Clownfish

Phylogeny

The clownfish lineage diverged from that of other living damselfishes around 35 million years ago (mya) during the late Eocene,^[5] and the most recent common ancestor of extant species is dated around 10.5 mya during the Early Miocene.^[10] A 2014 study placed their origin in the waters of the Malay Archipelago.^[11] Clownfishes experienced an increase in species diversification starting around 5 mya,^[5] with two major adaptive radiations; one centred on the Malay Archipelago and a later one in the waters of the western Indian Ocean.^[11] There is genetic evidence for high amounts of interbreeding between species through their evolutionary history.^[12] Clownfish speciation has been linked to their sea anemone hosts, species of which can be found in different habitats and thus drove ecological separation.^[13][10]

There are 28 living species of clownfish and two hybrids, Amphiprion leucokranos (white-bonnet anemonefish) and Amphiprion thiellei (Thielle’s anemonefish).^[14] More species possibly exist in the Pacific, possibly cryptic species.^[15][16] In 1972, Allen listed five major clades or complexes based on morphology; percula, akallopisos, ephippium, polymnus and clarkii with A. biaculeatus being in a clade on its own.^[17] A 2014 study lists two more major clades: Australian and Indian, with A. biaculeatus under the percula clade and Amphiprion chrysopterus and Amphiprion latezonatus being single-species clades.^[12] A 2021 study placed A. chrysopterus and members of the clade polymnus in the Indian clade,^[5] while a 2025 study found A. biaculeatus to be a single-species clade.^[10]

The following cladogram of the 28 clownfish species is based on a 2025 genetic study,^[10] with clade groupings based on a 2014 study.^[12][b]

Colour patterns

Clownfish have distinctive colour patterns consisting of a red, orange, yellow, brown or black background with up to three white vertical bars lined with black. Some species have a horizontal bar along the back while one species, A. ephippium, has no bars at all. Orange, yellow and red colouration is created by xanthophore pigment cells, black and brown by melanophores and the white bars by iridophores. Vertical bar formation starts at the front: species with only one bar have it at the head, those with two at the head and trunk, and those with three at the head, trunk and tail. Variations in the number of vertical bars between individuals of the same species occurs in A. melanopus, A. polymnus and A. clarkii.^[23][27] Numerous colour morph mutations occur particularly in captive clownfish, including melanism, a "misbar" (incomplete vertical bars) morph, and a "golden" morph which is caused by a lack of both melanophores and iridophores. There are also morphs with thickened and merged bars.^[28]

In 2018, Merilata and colleagues found that clownfish with only one or no vertical bars tend to be more specialised for anemone species with greater toxicity and shorter tentacles. Conversely, clownfish species with two or three bars are more likely to use more anemone species within their range, several of which have longer tentacles. The researchers suggest that vertical bars function in camouflage while warning colouration is more important for species that cannot hide in the tentacles of their hosts. This would be a unique case since it warns about another animal, namely the anemone. They found no evidence for the use of the bars in species recognition, noting the geographic and ecological overlap between the similar-looking A. percula and A. ocellaris.^[29] Conversely, Salis and colleagues supported species recognition, finding little overlap between species with the same bar numbers within various clownfish communities.^[23] A 2024 study also found evidence for this function as A. ocellaris can distinguish between individuals of different bar numbers.^[30]

• 
  A. ocellaris with three vertical bars
• 
  A. allardi with two vertical bars
• 
  A. mccullochi with one vertical bar
• 
  A. ephippium with no bars
• 
  A. perideraion with one vertical bar and a horizontal dorsal bar
• 
  A. sandaracinoss with a horizontal dorsal bar

Feeding

Clownfish are omnivorous, and mostly feed on planktonic food such as copepods and larval tunicates. Algae is also an important food source and makes up much of the diet of A. perideraion. Clownfish will also feed on the waste dischanged by the anemone.^[36][37] Feeding takes up most of a clownfish's acivity; around 90 percent in dominant A. chrysopterus specifically. Where predators are less common, clownfish may forage in an area as large as 20 m² (220 sq ft) around their anemone. Otherwise they are restricted to feeding in the water column above their host.^[38] The dominant pair in a clownfish group feed further from the anemone than the smaller subordinates.^[37]

Mutualism

Clownfish have a mutualistic symbiotic relationship with sea anemones.^[19][39] They acclimate themselves to their hosts by touching, nipping and fanning the tentacles over a period of minutes to days.^[40] The main benefit of living among anemones is protection from predators by anemone's stinging tentacles. Straying clownfish retreat to the safety of the tentacles when they encounter a potential threat and they are always near their hosts, with smaller fish rarely leaving the oral disc.^[33][19] Clownfish may even swim into the coelenteron (gastrovascular cavity), though Gerald Allen observes this to be uncommon. Nighttime is spend resting deep among the tentacles.^[19] A less important benefit for clownfish is nourishment from the discharged waste and parasites.^[33][19]

Anemones are less dependent on clownfish than the fish are on them, as is evident as many individuals of host species lack clownfish.^[33] Nevertheless, clownfish contribute to the survival of their hosts by guarding from anemone-eating fish such as the butterflyfish species Chaetodon lunula.^[33][41] Other benefits they provide include the removal of copepod parasites, increased oxygen flow via the rapid movements of the fish's fins and the attraction of additional zooxanthellae by clownfish waste.^[39][42] A 2005 study found that anemones grew and regenerated faster in the presence of clownfish, and attributed this to ammonium from clownfish waste.^[43] Experimental evidence finds that when clownfish are given small and large pieces of food, they will consume the former and give the latter to their anemone.^[44]

A total of ten sea anemone species are used by clownfish as hosts: Radianthus malu, R. crispa, R. magnifica, Stichodactyla mertensii, S. haddoni, S. gigantea, Cryptodendrum adhaesivum, Entacmaea quadricolor, Heteractis aurora and Macrodactyla doreensis. Some clownfish are generalist in their choice of hosts while others are more specialised. A. clarkii is the most generalised species and utilises all ten anemone species, while nine clownfish species — A. frenatus, A. chagosensis, A. pacificus, A. fuscocaudatus, A. latifasciatus, A. mccullochi, A. nigripes, A. sebae, and A. biaculeatus — use just one anemone species respectively. Desirable traits in a host include long tentacles to hide among. In addition, certain anemones like H. aurora and E. quadricolor have tentacles with knob-like structures which provide more surface area for the fish to conceal itself. R. magnifica can provide extra protection as clownfish can hide inside their soft body when it engulfs its tentacles. The potency of venom is also a desirable trait; highly toxic anemone species tend to have smaller tentacles and so provide less shelter but more protection.^[45]

Their ability to avoid being stung is attributed to their mucus coating.^[33] There is evidence that clownfish mucus mimics the molecules or bacteria of anemone mucus and lacks the trigger for the anemone’s nematocysts (stinging barbs). Mucus thickness may also play a role, but the evidence is ambiguous.^[40] There is dispute over how much of the mucus is innate to the clownfish and how much is gained from the anemone during the acclimation period.^[33] This may vary between species.^[46] In 2019, Roux and colleagues found evidence that clownfish exchange microbiota with their anemone hosts.^[47]

Social structure

A group of clownfish occupying an anemone usually consists of a breeding female and male along with some non-breeding individuals. Dominance in clownfish groups is based on size. The largest fish, the breeding female, is the most dominant. Next is the second-largest, the breeding male, followed by the third largest (the largest non-breeder), and so on.^[48] In A. percula in particular, individuals get around 26 percent larger going up the hierarchy.^[49] In this species the number of non-breeders ranges from zero to four, with group size depending on anemone size,^[50] as well as the size of the female as larger females allow for more members without unbalancing the size ratio between them.^[49] Members of a group are unrelated.^[51]

The male clownfish changes into a female (protandrous sequential hermaphroditism) when the previous one is lost, while the largest non-breeder becomes a male and the others rise in rank.^[48][52] New fish that join the group rank at the bottom.^[52] Non-breeders are forced to wait for their time to become breeders, since nearby anemones are occupied and they are too small to challenge the dominants.^[53] The dominant pair controls membership of the group and will drive away individuals when the anemone gets too full,^[50] particularly those that are close to them in size. Thus newcomers control their growth rate so they can remain smaller than their immediate superior to avoid getting evicted.^[52] Clownfish maintain their dominance hierarchy via displays, sound production and chasing. Sounds produced by clownfish include "clicks", "grunts", "pops" and "chirps". Dominants will chase their subordinates while producing a sound consisting of one or more long pulses. The subordinate submits to by emitting a sound with quicker pulses while shaking their heads.^[54] Clownfish appear to produce sounds via the jaws and teeth, which is amplified by the swim bladder.^[55][56]

One study of captive A. ocellaris found that the dominant pair are the most territorial while non-breeders are much less so. Both the male and female direct their aggression against intruders of the same sex, though resident males are more likely to display than to attack. Similarly, non-breeding intruders are more likely to be simply intimidated.^[57] Another study of the same species found that they were more aggressive towards fish with three vertical bars, followed by those with two, one, and none. This suggests that they recognise and see members of their own species as their main competition for anemones.^[30] A clarkii is recorded to share hosts with juveniles of other clownfish species such as A. sandaracinos and A. perideraion.^[35] A 2002 study found that dominant A. clarkii acted aggressively towards juvenile A. perideraion than those of their own species, particularly those of a larger size.^[58]

• A. clarkii dominant chasing subordinate while producing aggressive sounds
• A. frenatus subordinate head-shaking while producing submissive sounds

Reproduction and lifecycle

Clownfish breed year-round in tropical waters while in more temperate waters, like those around Japan, breeding occurs mostly in spring and summer. Only the dominant female and male reproduce, which mostly occurs during a full moon. In the days leading up to spawning, the pair perform courtship rituals which involve the male chasing and nibbling the female as well as erecting his dorsal pelvic and anal fins while staying motionless in front or alongside her. Both the female and male then prepare a nest by cleaning up a nearby rock. Here the female will deposit eggs for the male to fertilise. Clownfish lay up to a thousand eggs, which are conical in shape, 3–4 mm (0.12–0.16 in) long and stick to the rocky substrate by bundles of short fibres.^[59][60] The male tends to the fertilised eggs, cleaning and guarding them as well as fanning them with his pectoral fins.^[61]

Incubation lasts six to seven days.^[61] The eggs start out bright orange and progressively darken, and the eyes of the embryos develop and become visible. The fish break out of their capsules during nighttime.^[60] After hatching, clownfish enter the larval and pelagic stage of their development. This stage lasts up to 12 days, which is shorter than in other damselfishes, where it can last for 70 days.^[62][63] Larval clownfish are initially transparent, except for the eyes, yolk sac and some pigment spots.^[62] Over time they begin to metamorphosise; growing in size and developing their fins, sensory and internal organs, notochord flexion and colouration.^[63] Clownfish larvae can disperse widely across open ocean; A. omanesis has been recorded travelling over 400 km (250 mi) along ocean currents.^[64]

As they enter the juvenile stage, clownfish begin settling to the ocean floor and find an anemone host,^[63] while transitioning to a more diurnal lifestyle.^[65] Juveniles continue to grow and develop their adult colouration,^[63][23] but cannot produce gametes until they ascend to dominance within a group.^[63] The dominant male's gonads produce sperm but also possess dormant ovarian cells. When transitioning into a female, the gonads switch to producing ovaries.^[66] The transition from male to female starts with an increases in body size and feminisation of the brain, followed by gonadal changes and then behaviour changes. The process can last over four months.^[67] Clownfish can live for over 20 years.^[68] A. percula is estimated to reach 30 years, which is long for a fish of its size.^[69]

• 
  A. ocellaris male tending to eggs
• 
  Clownfish eggs closer to hatching
• 
  Development of A. ocellaris (above) and A. frenatus

Parasites

Parasites of clownfish include copepods, trematodes, nematodes and acanthocephalans. One study of four species near Nha Trang, Vietnam found that the most common parasites are the trematode Hysterolecitha nahaensis and the nematode Spirocamallanus istiblenni.^[70] Clownfish can also be infected with the protozoan Brooklynella hostilis, which causes "clownfish disease". Fish with this disease will stop feeding, breathe heavily, gasp, develop thick white mucus and lose their vibrant colours.^[71]