Clownfish (family: Pomacentridae), also known as anemonefish, are some of the most iconic marine fish in the world. They are highly popular among SCUBA divers due to their striking colour combinations of white, orange, yellow and brown. There are a total of 28 species of clownfish, which are all very similar in their habits. Perhaps the most renowned of these is the Common Clownfish (Amphiprion ocellaris), which was popularised in the Walt Disney movie ‘Finding Nemo’.
Clownfish are found in tropical and subtropical areas of the Pacific and Indian Oceans. The greatest diversity of clownfish is found close to Papua New Guinea, although the Great Barrier Reef in Australia is also known for a number of unique variants. Within their range, clownfish are restricted to shallow waters due to their fascinating relationship with only a handful of specific sea-anemone species upon which they depend. While most fish are repelled by the anemones’ poisonous stings, the clownfish acquires immunity to this defence, and is therefore able to use it for its own protection against predators. In return, clownfish keep their host anemone in a healthy state, and prevent them from being attacked by angelfishes and sea turtles, (which are also immune to the sea anemone’s stinging tentacles).
As a larva develops, it begins to use chemical signals present in the water to detect a suitable anemone to be used as its new home. These signals permit the young fish to select the right type of anemone, and ensure that it is different from its place of birth to avoid inbreeding. The period between hatching and seeking a new anemone ‘home’ is notably short (around 8–12 days), meaning that they do not disperse very far from their parents’ anemone.
The most significant impacts of climate change on clownfish are those affecting their coral reef habitat, and water temperature and chemistry. Such changes may affect clownfish in a number of ways.
Loss of habitat as coral reefs decline:
At today’s level of 387 ppm CO2, coral reefs are seriously declining and time-lagged effects will result in their continued demise. If CO2 levels are allowed to reach 450 ppm (due to occur by 2030–2040 at the current rates of increase), reefs will be in rapid and terminal decline world-wide from multiple synergies arising from mass bleaching, ocean acidification, and other environmental impacts (more details are available in the staghorn coral account).
Clownfish depend on sea-anemones, which are most frequently found on coral reefs. Reef-dependent species such as the clownfish will undoubtedly be affected by the decline in coral reefs. In 1998, one of the most severe global coral bleaching events in recorded history led to the complete disappearance of several sea-anemone species used by clownfish in the corals reefs around Sesoko Island, Japan, causing local population declines.
Disruption of navigation as ocean acidity rises:
Increases in ocean acidity levels have been shown to affect clownfish’s ability to detect the chemical signals necessary for navigating to and locating their anemone homes. This effect is known to be particularly severe in juvenile fish. Fish that are unable to locate a suitable hiding place are at a much higher risk of predation, and are much less likely to find other clownfish with which to mate.
Juvenile fish that are unable to locate new anemones to inhabit also have a much greater chance of returning to their original place of birth. While such individuals may be considered fortunate for at least finding protection, the likelihood of inbreeding in these fish is greatly increased. Adult clownfish, although less susceptible to the loss of chemical cues, can still become confused and lost when venturing away from their host anemone. An extended period away from their host commonly leads to the loss of their immunity to their anemones’ poison, and a much greater risk of predation. In order to regain this immunity, the fish must perform an elaborate ‘dance’, which may last up to several hours and further increases the chances of predation.
Ocean warming changes development rates:
All fish are ‘cold-blooded’ or ectothermic, which means that all aspects of their life-history are highly influenced by the surrounding water temperature. As ocean temperatures continue to increase we may expect to see a number of effects on clownfish. Juvenile clownfish have been shown to develop faster as water temperatures increase (assuming sufficient food is available). There are potential immediate benefits to individuals such as faster reproductive turnover, but more rapid growth will generally mean that individuals disperse shorter distances from their parents’ anemone before their development stage triggers the instinct to find their own anemone. The resulting decrease in dispersal distances means greater competition for local dwelling places, greater chance of predation and increased inbreeding.
A further threat to clownfish associated with warming ocean temperatures relates to their reproductive behaviour. Clownfish (as with many other fish species) are known to only reproduce within a very small temperature range. It follows, therefore, that an increase in temperature could discourage clownfish from breeding. High temperatures have also been shown to cause eggs to perish. Either of these outcomes, or a combination of the two, could have disastrous consequences for clownfish. In summary, a combination of habitat loss, disruption of their olfactory senses and direct effects to their physiology makes clownfish particularly vulnerable to the effects of climate change.
|Clownfish and Nemo Fish|
As ocean temperatures warm, clownfish may be forced to shift their ranges polewards to find cooler water. However, clownfish larvae travel only short distances from their parents’ anemone, and increased development rates caused by warming, coupled with the need for parallel dispersal in interdependent sea-anemone species, is likely to limit dispersal further.
To avoid the negative effects of warming on reproduction and egg survival, clownfish could potentially adapt behaviourally to time such events during cooler periods or seasons. However, the potential for such changes in clownfish are, as yet, unexplored. The problems caused to clownfish by habitat loss will require movement to new areas of suitable habitat.
Obviously such areas will be limited in number and, once again owing to the poor mobility of the species, will only be accessible if they are relatively close by. One species of clownfish has recently been shown to use soft corals as an alternative habitat, something only ever previously witnessed in captivity. Whether such behaviour could be adopted by other species of clownfish, and, if so, whether it would serve to alleviate pressure on clownfish, is currently unclear.