Despite Nemo getting all the fame, there are actually 28 different species of anemonefish. These 28 fish associate with only 10 out of a possible 1000 species of anemones, in a relationship that is referred to as a symbiosis. The anemonefish gain protection and shelter from the anemone and in return offer additional nutrients and increased oxygenation. But why are only 10 out 1000 species are special enough to act as hosts for these charismatic fishy friends?
Scientists believe there is a toxic key that underpins this relationship and aids anemonefish in finding their ideal symbiotic partner. Anemones utilise a range of different toxins for both feeding and defence however, different species have different toxicity levels.
Cryptodendrum adhaesivum has the highest toxicity levels of the 10 host anemones and is only host to 1 of the 28 species of anemonefish. Similarly, Heteractis malu while having the lowest toxicity of host anemones also only has one anemonefish species associated with it. Oppositely, Entacamea quadricolor, Heteractis magnifica and Heteractis crispa which all exhibit middle range toxicities have the highest number of anemonefish associates (13, 14, 12 respectively).
So why is a middle range toxicity preferred by so many anemonefish? Wouldn't a higher toxicity be more beneficial? Anemonefish are not born with a natural resilience to the toxic protection offered by anemones; it is something they develop overtime. The more toxic an anemone is the more energy and time needed for the anemonefish to develop and immunity to it. Amphiprion clarkii is the only anemonefish to associate with C. adhaesivum (the most toxic host) and is suggested to have a thicker mucus layer than other species; enabling them to withstand the higher toxcity. However, for other species it is too costly for them to try and withstand this level of toxin and therefore a middle range toxcity is prefered.
When threatened H. malu (the least toxic host) will retreat under the sand and burry itself. This is not ideal for the anemonefish it is supposed to be protecting! However only juveniles are found associated with this anemone as they are unable to compete effectively for more prized anemones (those in the optimal range). Once they mature they move up to better and more toxic anemones who offer better protection.
This indicates that the species anemonefish do not interact with, either have a toxicity above the optimal range or have toxicity so low that they have other defence mechanisms that don’t support a symbiosis with anemonefish.
Reference:
Nedosyko, A. M., J. E. Young, J. W. Edwards, and K. Burke da Silva. 2014. Searching for a toxic key to unlock the mystery of anemonefish and anemone symbiosis. PLoS One 9:e98449.
Original Post: http://www.atollvolunteers.com/toxic-key-anemone-popularity/
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