Caribbean Coral Reefs website hot button for mutualism part of Biology of Caribbean Coral Reefs website hot button for commensalism topic in BCCR hot button for symbiosis quizzes section of BCCR