Any organism that spends a portion or all of its life cycle intimately associated with another species is a symbiont, and that association is called symbiosis.
Symbiosis is a very general term and by using it, no intent is meant to imply the occurrence of unilateral or bilateral metabolic dependency. Language is dynamic and unfortunately most people use the term to describe a mutual benefit – usually not the case! Symbiotic associations are incredibly common place – no organism lives alone. Take for example us; humans are made up of about 10 trillion cells with only about a tenth of these being human! In nature there are many levels of interaction and interdependence between pairs of species. Perhaps best known because it is so evident is the predator-prey relationship. A tuna devours a mackerel. Straight forward and without twists. Tuna is fed and mackerel is food. Symbiotic relationships, although much more prevalent, are also much more complex and this complexity makes them harder to classify. see, symbionts clinging and moving around on their surfaces. Once they are pointed-out , you will see how obvious they truly are.
Science in its inexorable march towards order, breaks things down into neat categories and as usual because of bias, insufficient knowledge, and the fact that the most intimate associations change back and forth with time, over burden us with an excess of silly categories. For this reason I recognize only 4 categories:
The smaller (phoront) is mechanically carried or held by the larger host. No real dependencies. A remora hitching a ride on a shark. Also in this category are refugees where survival is aided by shelter and a great example is the pearl fish which spends the daylight hours in the gut ( which it enters through the anus) of a sea cucumber and then exits under the cover of darkness to forage on the reef.
Both the host and the commensal eat as the same table. In other words, the close spatial proximity of the partners permits the commensal to feed on substances captured by the host. Host is said to be unaffected. An example would be that same remora that was involved as a phoresis now dislodging, because the shark made a messy kill, and is feeding on scraps. This is a perfect example of how poorly associations “fit” into artificial categories. Each association must be graded over the long run and evaluations made recognizing which association is most important.
In this relationship the parasite, usually the smaller of the two, gets its nutrition directly from the host’s body. The parasite is metabolically dependent on the host and the host is said to be harmed. However long term parasitic relationships typically do little harm to their hosts and usually the parasites are highly specific to not only just one host species but often a particularly small site of the host. Parasites can be full or part timers – tapeworms and mosquitoes.
In this most highly evolved of all the interrelationships, both organisms benefit. In many cases benefits received from the other organism is essential to their survival. A prime example is the association between hard corals and their zooxanthellae, which are actually a type of animal which can photosynthesize. Niether the corals or the zooxanthellae would survive very long if separated.
Symbiosis is a fascinating subject and there is little doubt we have yet to discover many associations among reef creatures we as divers see on every dive. Keep an eye out.
About the Author: Dr. Tom Byrnes has been running dive charters in Grand Cayman for 30 years making him one of the most knowledgeable captains on the water today. Cayman Marine Lab started as a strictly academic endeavor teaching college students tropical marine biology but as it turns out the hunger for a marine biologist to show and teach tourist divers the secrets of the reefs and various other marine habitats was overwhelming. CML is the adventure dive operation in that we go out of our way to bring you to the least dived reefs Cayman has to offer…and what a difference it makes.
This video is about Symbiosys