Acanthurus japonicus: technical profile

Acanthurus japonicus, commonly known as the white-cheeked surgeonfish, is one of the most recognisable and appreciated representatives of the Acanthuridae family. Native to the western Pacific, this species occupies a key ecological role in coral reefs, where it acts as a natural regulator of algal growth.

Its elegant body, its contrasted livery and its generally peaceful temperament have made it popular in the aquarium world, although it is not suited to newcomers because of its physiological requirements. Although its distribution in nature is relatively wide, the survival of this species depends strongly on the health of the coral reefs, today threatened by climate change, by the acidification of the oceans and by the destruction of coastal habitats.

Acanthurus japonicus is a fish that combines aesthetics and biological function: its feeding based mainly on benthic algae contributes to maintaining the balance in the reefs, preventing phenomena of algal overgrowth that can damage the corals. At the same time, its territorial and social behaviour makes it fascinating to observe, both in nature and in the tank.

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🏷️ Common name

• White-cheeked surgeonfish
• White-faced surgeonfish (English)
• Japan surgeonfish
• In Japanese: Shiroho-tsuno-uo (白頬ツノウオ)

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🔁 Synonyms

There are no officially validated scientific synonyms for this species. The name accepted and currently in use in the scientific literature is:

Acanthurus japonicus (Fowler & Bean, 1929)

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🔬 Morphology

The body of Acanthurus japonicus is typically elliptical, with a hydrodynamic line perfect for active swimming. It presents a visually harmonious appearance, with sharp and strongly contrasted colourings, which increase with sexual maturity.

Morphological characteristic	Description
Body	Fusiform, flattened laterally
Caudal fin	Forked, symmetrical and mobile
Dorsal and anal fin	Elongated and sharp, edged with white
Colouring	Brownish-black ground with white borders and yellow inserts
Eye	Wide, lateral, optimal for detecting predators
Cheek	Well-defined white area, from which the common name
Caudal spines	One per side, sharp and retractable, used for defence

📌 The livery reaches its maximum splendour in mature specimens, above all if fed correctly with diatoms, fresh algae and spirulina.

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♀♂ Differences between males and females

In Acanthurus japonicus, sexual dimorphism is reduced. However, slight differences can be observed, especially during courtship:

Characteristic	Male	Female
Size	Slightly larger	More compact
Behaviour	More dominant and territorial	Less aggressive
Colouring	It tends to intensify in the reproductive period	More constant
Courtship	Active protagonist, it performs circular movements and vibrations	Receptive but selective

📌 Certain identification of the sex often requires prolonged behavioural observation or gonadal investigations in a controlled environment.

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🌊 Way of life

Acanthurus japonicus is a diurnal species, very active and constantly engaged in the search for food. It usually lives in small groups or in pairs, close to rocky substrates or well-oxygenated coral reefs.

In nature:

• It moves nimbly among the fissures of the reef
• It grazes incessantly on benthic algae
• It avoids zones with strong turbulence
• It sleeps wedged among rocks or coral branches during the night

💡 Its social behaviour is generally peaceful, although it can become competitive in restricted spaces or in the presence of other Acanthuridae.

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🧱 Territoriality

• The species shows moderate territoriality, mainly towards other surgeonfish or similar herbivores.
• Aggressive behaviour is limited to defence of the grazing territory.
• If kept in an aquarium with ample space and hiding places, A. japonicus is tolerant and collaborative.

📌 Introduction into an already populated tank must be done with care, avoiding other simultaneously dominant tangs, such as Acanthurus leucosternon or A. lineatus.

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⏳ Life expectancy

Environment	Average longevity
In nature	10–12 years
In the aquarium	8–10 years (in optimal conditions)

With correct feeding and excellent water quality, some specimens even pass 12 years in captivity.

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📏 Maximum size

• Maximum length in nature: 21–22 cm TL
• Average length in the aquarium: 18–20 cm
• Relatively slow growth: it reaches adult size in about 2 years

📌 It is fundamental to consider its progressive growth over time: initially it may seem small, but it always needs large tanks right from when it is young.

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🧾 Description

The body is basically very dark brown, with brilliant white finishes on the dorsal and anal fins. The white cheek is the main distinctive feature, positioned immediately behind the eye.

On the caudal peduncle there is a well-defined yellow-orange patch, outlined by black pigments. The caudal fin is transparent, edged with white.
The eyes are large and mobile, capable of observing laterally with precision.

In swimming, the whole livery produces an extremely scenic contrast, above all when the fish moves among coloured corals.

📉 Status on the IUCN red list

According to the IUCN Red List, Acanthurus japonicus is classified as Least Concern (LC), that is at low risk of extinction.

This evaluation is based on:

• 🌍 Wide geographical distribution in the western Pacific
• 🐠 Stable populations in nature
• 📈 No evidence of significant global declines

Despite this, some potential risks are localised:

• Degradation of the coral reefs
• Fishing for the ornamental trade
• Impacts of marine climatic warming

📌 It is important to monitor the species in the coming decades, especially in regions such as Indonesia and the Philippines, where greater human pressure on the reefs is observed.

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🌐 Habitat – origin and geographical distribution

Acanthurus japonicus is native to the tropical waters of the western Pacific, in particular:

Area	Countries and islands
Japan	Ryukyu Islands, Okinawa
Philippines	Palawan, Luzon, Mindoro
Indonesia	Bali, Sulawesi, western Papua
Micronesia	Palau, Guam
Taiwan and Vietnam	Coastal zones with developed reefs

It prefers sheltered coral reefs, gently sloping reef slopes, lagoon channels and rocky zones with an abundant presence of benthic algae.

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🌡️ Temperature in nature

The temperature of the water in its natural habitats varies between:

• 25 °C and 29 °C throughout the year
• Maximum seasonal peaks: 30–31 °C in summer
• Relative minimums: 24 °C in winter or during El Niño events

📌 The species shows sensitivity to rapid thermal swings and to temperatures persistently above 30 °C, which can alter feeding behaviour and immune resistance.

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⚗️ Environmental parameters: nature vs aquarium

Parameter	In nature	Recommended in the aquarium
Temperature	25–29 °C	25.5–27.5 °C
Salinity	33–35 ppt	34 ppt (SG 1.025)
pH	8.0–8.3	8.1–8.4
KH	6–10 dKH	7–9 dKH
Oxygenation	High	High (with an efficient skimmer)
Circulation	Moderate/strong	Alternating, reef simulation
Nitrates	< 2 ppm	< 5 ppm
Phosphates	Traces	< 0.03 ppm

💡 Using a refugium with algae and activated carbon improves the stability of the nutrients and of the pH.

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🧪 Keeping in the aquarium

Acanthurus japonicus is suited only to expert aquarists, because of its environmental requirements.
It requires:

• A mature tank, with at least 6 months of biological stability
• A layout with live rock, caves and free space for swimming
• A predominantly vegetable diet (nori, spirulina, fresh algae)
• Clean water, rich in oxygen and with constant microcurrents

🤓 It tends to become stressed if introduced into tanks that are small, noisy or with conspecifics that are too dominant.

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📐 Minimum dimensions of the aquarium

For a single adult specimen:

• Minimum recommended: 400 net litres
• Ideal: 500–600 litres, with at least 130–150 cm of frontal length

🔁 In smaller tanks, chronic stress can lead to loss of colour, immunodeficiency and aggressive behaviours.

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👥 Minimum population in the aquarium

In a closed environment it is preferable to keep a single specimen. However:

• In very large tanks (>700 litres), it is possible to introduce a pair or several young individuals, provided they do not cohabit with other similar Acanthurus.

📌 Avoid simultaneous introduction with A. leucosternon, A. nigricans or A. achilles, with which it shares its ecological niche and appearance.

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🧩 Compatibility with other species

It is 100% reef-safe and compatible with:

• Clownfish
• Chromis
• Anthias
• Gobiids
• Blennies
• Peaceful wrasses

⚠️ Variable compatibility with other territorial surgeonfish. Better to avoid: A. sohal, A. lineatus, A. olivaceus in tanks below 800 litres.

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🥬 Feeding in nature and in the aquarium

In nature it is an obligate herbivore: it feeds mainly on:

• Benthic filamentous algae
• Diatoms and microbial films
• Organic detritus rich in cellulose

In the aquarium it must receive:

Type	Examples
Vegetables	Nori, spirulina, sea lettuce, fresh algae
Commercial	Granules and pellets for herbivorous surgeonfish, vegetable-based products
Occasional	Mysis, artemia (1–2 times a week)

⚠️ The absence of vegetable fibre can cause digestive problems, apathy or aggressiveness.

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💞 Reproductive behaviour

In nature:

• Reproduction takes place during favourable lunar periods
• The males court with spiral movements
• The release of eggs and sperm is synchronous and pelagic

In the aquarium:

• No documented cases of natural reproduction are known
• The spatial and light conditions are not yet easily reproducible

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🐣 Protection of the fry

• The eggs are pelagic, fertilised in the water column
• The fry derive from a planktonic larval stage called acronurus
• There is no form of parental care after spawning

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⚠️ Risks of hybridisation

Acanthurus japonicus can hybridise in nature with:

• Acanthurus nigricans
• Acanthurus achilles

This happens above all in zones of geographical overlap or in areas impoverished of habitat. The hybrids are often fertile but they present:

• Unstable colourings
• Less defined liveries
• Lower hardiness

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🛡️ Hardiness

• It is considered fairly hardy, if introduced into mature and stable tanks
• Sensitive to: Cryptocaryon irritans, transport stress, swings of pH and temperature
• Prophylaxis in quarantine recommended (28–30 days)

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💡 Lighting in the aquarium

• Medium or strong intensity, with a full spectrum
• Ideal values: PAR 150–250
• A photoperiod of 10–12 hours is preferable, with gradual switching on and off

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🌊 Current in the aquarium

• An alternating laminar current is necessary, similar to that of the reef slopes
• Position the pumps so as to generate zones of:
  • Medium flow for feeding
  • Low-flow refuges for rest

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🛒 Availability in the trade

• Very available in the marine aquarium trade
• Average price: 60–120 €, depending on the size
• More widespread in Europe than in the USA
• Prefer specimens acclimatised in captivity or reared in a quarantine tank

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⚖️ Pros and cons

Pros	Cons
Spectacular and distinctive aesthetics	It requires large tanks
Reef-safe	Sensitive to stress
Interesting behaviour	Not suited to beginners
Useful against algae	Incompatible with other dominant surgeonfish

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❓ FAQ

Can I keep it in a 200-litre aquarium?
No. It is too small. A minimum of 400 net litres is required in order to guarantee space and stability.

Is it compatible with corals?
Yes. It is completely reef-safe and does not touch the polyps, although it can momentarily disturb some LPS while it looks for algae.

Does it eat only vegetables?
Yes, it is an obligate herbivore. It must be fed with algae, spirulina, nori and only occasionally with protein food.

Can it live with other surgeonfish?
Only in very large tanks and with strategic introduction. It is better to avoid dominant conspecifics.

Does it suffer from transport?
Yes. It is sensitive to initial stress. Quarantine and gradual adaptation to the tank are fundamental.

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🧰 Practical boxes

📦 Pre-introduction checklist:

• A tank of 400+ litres, mature for at least 6 months
• Medium-strong lighting
• Modulated water flow
• A vegetable diet already established
• Absence of other aggressive surgeonfish

🔍 Signs of stress:

• Loss of colour
• Accelerated respiration
• Loss of appetite
• Excessive aggressiveness or apathy

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🧠 Conclusion

Acanthurus japonicus is one of the most fascinating species of the genus Acanthurus. It combines iconic aesthetics, a key ecological role and dynamic social behaviour. It is ideal for large, mature and well-managed reef tanks, where it becomes a central presence and useful against algae.

Although it is not recommended for newcomers, it is extremely rewarding for expert aquarists, representing a perfect balance between beauty, biological function and behavioural interaction. In nature, its survival is closely linked to the health of the coral reefs, confirming the symbolic and environmental value of this species.