Fish
Ribbon Eel
Rhinomuraena quadra · also called ribbon eel, leaf-nosed moray eel
The ribbon eel is a moray unlike any other: as thin as its name suggests, with a tall fin running the length of its body and a snout that ends in leaf-shaped flaps and flared, fan-like nostrils. Over its life it runs through three colour schemes — black, then electric blue, then yellow — and changes sex along the way.
Overview
Ribbon eels live across the tropical Indo-Pacific, from East Africa to the central Pacific, in burrows in sand and rubble near coral reefs, usually with just the head and forebody protruding and weaving from side to side. A large female can reach 1–1.3 m but is only pencil-thin. They ambush small fish and shrimp with a fast strike.
Three colours, one eel
Colour tracks life stage. Juveniles are jet black with a yellow fin. Functioning males are brilliant electric blue with a yellow snout. The largest individuals turn almost entirely yellow as they become females — the ribbon eel is a protandrous hermaphrodite, male first and female later, with the colour change happening alongside the sex change. The three forms were once thought to be different species.
The gaping mouth
A ribbon eel constantly holds its mouth open and works its jaws. This looks like a threat but is simply how it breathes, pumping water over its gills while it sits still in the burrow.
Status
The ribbon eel is Least Concern, widespread and locally common. It is collected for the aquarium trade despite a very poor survival record there — most refuse to eat and slowly starve — so keeping one is discouraged.
Ribbon Eel: common questions
Why does the ribbon eel change colour?
The colour change tracks the eel's life stage and, crucially, its sex. A young ribbon eel is black with a yellow fin. When it matures into a functioning male it turns a vivid electric blue with a yellow snout and dorsal fin. As it ages further and becomes a female it loses the blue and turns almost entirely yellow, sometimes with black edging. Because these look like completely different animals, black, blue, and yellow ribbon eels were once described as separate species before the sequence was worked out.
Does the ribbon eel really change sex?
Yes. It is a protandrous hermaphrodite, meaning it starts adult life as a male and later becomes a female. All the larger, yellow ribbon eels are females that were previously blue males, and the smaller blue eels are males that will make the switch if they live long enough and grow large enough. Whether every individual completes the change, and exactly what triggers it, is still not fully understood, partly because the eels are so difficult to observe over a lifetime in the wild.
Why does a ribbon eel keep its mouth open?
It is breathing. Like other morays, the ribbon eel lacks the gill-pumping cover that most fish use and instead draws water over its gills by repeatedly opening and closing its mouth and pumping with its throat. Because it spends most of its time anchored in a burrow with only its head exposed, an observer sees this constant slow gaping and jaw movement, which can look like a threat or an attempt to bite but is just respiration.
Can you keep a ribbon eel in an aquarium?
It is strongly discouraged. Ribbon eels have a very poor record in captivity — most refuse to eat, and even those that accept food often stop after a while, so the majority slowly starve to death within months. They also almost never show the natural colour and sex changes in a tank. Because wild-caught ribbon eels are still collected for the trade despite this, buying one generally means removing an animal from the reef that will not survive.