What is symbiosis?

Symbiosis is the interaction between two dissimilar organisms living in close physical association.

The relationship between hosts and associates is discussed as observations are made of multiple examples of organisms living in close association with one another. Video courtesy of NOAA Ocean Exploration, 2017 Laulima O Ka Moana. Download larger version (mp4, 186.9 MB).

Symbiotic relationships are an important component of life in the ocean. In such relationships, plants or animals of different species may be dependent on one another for survival. They may share habitats or lifestyles or interact in a way in which they benefit from the presence of another organism.

We often refer to animals living in tandem as "associates." The relationship between associates and their hosts can be described as mutualistic, commensal, or parasitic. In a mutualistic relationship, both animals benefit from living together. Commensal organisms cause no harm to their hosts, but receive some benefit from living with them. Parasites actually feed off their host organism, thus causing harm to the host.

Although there are many ways organisms interact with one another, most symbioses involve clever means by which to obtain food or protection. For example, ophiuroids (brittle stars) are often found living within the branches of corals, using their hosts to get further up off the seafloor into the water column to feed or for protection. At hydrothermal vents, chemosynthetic bacteria live inside of animals in a mutualistic symbiotic relationship where the animals support the existence of the bacteria and the bacteria provide food to the animals in an environment where light does not penetrate. And different types of marine parasites, including worms, isopods, and copepods, infect a variety of host species, including crabs and fishes.

Deep-sea symbioses are poorly understood and less well documented than the symbiotic relationships frequently encountered in shallower habitats.