March 1, 2025, 9:28 am | Read time: 4 minutes
The symbiosis between clownfish and sea anemones is one of the best-known examples of a marine community. Until now, it was known that the anemones offered the fish protection and that the fish defended them in return. However, a study now shows that anemonefish, such as the yellowtail clownfish, also bring food to their hosts.
Symbioses — partnerships between species with mutual benefits for both — are widespread in nature. Take leafcutter ants and aphids, for example. The ant defends ‘its’ aphid, which in return provides sweet nectar. Crocodiles and wading birds also have a similar relationship. The bird nests near the crocodile, which deters predators from the nest. In return, the crocodile occasionally receives a fish from the bird’s catch. If you look at the sea, the most famous symbiosis between clownfish and anemones has probably been there since the Disney animated film Finding Nemo.
Clownfish Make Up for a Lack of Resources for Their Anemone
It was previously known that anemones offer fish a safe space, for which they then defend the flower animal accordingly. However, a study reveals for the first time that Clark’s anemonefish, also known as the yellowtail clownfish, actively shares food with its host, the bubble-tip anemone (Entacmaea quadricolor), which in turn promotes the growth of the anemones.
Anemonefish are known to defend their host anemones, aerate them, and remove debris. Previous studies have suggested that anemones may also absorb nutrients from the fish’s waste. However, until now, there has been no evidence that the fish specifically feed their anemones.
Researchers at Osaka Metropolitan University in Japan investigated whether anemones accept food from fish, the impact of food size and type on their behavior, the anemone’s response to hunger, and whether additional food contributes to the growth of the anemone.
Just “Food Cutters” or Active Feeding?
Previous theories have suggested that anemonefish may bring larger pieces of food to the anemone in order to consume them themselves later or use the anemone as a “food cutter.” However, there was a lack of clear evidence. This study aimed to determine whether the fish intentionally provide food for their host anemones.
From 2020 to 2021, researchers in Japan conducted field studies to investigate the feeding behavior of anemonefish systematically. Several experiments were conducted for this purpose. Anemonefish were offered food (e.g., krill), which was attached to a string. It was then documented whether the food was brought to the anemone and consumed by it.
In the next experiment, the clownfish were given krill in various sizes from 3 to 50 millimeters to determine whether they ate food of a certain size themselves or passed it on. The researchers then investigated whether a food preference could be determined. Fish, squid, mussels, algae, and sea urchin shells were tested to find out which food was passed on to the anemone.
To test the starvation hypothesis, the scientists looked at whether the anemonefish gave up more food as they became more satiated. In a final experiment, the anemones were fed differently. Food was either given to the clownfish, fed directly to the anemones, or there was no feeding (control group). During these tests, the growth of the anemones was measured for three months.
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Clownfish Give Anemones the Biggest Bites
The study’s findings, published in the journal Scientific Reports, conclusively showed that Clark’s anemonefish actively share food with their symbiotic anemones. The various experiments revealed exciting findings:
- Food intake by anemones: In 17 out of 26 cases, food was taken up by the anemones. In the other cases, food intake was disrupted or withdrawn by the fish.
- Influence of food size: Small pieces of krill (under 7 mm) were eaten by the fish themselves. Larger pieces (over 7 mm) were almost exclusively passed on to the anemones.
- Food preference: The fish preferred to give animal food to their anemones. At the same time, they either ignored algae or sponges or ate them themselves.
- Influence of hunger: The fuller the anemonefish were, the more frequently they passed food on to the anemone.
- Growth effect: Anemones that were fed indirectly by the fish grew significantly faster than the control group — their growth was comparable to anemones that were fed directly.
This study provides the first clear evidence that anemonefish supply their “dwelling” with food. This indicates that the fish actively contribute to the survival of the anemone, potentially as an evolutionary strategy. This is because larger anemones can provide more protection. Alternatively, the anemonefish might ‘cultivate’ the anemones in this manner, effectively creating a suitable habitat for themselves.
However, there are still some unanswered questions. It is possible that this behavior only occurs under certain environmental conditions, such as the threat of malnutrition due to food shortages. It is also unclear whether the fish consciously promote growth or whether it is a side effect of their diet. 1