Marine ecosystems are complex and dynamic places in which species interact in a myriad of ways: they both compete and cooperate for protection, shelter, food and various other resources. For certain species, these interactions may have negative effects (such as competition or predation) or positive ones (in the form of symbiotic relationships).

The term “symbiotic” has long since made its way into everyday parlance; we use it, often in a casual way, to loosely describe all sorts of mutually beneficial relationships in our lives including business, social and personal matters, but what does it actually mean?

Candy-striped Shrimps with Crimson Anemone

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The word "symbiosis" literally means "living together". In biology, it refers to a close and usually obligatory association between two organisms of different species. While it is often assumed that this relationship is mutually beneficial, this, however, is not always the case. Symbiotic relationships include mutualism, commensalism and parasitism. In mutualism, there is a cooperative or mutually dependent relationship where both organisms benefit. In commensalism, one benefits and the other is unaffected. In parasitism, one benefits at the expense of the other.


Two organisms of different species are said to exist in a mutualistic relationship when each individual benefits from the activity of the other. This relationship can be either external or endosymbiotic, whereby an organism resides in another’s body or cells, such as the photosynthetic algae harbored in corals.

An example of this that would be well-known to divers is the relationship between sea anemones and anemonefish. The sting of the anemone’s tentacles provides the fish with protection from predators, while the fish defends the anemone against butterflyfish, which feed on anemones.

Meanwhile, but less obviously, waste ammonia from the fish also nourishes the algae residing in the anemone’s tentacles. Since these algae are also in an endosymbiotic relationship with their host coral—they provide photosynthates and get protection and supply of inorganic nutrients in return—it goes to show that there is often more than one aspect to symbiosis, and many relationships can exist concurrently, often in an intermeshed manner.


Commensalism is a symbiotic state in which one organism benefits from the other without affecting it. The term stems from the Latin commensalis, meaning “sharing a table”. In biology, it describes the relation between individuals of two species in which one species obtains food or other benefits from the other without either harming or benefiting the latter. The benefiting species, the commensal, may obtain nutrients, shelter, support or locomotion from the host species, which is usually significantly larger and mostly unaffected. However, commensalism is often a tricky case to argue. As any close interaction between two organisms is unlikely to be completely neutral for either party, relationships identified as commensal may in fact be subtly mutualistic or parasitic.

Phoresy—an association in which one animal is attached to another exclusively for transport—may hinder the host by making its movements more difficult or cause it to expend extra energy. Barnacles on whales, remoras attached to sharks or algae growing on turtles’ shells are examples of such commensal relationships. Other forms of commensal relationships that are less direct include one organism using another for housing (inquilinism) or using something another created, after its death (metabiosis) such as is the case with hermit crabs, which use shells from gastropods to protect their bodies, as shown on the image on the left.

parrotfish with cleaner wrasse
Parrotfish with cleaner wrasse


An interaction between species wherein one, the parasite, benefits at the expense of the other and will often live in or on its host for an extended period of time. Although parasitism applies unambiguously to many cases, it is part of a continuum of types of interactions between species; and in many cases, it is difficult to demonstrate that the host is harmed. Parasites that live on the outside of the host, either on the skin or the outgrowths of the skin, are called ectoparasites (e.g. lice, fleas and some mites) while those that live inside the host are called endoparasites (e.g. parasitic worms).


How do symbiotic relationships affect the ecosystem? An ecosystem is the interaction between living and non-living things in a particular environment. Ecosystems can be likened to complex marketplaces through which resources flow, are traded and passed on between entities in a multidimensional web.

Whether the currency of this market should rather be viewed as energy, information or entropy, is the subject of a never-ending scientific discourse. Suffice it to say, all organisms and parts within this place are interacting all the time, and adjustments must occur if the organism is to survive. Since cooperation is fundamental to achieve integral participation, good results and ultimately, survival, symbiosis has been key in natural evolution.

Competition, predation and symbiosis are three processes that organise ecological communities. All these interactions vary in strength and duration from intimate, long-lived symbioses to brief, weak interactions through intermediaries.

While competition and predation controls the abundance and diversity of species in a community, symbiosis links species and serves to increase community organization, and with it, complexity and specialization in an ongoing process, until resources get limited or a structural collapse occurs, i.e., due to catastrophic events. ■


Press releases from Divers Alert Network (DAN)