Delve into the unique and complex biology of the clownfish, arguably the world’s most famous fish says Kush Patel
Straddling the equator the Maldives achipelago consists of more than 1,000 islands ranging from Malé, the densely populated island capital, to tiny sandbars that are drowned at high tide.
Born of volcanic eruptions that have long ceased and settled as 26 atolls, the Maldives is the epitome of paradise, with flour-soft sand, aquamarine waters and palm trees whispering in the breeze. But its true magic lies hidden under the water’s surface, where gardens of coral are home to a multitude of jaw-droppingly vibrant and beautiful species.
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I’m here to discover this underwater realm. Rey Gelera, resident marine biologist on Anantara Kihavah, guides me over the reef. As we dive deeper, he points out a small group of familiar, tangerine-hued fish decorated with white vertical bands.
They are, of course, clownfish, immortalised in the movie Finding Nemo, which tells the story of a male ocellaris clownfish (Amphiprion ocellaris) who has lost his partner to a barracuda (one of the world’s deadliest fish), and his son, Nemo, to the aquarium trade.
What are clownfish and how many species of them are there?
There are 28 species of clownfish, also called anemonefish, which live in the warm waters of the Indian and Pacific Oceans, particularly in coral reef habitats. These colourful creatures have become instantly recognisable and adored, but behind their flamboyant costume is a unique lifestyle, honed by evolution, complex social dynamics and mind-bending genetics.
I watch an individual prancing within a venomous anemone, which is home to these fish. This is the Maldivian (or black-finned) clownfish, endemic to the waters around India, Sri Lanka and the Maldives.
What do clownfish look like?
A single white stripe adorns each cheek, breaking up an uniform orange that blends into black around the lower fins. This anemone is hosting four other fish, each a slightly different size. The largest is about 8cm long and female.
How to clownfish mate and reproduce?
The rest are male and their size relates to their social stature in that particular group. Only the largest male gets to breed with the female; the other non-related males must wait on the sidelines.
“The dominant breeding pair puts pressure on the other fish with the threat of eviction,” says Peter Buston, a professor of marine and evolutionary ecology. “This ‘bullying’ actually maintains the size and
sex of the other males by controlling their gene expression.”
How and why clown fish are able to change sex
Clownfish are renowned for their intriguing social dynamics. They are protandrous hermaphrodites, which means they are born male and stay male “until the female dies,” explains Peter. “The dominant male then becomes the female of that group, and each male climbs up a rank – so the second male now becomes the dominant male.”
Recent studies have shown that the sex change in clownfish is preceded by alterations in the brain; the physical switch to becoming a female can take months or even years. Taking account of this science means that Nemo’s father, Marlin, would have been in the process of transitioning into a female during his mission to rescue his kidnapped son.
I’m curious as to why a dominant male would accept other males on his patch, especially as they don’t help to care for the young. As it turns out, the non-breeding males enhance the health of the sea anemone by providing nitrogen from their excrement.
This, in turn, benefits the breeding pair, “so they don’t mind the others,” says Peter. It seems waiting for their turn to breed is the main reason that the non-breeding males stick around. Besides, leaving your anemone home to find another is risky in a reef packed with predators.
hey are protandrous hermaphrodites, which means they are born male and stay male “until the female dies,” explains Peter. “The dominant male then becomes the female of that group, and each male climbs up a rank – so the second male now becomes the dominant male.”
The reef I’m diving is certainly chock-a-block with hungry mouths – triggerfish with warrior-like markings, enormous groupers, moray eels that slither into every crevice, and lots of sharks. The presence of predators is a sign of a healthy ecosystem, indicating plenty of prey and favourable habitat.
How pollution is affecting the ocean
However, despite this abundance of life, there are signs that not all is well. Among the shoals of fish, a plastic bag lingers. On the shore, the high tide has dumped a menagerie of litter, including rubber sandals and an old chair.
Hugo Tagholm, executive director of Oceana UK, elaborates on the scale of the plastic pollution problem: “Currently, the equivalent of a garbage truck of plastic enters our ocean every minute. By 2030, this will increase to two trucks every minute,” he says. Hugo stresses the contribution of discarded, or ghost, fishing gear, one of the dominant causes of marine pollution. “This ghost gear has a profound impact on marine life – everything from birds to whales gets caught in it or swallows it,” he says.
The Olive Ridley Turtle Project has found 601 turtles entangled in 732 ghost nets in the Maldives. Along with the International Pole And Line Foundation, it is actively encouraging local fishermen to pick up this discarded gear to protect wildlife. Other organisations, such as The Ocean Cleanup, are collecting rubbish from ocean garbage patches and preventing more from entering our oceans from rivers, while larger movements, such as The Global Plastic Treaty, aim to create a circular economy where plastics are reused and recycled.
Chemical pollution is also a concern. Scientists have found that certain chemicals, including those from plastic bottles, cause the feminisation of clownfish. This is a serious issue for a species where gender plays a crucial role in its natural history and social life.
The current draws me closer to another anemone, its arms waving ominously. These appendages house nematocysts – paralysing, toxin-laden harpoons used to catch passing prey, which is then guided into its mouth. It is hard to comprehend how clownfish can live with such a species, but evolution has endowed them with a solution: a coat of mucus that develops over time, after repeated contact with their host. Shielded from the sting, they appear to revel in the fronds’ protective embrace.
Clownfish habitat
Each clownfish species has co-evolved to live within one or more of 10 different anemones for almost their entire lives. In fact, this mutualistic relationship is one of the key triggers for the fish’s diversity.
Different anemones live in different habitats, so clownfish have adapted and diversified accordingly. In a wonderful example of symbiosis, the fish provide their hosts with cleaning services and protection, chasing off anemone predators such as butterflyfish, whose long mouthparts can bypass the plant’s sting. Nitrogen from the fish’s excrement also enables the algae within the anemone’s tissues to carry out photosynthesis, which nourishes both algae and anemone. In return, the plant provides a safe home for both algae and clownfish.
But this mutualistic lifestyle features a rather macabre intruder. A type of crustacean called an isopod can sometimes be found inside the fish. It lodges itself onto the fish’s tongue, feeding on its blood. Under most circumstances, clownfish can still feed and survive, but in warmer waters or stressful environments their survival is compromised.
What do clownfish eat?
As omnivores, clownfish enjoy a varied diet consisting of algae, zooplankton, worms and small crustaceans
What do clown fish eggs look like?
At another anemone, I come across Clark’s clownfish, emblazoned in a dark black suit with bold white stripes and fringed in honeyed orange. Sheltered by a small overhang, I notice tiny specks undulating in the current. As I get closer, they morph into pairs of eyes, cocooned in transparent capsules – they are clownfish eggs!
The male gives them plenty of attention, checking over each individual, fanning them and keeping them clean. Male clownfish provide 90 per cent of parental care, while the females concentrate on defending the territory. Peter Buston recently discovered that when the eggs hatch at night, “the breeding pair provides safe passage for the vulnerable fry into the open ocean by nipping at the anemone’s nematocysts”. This usually happens during a full or new moon, when the strong current sweeps young away.
As I scrutinise these tiny fish living out their lives in the underwater world, a shadow drifts over us like a plane coming in to land. A seven-foot nurse shark oscillates from side to side, making me flinch. Its size is both thrilling and unnerving. On its belly are two remoras, clamped on via suckers on the tops of their heads. Twisting around, I celebrate the moment with my wife, who is as awestruck as I am.
Having observed one of the reef’s tiniest residents and one of its largest, it seems to me that the Maldives has it all. We encounter hawksbill turtles plucking algae off the coral, angelfish parading their finery and neon parrotfish munching on coral (they produce so much sand in their excrement that 85 per cent of Maldivian beaches are effectively parrotfish poo).
What threats do clownfish face?
Fish are above, below, to the left and right of us, feeding, schooling and chasing one another. They are also quite noisy. Listening intently, I can make out the various pops, squeaks and whistles of fish communication. Indeed, so significant is the sound that scientists are using this ‘soundscaping’ to judge and monitor the health of coral reefs. The more sound there is, the greater the biodiversity. This reef is protected and flourishing, reflected in the din around me.
Elsewhere in the world, clownfish are faring less well. Finding Nemo resulted in an increase in demand from the aquarium trade for these species, and scientists simultaneously reported a reduction in wild populations, such as on the Great Barrier Reef. Ironically, the conservation message from the film – that clownfish don’t like living in tanks – was missed by some. Clownfish are one of the most frequent species taken from the wild, making up 43 per cent of the global marine ornamental trade. “I hardly see clownfish when I swim on reefs close to Malé,” says my guide Rey.
Coral bleaching is a threat, too, with mass coral-bleaching events increasing in frequency due to climate change. Bleached anemones affect the metabolism, growth, behaviour and reproduction of their resident fish, frequently resulting in local extinctions. Innovative coral restoration programmes are in progress – including one run by Rey – which aim to expand reefs by transplanting live coral in the hope it will form new colonies. Scientists are also identifying and propagating heat-tolerant coral species, stabilising reef rubble (fragments of reef on the ocean floor that still provide habitat) to promote reef recovery, and even creating clouds to reduce sunlight-induced heat damage.
These strategies can buy time, but we must do more. The need to save Nemo and his friends is now more urgent than ever. The future of this colourful, iconic and biologically fascinating little fish, and its ocean home, is firmly within our hands.
Kush Patel has a deep interest in the natural world and aims, through his wildlife and travel writing, to inspire others to preserve our rapidly diminishing wilderness.