Fish
Mandarinfish
Synchiropus splendidus · also called mandarin dragonet, mandarin goby (incorrectly)
The mandarinfish is often called the most beautiful fish in the sea — a tiny, slow reef dweller painted in swirling orange, green, and electric blue. That blue is not the usual trick of light; the mandarinfish is one of only two animals known to actually manufacture blue pigment.
Overview
Mandarinfish live in sheltered inshore reefs and lagoons of the western Pacific, from southern Japan to northern Australia, always in areas with plenty of rubble, coral, and hiding places. They are members of the dragonet family, not the gobies they are sometimes sold as. Each fish is a bottom-hugging micro-predator, hopping and gliding over the reef on its pelvic fins, picking at the substrate all day for minuscule prey. They are secretive, spending the night and the middle of the day tucked into crevices.
Colour without a trick
Blue is the rarest colour in animals, and almost every “blue” creature is really producing it with structural colour — nanostructures that scatter blue light over a dark pigment base. The mandarinfish and its close relative the psychedelic mandarin are the only known vertebrates with true blue pigment cells (cyanophores). This makes the mandarinfish a genuine oddity of animal colouration, and biologists still do not know why it and only it went down this route.
Slime instead of armour
The mandarinfish has no scales. It relies instead on a thick coat of mucus that is bitter, foul-smelling, and toxic, which protects it from predators and parasites and lets a small, slow, gaudy fish get away with being so visible. The trade-off is fragility: damage the mucus layer and the fish is highly prone to infection.
The dusk dance
Mandarinfish spawn at twilight. Males show off their colours and fins, a female picks one, and the pair rise together off the reef into open water, bodies pressed side by side. At the top of the ascent they release eggs and sperm into the current and immediately drop back to shelter. The eggs drift as plankton for a couple of weeks before the larvae settle to the reef.
Status and the aquarium trade
The mandarinfish is Least Concern and common where suitable reef habitat exists. Its main pressure is collection for the marine aquarium trade, where its beauty makes it highly sought after despite its difficult care — many wild-caught mandarinfish starve in home tanks because they will only eat live micro-crustaceans. The growth of captive breeding, producing fish that accept prepared foods, is easing that pressure.
Mandarinfish: common questions
Why is the mandarinfish's blue special?
Almost every blue animal — a blue jay, a blue morpho butterfly, a blue tang — is not actually blue. Its colour comes from nanostructures in the skin, scales, or feathers that scatter blue light; the underlying pigment is brown or black. The mandarinfish is one of only two known vertebrates (the other is the closely related psychedelic mandarin, Synchiropus picturatus) that produces a genuine blue pigment, held in cells called cyanophores. Why these two small fish evolved a true blue when nothing else did is not understood.
Does the mandarinfish have scales?
No. Instead of scales it is covered in a thick layer of slimy mucus that smells and tastes foul and contains toxins. This coating protects the slow-moving, brightly coloured fish from being eaten and from skin parasites, and it is part of why the mandarinfish can afford to be so conspicuous. It also means the fish is very vulnerable to skin infections and abrasion if the mucus layer is damaged, which is one reason it does poorly with rough handling.
How do mandarinfish mate?
In one of the more photogenic courtships on the reef. As the light fades, males display their fins and colours to females; a female chooses a male, and the pair align side by side, press their bodies together, and slowly rise off the bottom — sometimes a metre or more into open water. At the top of the ascent they simultaneously release a small cloud of eggs and sperm, then dart back down to cover before predators arrive. Divers regularly gather at known "mandarinfish spots" at dusk to watch this.
Why is the mandarinfish hard to keep in an aquarium?
It is a picky, non-stop micro-predator. A single mandarinfish spends its whole day slowly hunting tiny live crustaceans ("copepods" or "pods") over rock and sand, and it will usually not eat flake, pellet, or frozen food. To sustain one you need a large, well-established reef aquarium with a big refugium continuously breeding pods, or a diligent regime of feeding cultured live foods. Many mandarinfish sold to beginners slowly starve. Captive-bred mandarinfish that will accept prepared food are now available and much easier.