The greatest threat to the common clownfish is global climate change, which threatens this species through a combination of habitat loss, disruption of its senses and direct effects on its behaviour (8).
The expected rise in ocean temperature associated with climate change threatens coral reefs by increasing the frequency of bleaching events, in which the stressed coral expels its symbiotic algae known as ‘zooxanthellae’, often resulting in the death of the coral. Climate change may also lead to more frequent severe storms, which can damage reefs. Rising carbon dioxide levels may also make the ocean increasingly acidic. Such stresses can make corals more susceptible to disease, parasites and predators (9) (10) (11). The common clownish may adapt to this loss of habitat by moving to new reefs, but its ability to do so is limited by the great rate at which its habitat is being degraded and by its general lack of mobility (8).
Increases in ocean acidity levels have also been shown to affect clownfishes’ ability to detect the chemical signals necessary for navigating to anemones. Clownfish that are unable to locate a hiding place are at a much higher risk of predation, and are much less likely to find other clownfish with which to mate. Furthermore, an extended period away from their host commonly leads to the loss of their immunity to the anemones’ poison, which also leads to a greater risk of predation. This effect is known to be particularly severe in juvenile clownfish (8).
A further threat to clownfish associated with climate change relates to their reproductive behaviour. Clownfish only reproduce within a very small temperature range. Therefore, an increase in temperature could discourage clownfish from breeding. High temperatures have also been shown to cause eggs to perish and to cause juvenile clownfish to develop faster. Individuals will therefore reproduce earlier, meaning they will disperse only a short distance from their parents’ anemone before their development triggers them to find their own anemone. This could result in greater competition for host anemones, greater chance of predation and increased inbreeding (8).
With an estimated 20 percent of the world’s coral reefs already destroyed, the common clownfish also faces many of the other threats that are affecting coral reef communitites globally. Worldwide, there is increasing pressure on coastal resources resulting from human population growth and development. There has been a significant increase in domestic and agricultural waste in the oceans, poor land-use practices that result in an increase in sediment running onto the reefs, and over-fishing, which can have ‘knock-on’ effects on the reef (9) (10) (11).