The oceans house a wide array of marine symbioses, where different organisms depend on one another for survival - although both organisms don't always benefit from the arrangement. Some examples of apparently mutualistic symbioses include the relationship between zooxanthellae dinoflagellate algae and the range of organisms that host them, including hard and soft corals, giant clams, and anemones. The corals and clams are fed extra sugars created by algal photosynthesis that leak to the hosts, while the algae source nutrients from the animals' waste products, and have a relatively secure home. Unfortunately, these symbioses can break down due to environmental stressors like climate change, causing bleaching.

A similar form of symbiosis prevails at hydrothermal vents, where a range of animals including massive Riftia tube worms and fuzzy-clawed crabs host chemosynthetic bacteria. Instead of using solar energy to photosynthesize, these bacteria create sugars using chemicals like hydrogen sulfide. Extra sugars are leaked to their hosts, or the animals eat the bacteria-for instance Yeti Crabs farm and then eat bacteria off their hairy arms.

Another interesting food-acquisition symbiosis exists between Osedax annelid worms and Oceanospirillales bacteria. The worms have root-like structures that they use to burrow into bones of dead animals (like whales) on the seafloor, as well specialized tissues that house symbiotic bacteria. The bacteria in turn degrade the bone material. By living inside the worm tissues, the bacteria thus gain access to fresh bone material more readily, and feed both the worms and themselves as they snarf it down.

Other symbioses result in mutual protection from predators. Several species of low-vision snapping shrimp dig tunnels in sandy sediment, which they share with particular species of goby fish that cannot burrow. The fish look out for predators, and warn the shrimp to retreat into the shared burrow by flicking their tails against the shrimp's antennae. Some anemones have a symbiotic relationship with anemonefish (AKA clownfish, or "Nemo"). The fish have evolved a special protective mucus layer that allows them to hide in the stinging anemone tentacles, gaining protection from would-be predators without a sting-guard. The anemonefish chases away would-be anemone predators like butterflyfish.

Not all symbiotic relationships are just peaches and cream, however. Many organisms are parasitic, meaning that they benefit from living on or in another organism, but that organism does not gain from the arrangement. Alexis wrote a while back about fish lice, for example, which can eventually kill small fish if they become too numerous. Corals host not only their algal zooxanthellae symbionts, but often a host of other organisms - sometimes to their detriment. Vermetid snails secrete tubes, and can attach to hard corals and impede their growth, perhaps by casting annoying or suffocating mucus nets over their surfaces. Interestingly, some corals also host "guard crabs" that appear to protect vermetid-hosting corals from the negative effects of the snails - possibly by inadvertently (or purposefully) removing the snail mucus nets from coral surfaces!

What are your favorite symbiotic marine relationships? Do both partners benefit from the interaction?

References

Goffredi, S. K., Orphan, V. J., Rouse, G. W., Jahnke, L., Embaye, T., Turk, K., ... & Vrijenhoek, R. C. Evolutionary innovation: a bone‐eating marine symbiosis. Environmental Microbiology 7 1369-1378 (2005)

Karplus, I., & Szlep, R. Goby-shrimp partner specificity. I. Distribution in the northern Red Sea and partner specificity. Journal of Experimental Marine Biology and Ecology 51 1-19 (1981)

Porat, D., & Chadwick-Furman, N. E. Effects of anemonefish on giant sea anemones: expansion behavior, growth, and survival. Hydrobiologia 530 513-520 (2004)

Preston, J. L. Communication systems and social interactions in a goby-shrimp symbiosis. Animal Behaviour 26 791-802 (1978)

Shima, J. S., Osenberg, C. W., & Stier, A. C. The vermetid gastropod Dendropoma maximum reduces coral growth and survival. Biology Letters 6 815-818 (2010)

Stier, A. C., McKeon, C. S., Osenberg, C. W., & Shima, J. S. Guard crabs alleviate deleterious effects of vermetid snails on a branching coral. Coral Reefs 29 1019-1022 (2010)

Thurber, A. R., Jones, W. J., & Schnabel, K. Dancing for food in the deep sea: bacterial farming by a new species of yeti crab. PLoS ONE 6 e26243 (2011)