The Fish That Changed Color Before My Eyes

A firsthand encounter with the Blue-tail Unicornfish in the Maldives

While diving last January in the Maldives, I experienced one of those rare underwater moments that makes you question your own perception.

Mid-dive, as I moved slowly along the reef, looking for something interesting to shoot with my underwater camera, I glimpsed a large reef fish hovering close to the rocky sea bottom. Its appearance was striking but familiar - until, when I started getting closer to it, it wasn’t. The same fish now looked entirely different. Its body coloration had shifted so noticeably that my first reaction was disbelief. I genuinely thought my eyes were playing tricks on me.

I watched carefully, replaying the moment in my head, trying to rationalize what I had just seen. Changes in light, angle, or water clarity can easily deceive a diver. But this felt different. The transformation was too distinct, too deliberate.

Upon resurfacing, my curiosity prompted further inquiry.  I asked our dive guide whether there were any fish known to change color so dramatically. To my surprise, he wasn’t aware of any species in the area that could do so. That response only deepened the mystery.

Fortunately, I had been filming the dive with an action camera, and at the same time I was using my underwater photography setup to capture still images of the fish as it moved calmly along the reef. When I later reviewed both the video footage and the sequence of photographs, I saw exactly what I had witnessed underwater - the same individual fish transitioning between two markedly different color phases. What initially seemed like a fleeting impression was now unmistakable. Frame by frame, the fish appeared in distinctly different forms: one was pale and silvery and the other was darker and richly saturated. The video confirmed the continuity of the transformation, while the still photographs froze precise moments in time, providing clear visual evidence of a change that could easily have been dismissed as a trick of light or perception. Confident that this was more than an illusion, I posted a short clip on Instagram and asked a simple question - has anyone seen this fish before, and does anyone know what it is? One of my followers took the time to research the footage carefully, and the answer came back with a name: the Blue-tail Unicornfish (Naso caeruleacauda).

Meet the Blue-tail Unicornfish

The Blue-tail Unicornfish is a member of the surgeonfish family and is found throughout the Indo-Pacific, including the Maldives. Like other unicornfish, it is named for the subtle horn-like protrusion that develops on the forehead of mature individuals, though this feature can be less pronounced or absent in some specimens.

At first glance, this species appears relatively understated compared to more flamboyant reef fish. Its body is large, laterally compressed, and built for cruising open reef slopes and outer reef walls. But what makes it truly remarkable is not its shape - it’s its ability to change color rapidly.

A Fish with Multiple Faces

The color change I observed is not random. The Blue-tail Unicornfish is known to exhibit distinct color phases, shifting between darker and lighter forms depending on its physiological and behavioral state.

In its darker phase, the fish displays a deeper brown to olive coloration, often with a rich yellow underside and a vivid blue tail. In contrast, during a resting, calm, or low-stress state, the same individual may appear pale, silvery, or grayish - almost like a completely different species.

These changes can occur within seconds, driven by specialized pigment cells in the skin called chromatophores. By expanding or contracting these cells, the fish can alter how light is reflected from its body. Unlike permanent coloration, this is a dynamic, reversible process.

Why Change Color at All?

Color change in reef fish serves multiple purposes:

• Communication – signaling calm, stress, or alertness to nearby fish

• Camouflage – blending with the reef or open water depending on activity

• Social behavior – interacting with other unicornfish or avoiding conflict

• Physiological regulation – responding to environmental or internal stimuli

In the case of the Blue-tail Unicornfish, these transitions are subtle enough that many divers never notice them - or only ever see one phase. Witnessing the transition itself is rare, which explains why even experienced guides may not immediately recognize it.

Seeing Versus Observing

What struck me most about this encounter was how easily such a phenomenon can go unnoticed. We often think of dramatic color change as the domain of octopus, cuttlefish, or chameleons. Fish, by contrast, are assumed to be static in appearance.

This experience was a reminder that the reef still holds behaviors we overlook, not because they are rare, but because we are not always watching closely enough.

Having video evidence allowed me to slow the moment down, study it frame by frame, and confirm what I had seen. It also opened the door to collective knowledge - without social media and the curiosity of one engaged follower, this fish might have remained an unanswered question.

Why This Matters

Encounters like this highlight the value of observational diving and underwater photography - not just as art, but as a form of documentation. Many behavioral traits in marine life remain under-reported simply because they occur quickly or subtly.

For divers, photographers, and marine enthusiasts, the lesson is simple: trust your observations, ask questions, and keep recording. Sometimes the most fascinating discoveries are not rare species, but familiar animals revealing unfamiliar behaviors.

The Blue-tail Unicornfish didn’t just change color that day - it changed the way I look at the reef.

Before the change

After the change