A new study on reef-dwelling clownfishes has revealed that their vibrant colours and patterns may have evolved to warn potential predators of their venomous anemone hosts.

The mutualistic symbiotic relationship between clownfish and anemones has long been known, with each providing a number of benefits to the other.

Immune to the potent effects of the stinging nematocysts in the anemone tentacles, the clownfish is afforded protection from predators, whilst at the same time the anemone provides leftover food scraps to the clownfish from the prey it catches. In turn the anemone absorbs nutrients from the waste the clownfish produces, aiding in growth and regeneration. The continual movement of the clownfish amongst the anemone increases water circulation around it, which allows for metabolic benefits and increased growth.

Although clownfish are famed for their vibrant orange and white stripes, until now, no-one has really looked into a possible link between their markings and the toxicity of the anemones they call home. The in-depth study, published in the Journal of Evolutionary Biology, collates data from 27 species of clownfish and 10 known species of host anemone. They looked for associations between certain fish colour/pattern traits and the extent of protection offered by the anemone, including tentacle length and degree of toxicity.

Interestingly, they discovered that clownfish with fewer stripes occupied a smaller number of different anemone species, and were inclined to choose hosts with shorter tentacles. Fish with additional stripes preferred to take up residence in a greater variety of anemone species, and those generally had longer tentacles. At the same time, they found that the anemones with shorter tentacles tended to possess more potent nematocysts, all of which suggests a complex evolutionary relationship between clownfish stripes and anemone type/toxicity.

It has long been recognised that vivid, aposematic colouration and patterns on an animal can be used to warn potential predators of danger before they attack. But for one animal to use its colouration and pattern to warn predators of the toxicity of another species is especially rare, and is perhaps due to the unique arrangement of this remarkable symbiotic relationship. It suggests that the clownfish are alerting predators to the fact they are allied with the venomous anemones using their bright colours/markings.
However, there is still much more to learn about this intriguing relationship: for example, how the striped clownfish patterns appear to the eyes of predators when viewed against the host anemone, and how this affects predatory behaviour.