Nature and Science
Author: Fredy Brauchli
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Symbiosis and Commensalism
Relationships between different organisms are often the secret to survival in the reef. Such dependency is also essential for the smallest ocean dwellers.
The term “symbiosis” is interpreted today as partnerships of different organisms that are mutually beneficial. In contrast, relationships of two different organisms, where one of the partners benefits without harming the other one, is called “commensalisms” or “probiosis”. The most familiar form of symbiosis in the ocean world is the one of sea anemones and fish or other animals.
Survival Strategy of Reef-Building Corals
A well known fact, coral reefs consist of huge accumulations of coral polyps. In the course of centuries they have created the gigantic structures in the middle of the oceans. This was only possible thanks to a very sophisticated form of symbiosis. The coral polyps have a symbiotic relationship with the single-celled algae called Zooxanthellae, which they store in their inner most layer. They function as solar cells. They convert the energy of the sun by means of photosynthesis into chemical energy and produce nutrients like sugar, fat, and oxygen for the coral polyps. The oxygen is needed by the coral animals to breathe. The carbon monoxide necessary as well exists in soluble form in the marine waters. The algae need nitrogen and phosphate to be able to carry out the photosynthesis. Both are available from metabolic processes of the coral polyps. Thus both organisms benefit from the symbiosis partner.
Life Threatening Danger for the Symbiosis Partners
This form of symbiosis bears one important disadvantage. The corals depend on the algae (Zooxanthellae). If the temperature of the ocean rises above average - i.e. due to meteorological phenomenon – it can present a life threatening situation for the corals. The algae stop carrying out their photosynthesis due to the increase in temperature, which in turn presents a life threatening danger for the coral polyps because the Zooxanthellae are on the verge of dying. The fatally affected reefs can bleach out and ultimately be killed. The same is true for clams (i.e. Tridacna) and anemones, which have stored the same algae as the corals.
Solar-Powered Sea Slugs
The Anemone Slug uses solar energy in an almost futuristic way. It feeds on corals, which have stored single-cell algae (Zooxanthellae). These algae are not digested but ingested in the form of complete cells by the slug’s body. How is works is currently unknown. The algae are stored inside the body appendages of the slug where they continue to live and carry on their photosynthesis. The slug itself benefits just like the coral polyps from the metabolites. This process is so efficient that the slug does not need to ingest more nutrients for quite some time but lives off the “solar engine” of its algae symbiosis partner.
Symbiosis Between Hermit Crab and Sea Anemone
One form of symbiosis especially attractive for divers and snorkelers is the one between the hermit crab and the sea anemone. Hermit crabs use empty shells of snails as protection and carry one or more stinging sea anemones on top of their shell to protect themselves from enemies. On the other hand, the anemone benefits from the remaining nutriments of the hermit crab.
Shrimps as Cleaners of Sea Cucumbers, Clams, and Corals
Certain crab species not living in cleaning stations to clean fish live instead on sea cucumbers, clams, anemones and corals. The shrimps either benefit from the camouflage or from the protection the stinging tentacles of their host will provide. In return they offer their services as cleaners. As such, they clean anemones and corals from parasites like sand and clam particles.