What is symbiosis?

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

A pair of commensal amphipods living on a sponge stalk. There is also a commensal ophiuroid at the top of the photo.

A pair of commensal amphipods living on a sponge stalk. There is also a commensal ophiuroid at the top of the photo. Image courtesy of the NOAA Office of Ocean Exploration and Research, 2016 Deepwater Exploration of the Marianas. Download image (jpg, 96 KB).

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 specific way to 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 ways 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 relative to symbiotic relationships frequently encountered in shallower habitats.