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Commensalism
COMMENSALISM - A symbiotic relationship in which one organism benefits while the other is
neither harmed nor benefited.
Examples of commensalism:
Moss growing on trees benefits by being raised above forest floor competition, while the tree
doesn't get much out of the deal either way.
A desert holly shrub provides shade for young creosote bush.
A cactus wren builds its nest in a cholla cactus to protect its young from predators such as
raven. There is no harm to the cactus.
Commensalism :
We will begin by looking at commensalistic relationships, as there are some very interesting examples of
this form of symbiosis where we have been diving, here in Papua New Guinea.
Commensalism usually occurs between a species that is either vulnerable to predation or with an
inefficient means of locomotion, and another species with a relatively effective system of defence. For
example certain Goby species live amongst the spines of toxic sea-urchins such as Astropyga radiata, so
gaining protection from their host; while a similar relationship occurs between Man-of-war fish and the
‘Portuguese Man of War’ jellyfish. In both instances the fish benefit, while their host is neither advantaged
nor disadvantaged by the relationship.
Crabs and Shrimps often form commensalistic symbiotic relationships with anemones in tropical waters,
again for the purposes of protection from predation. For instance the Anemone crab, Neopetrolisthes
oshimai, which is a filter feeding Porcelain crab, lives and captures its food from within the tentacles of
giant anemones.
While diving in Papua New Guinea we have been fortunate enough to see several interesting
commensalistic relationships. Apart from the Anemone crabs, we have also filmed Imperial shrimps,
Periclimenes imperator, hitching a ride on the large sea cucumbers found here, (genera Stichopus). The
shrimps get transported through a large area of potential food by their host with only a minimal
expenditure of energy on their part. They can be observed getting off their host cucumber to feed in
productive areas, and back on for a ride to the next spot! The Imperial shrimp also rides on large
nudibranchs such as genus Dendrodoris, which although slow moving, afford the shrimp with protection
by virtue of their toxic chemical secretions and warning colouration. Although this is currently classed as a
commensalistic relationship, it is possible that the Emperor shrimp may assist the nudibranch by
removing parasites.
One especially amazing example of commensalism that I have yet to witness occurs between the
Pearlfish and a particular species of sea cucumber. The Pearlfish spends the day inside the alimentary
tract (‘intestines’) of the sea cucumber, and at night emerges from its anus to feed on small crustaceans!
In this manner it gets a safe place to live; and while not appearing to gain any benefit from the
relationship, the cucumber is not harmed.