The Yellow Boxfish

Marine life around Koh Tao - Yellow Boxfish

The Yellow Boxfish

The Yellow Boxfish (Ostracion Cubicus) is a member of the Boxfish family and is understood to grow no larger than 45cm. Recreational divers can find these fish as deep as 40 metres, however they tend to spend most of their time in the shallower and warmer parts of the world’s oceans like the Pacific Ocean, Indian Ocean and also parts of the south eastern Atlantic Ocean.

Reef Grazers

Boxfish tend to spend most of their days on the reef, their main diet when not grazing on algae is small invertebrates, worms, crustaceans and sponges for example. Using its puckered lips, it blows small bursts of water at the corals and sand to reveal anything that might be hiding.

Box Shaped, and Yellow!

The Yellow Boxfish is in fact, a yellow, box-shaped fish – who would have guessed it? The Juveniles area bright yellow colour with black spots, which helps them to survive by telling nearby shopping predators that they are poisonous (an evolutionary phenomenon known as ‘aposematism’).

However as they begin to mature the number of spots decreases and the yellow blends into almost a dirty, mustard colour. It’s not just the colours that change either, their body also elongates as they grow creating many layers of fused scales that act like a coat of armour.

Impressive Defence

Their body frame is solid and only has necessary openings for the eyes, fins, mouth and tail. These quirky fish may look cute and innocent, but are in fact more like an aluminous swimming Swiss-army knife, packed with defensive gadgetry! The most obvious of these is being a living tank, not too many hunting predators that I can think of would rush to eat a boxfish, which I can only assume is like biting down on a kneecap, which is a bad decision all round, wouldn’t you agree?

A Toxic Nightmare

Another trick the Boxfish holds up its salty sleeve is their chemical defence system. Being closely related to the Pufferfish this is no surprise, however the toxins they hold ‘Pahutoxin’ also known as ‘Ostracitoxin’ works in a way like no other known fish toxins. When in danger the Boxfish secretes this toxin from special cells covering their entire skin and quickly disperses this clear mucus in what is effectively a cloud of regret for any engaged predator!

The pahutoxin has what is known as a ‘haemolytic’ effect, meaning it bursts red blood cells. The toxic mucus binds to the gills and slowly asphyxiates its victim all at a concentration of 10 parts per million. To put it into more relative context of how potent pahutoxin is,10 parts per million is roughly the same as you spitting your mouthwash out on a morning but instead of the sink, into a 2.5million litre sized Olympic swimming pool!

Master of Awkwardness

It’s quite clear the boxfish has made an evolutionary exchange, surrendering all abilities involving speed and agility for its reinforced defensive toolbox. Severely lacking in speed they never fail to amuse me as juveniles, constantly looking like they’re in a state of awkward confusion on how to use their own fins and not knowing which direction to go in.

They are also certainly not up for any master of disguise awards either, however ironically enough their manoeuvrability skills are second to none. I must admit whenever I find one I spend more time waiting to see it again to point out to my divers than I do actually looking at it. This involves me either upside down franticly searching for it or waiting for it to reappear from whatever it has taken refuge in, under or behind.

As effective as this evolutionary route has been for the boxfish in terms of its own day to day survival, it has also been beneficial in long term survival. Betting your reproductive future all on the fact you are an unpalatable box of bones isn’t a common technique used by fish, however the Boxfish have proved it to be a successfully sustainable method and is another reason why they are different to their fellow gilled brethren.

Kaleidoscopic Spring

Boxfish are known to reproduce in spring or when the waters are beginning to warm. They usually are in a harem or small group and the male takes on the duty of protecting the territory and females involved.

Research has shown that on sunset or sunrise the males spend their time flashing bright colours and dancing around the female, once she is satisfied they quickly swim to the surface or shallow open water, this is the most amazing part, the male then hums to the female, loud enough to be heard be a diver.

The eggs are then fertilised and released and the Boxfish return back to the safer parts of the water.

This Pelagic spawning technique means that once the eggs are released, they are taken by the currents and heavily dispersed into the open ocean. The Boxfish takes more of a quantity over quality approach. The female can produce a number of eggs daily for around a month.

One benefit of this technique is that predators cannot feed on the whole batch of eggs however, the chances of a hatched egg drifting to a suitable place of refuge for a new born to live and survive are slim meaning there is a high mortality rate in the reproduction of these fish.

Human Impacts

Boxfish have gone to extreme and patient methods of evolution to survive and have their own unique advantage. Although they are not at this minute in danger as a species it is important we keep awareness. In the ‘Ostraciidae’ family there are 23 exact species and all, in one way or other use toxins as their defence and survival.

These toxins contain compounds very similar to detergents and so we must acknowledge that polluting our oceans with waste water and run off containing soaps and other cleaning products can in the long term disrupt and restrict the way in which thetoxin is effective in a natural setting. This in-turn would put the whole species at risk and leave them defenceless and eventually into population decline.

Boxfish on Koh Tao can be spotted at numerous dive sites around the island, common sites like White RockMango Bay and Twins (North around Koh Nang Yuan) all have juveniles living there. Crystal’s very own dive site, Junkyard Reef, has a permanent resident also. This is a fully matured adult and you can see the difference in change as they grow.

If you are interested in seeing these during your dive, you can ask our team at Crystal and they would be happy to advise you on suitable dive sites.

Believe it or not….

In 2006, Mercedes-Benz unveiled its Bionic concept car which was inspired by the shape of the Yellow Boxfish!

Author: Kieran Hooley (PADI DM #364105)

Crystal Dive Koh Tao

7/1 Moo 2
Tambon Ko Tao
Koh Tao
Surat Thani

E-mail: [email protected]

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