Yellow Boxfish

Learn about the Yellow Box Fish on Koh Tao

Yellow Box Fish are one of the most adored and uniquely shaped fish you can find whilst snorkeling or scuba diving around Koh Tao.

Where to find the Yellow Box Fish on Koh Tao

Juvenile Yellow Box Fish can be seen at many dive sites around Koh Tao including Buoyancy World and Aow Leuk Bay to deeper dive sites such as Southwest Pinnacle and Sail Rock.

Artificial dive site Junkyard Reef is a very popular location to see the Adult Box Fish, using many of the structures as a residence, a grazing area and nursery.

If you are lucky and quiet, you may see one of them asleep in one of the many toilet basins that have been sunk there.

How to Identify a Yellow Box Fish

Yellow Box Fish are in fact illuminous yellow marine animal with black spots and shaped like a small box, which makes identifying them quite easy however, they are very shy and finding them can be quite a challenge.

As a Yellow Box Fish begins to mature their spots start to slowly disappear and their bright coloured body fades away into a darker, more earthy mustard colour.

As they grow, their body will continue to transform and their box-like body gradually elongates into more of a rectangular shape and their caudal (tail) fin becomes a lot more prominent.

Yellow Box Fish Characteristics & Behaviour

It is believed that Yellow Box Fish will grow no larger than 45cm and can be seen by scuba divers at depths down to 40 metres.

You will generally find them on the reef grazing and their diet is a mixture of algae, worms, sponges and other small invertebrates or crustaceans.

As a juvenile you will see the Yellow Boxfish in very sheltered areas on the reef and due to their small size and lack of growth in their fins, they are very vulnerable at this stage of their life.

Coral Reef Grazing

When in danger a Yellow Box Fish can secrete a neurotoxin known as Pahutoxin or Ostracitoxin around its entire body from special cells, which can paralyse anything nearby so it can make a quick escape from a predator.

Yellow Box Fish are a very solitary fish and generally you do not see a pair of them together.

Even as a juvenile, they are left to fend for themselves as the mother can produce eggs for up to a month and will release them near the surface.

It is believed that by releasing the eggs near the surface it allows ocean currents to disperse the eggs, which means there is less chance of predators finding them all in a group.

Interesting Facts about the Yellow Box Fish

In what can be considered a mating ritual, studies have shown that during sunset and sunrise hours, the male Yellow Box Fish likes to dance around a female and flash a range of bright colours.

During this intricate dance routine, the male makes a humming sound to the onlooking female, which is loud enough to be heard by a scuba diver.

Pahutoxin found inside a Yellow Box Fish works by what is known as the Haemolytic effect, which effectively bursts red blood cells and the toxin binds to the gills of a predator and slowly asphyxiates the victim.

Beautiful Yellow Box Fish

The Pahutoxin secreted by the Yellow Box Fish, is one of the most potent toxins of any species and it is secreted at 10 parts per million, which is the equivalent of a shot glass in an Olympic sized swimming pool.

Yellow Box Fish body shape comes from their shell-like bone structure that is called a carapace, which is made up of fused plates and covers the entire body with holes for their eyes, mouth and fins.

In 2005, Mercedes-Benz car manufacturer designed and produced a Bionic car, which was based on the shape of a Yellow Boxfish due to research showing how hydrodynamic their body shape really is.