Mandarinfish at Dusk
My best diver last week spent fifty minutes staring at a sea urchin. She hovered a couple of centimetres above the sand like a frozen statue and refused to look at the blue-ringed octopus I found 10 metres away. We do this every evening because the reef runs on a tight schedule and the headline act does not show in daylight. Sipadan is the big amphitheatre but the real theatre happens in the rubble at dusk.
Table of Contents
- The biological clockwork
- The anatomy of a dragonet
- The science of true blue
- The courtship dance
- The photographer's darkroom
- Paradise reef and the staghorn fortress
The biological clockwork
The transition starts when the water turns the colour of old coins. Daytime foragers pack up and the night crew takes over the reef slope. Sipadan has no night diving and the boats are off the water by 4pm. That leaves dusk to Kapalai and Mabul. We drop into Mandarin Valley at 5:30pm as the shadows lengthen across the sand. The site slopes gently to 20 metres and features scattered coral bommies alongside the remnants of a wrecked fishing boat. We bypass all of it and head straight for the spiky barricades of the diadema sea urchins. You settle on the sand and wait.
The waiting game tests your discipline. Mandarinfish feed along the bottom throughout daylight hours but they stay hidden deep in the coral. They spend the day working the rubble for tiny crustaceans and snails. It is the sort of food you need a microscope to admire. They wait for the light to fade before they expose themselves for the evening ritual. Experienced guides navigate to the urchins first and the animal second. The fish maintain tiny home ranges and keep to their postal codes night after night. I often see the residents emerge as the light fades when I hold my position over their patch of rubble. Patience has paid off for me often enough to keep me coming back.
The anatomy of a dragonet
People call them gobies out of habit but they belong to a different family tree altogether. The mandarinfish is a dragonet with a broad flattened head and fins under its belly shaped like oversized snowshoes. They don't really swim. They march across the seabed. A cruising dragonet resembles a wind-up toy on a mission through the coral rubble. They navigate the loose jagged rocks using those pelvic fins to walk rather than glide. You rarely see them venture into open water because they top out at 8 centimetres and the ocean is full of hungry mouths. It is safer in the rocks.
They survive the food chain by wearing chemical armour. A mandarinfish has no scales to protect its skin. It relies on special glands that pump out a thick layer of bitter mucus. This foul-smelling slime acts as a liquid forcefield against parasites in the dense rubble. The toxic barrier allows them to forage slowly without constantly fleeing to cover. Predators leave them alone. The brilliant pattern is a biological billboard that advertises a terrible meal. It works.
The science of true blue
That colour is a famous anomaly in the vertebrate kingdom. Most blue fish wear an optical illusion made of tiny crystals that bounce light back to the eye. The mandarinfish manufactures the real thing in its own tissues. It uses novel light-absorbing cells loaded with true blue pigment. The fish wears its colour like wet paint rather than a reflection. The blue stays blindingly bright even when the sun drops and the reef goes dark. It is true blue.
Scientists mapped their whole genetic code in 2021 to understand this pigment factory. The result of all that coding is a fish painted like a carnival float that hides in a pile of dead coral. The bright colouration works as a warning sign in a predator-heavy environment. Evolution gave them a neon sign and a toxic coat to back it up.
The courtship dance
The rubble below the Kapalai jetty operates as a ruthless singles bar. The big dominant males court up to eight females in a single evening. The females pick one partner for one dance before calling it a night. The males spend the last hour of daylight fighting for territory and flashing their tall first dorsal fins like raised flags. Small subordinate males lurk on the periphery while the heavyweights show off. Size wins.
When a female accepts a mate, she aligns her cheek with his. The pair lock together like two magnets snapping into place. The rise happens fast. They lift off the safety of the bottom and spiral upwards. They hover a few feet up for two seconds before they release their payload. The eggs and sperm mix in a tiny puff of smoke and the parents dart instantly back into the urchin spines. The female leaves behind a fragile cloud of 12 to 205 eggs that drift in open water. Each egg measures roughly 0.7 millimetres across and drifts in the current like dust in a sunbeam. The larvae hatch and ride the local water movement for 14 days before settling back onto the reef. It is a brief childhood.
The photographer's darkroom
Shooting this event requires the discipline of a sniper. The fish hate bright lights. A standard dive torch over 150 lumens will end the show immediately. The mating pair will abandon the rise and vanish deep into the coral. You have to treat the reef like a photographic darkroom. Divers equip themselves with focus lights that have a dedicated red mode because the fish ignore red wavelengths. You kneel in the sand bathed in a ruby glow and track the action through the viewfinder.
The technical setup separates the professionals from the hopefuls:
* A 105mm macro lens on a full frame camera or a 60mm on a crop sensor to maintain a respectful working distance.
* An aperture stopped down between f/11 and f/22 to keep the moving fish in sharp focus.
* A shutter speed locked between 1/125s and 1/250s to freeze the erratic helical ascent.
* An ISO pushed to 1000 to scrape the last ambient light from the dusk water.
* A strobe power dialled down to a quarter or a -3 setting to allow rapid recycling without turning the translucent eggs into a white blur.
You decouple your autofocus to a back button so the camera tracks the spiral ascent without hunting for a subject when you press the shutter. Getting too close is a rookie mistake that ruins the dive for everyone on the boat. Keep your distance.
Paradise reef and the staghorn fortress
Kapalai shares this daily event with the neighbouring island of Mabul. Paradise 1 and Paradise 2 sit in 5 to 18 metres of water and swap the Kapalai urchins for sprawling staghorn coral gardens. The staghorn acts as a brittle fortress for the resident dragonets. Dive boats arrive by 5pm to secure a prime position around the sunken wooden wrecks that break up the sand flats. Paradise 2 features a 12-metre wooden boat encrusted with feather stars that attracts schools of batfish and passing eagle rays.
Mabul is a muck diving capital and the surrounding rubble is packed with a secondary cast of characters:
* Flamboyant cuttlefish flashing warning colours like miniature neon signs.
* Blue-ringed octopuses hunting crustaceans in the discarded shells and tyre treads.
* Painted frogfish doing their patient ambush act on the edge of the staghorn.
* Giant moray eels lurking like gargoyles in the wooden wreckage.
The secondary cast has often made the dive for me while the dragonets stayed out of sight. I often find something else to watch.
The etiquette remains the same no matter which island you dive. You establish neutral buoyancy early and you hover statically without kicking up the silt on the bottom. A careless flutter kick acts like a snow globe shaking over the reef and destroys the visibility for twenty minutes. You control your breathing to keep your exhaust bubbles away from the coral. You never use a pre-flash on your camera. The whole experience depends on patience and staying perfectly still. Move slowly.
The ride back to bed is short after every one of these dives if you stay on the islands. Check the Mabul resorts page or read the Mabul muck diving guide to see what else lives in the rubble. Send us your dive preferences form so we can plan the week around it.