I am talking about the cute yellow boxfish. They are well known for their cube like body shape and bright yellow and black spotted coloration. Only few people know that this cute little box fish can secret a toxic mucus when it feels threatened.

Did you know that boxfish have an armoured and rigid body called a carapace, which covers the head and body. Yes, it is a similar style carapace as the one of a turtle. It has openings for the mouth, nostrils, gill opening, anus, caudal peduncle, and fins. The rigid body shape provided by the carapace was thought to help the fish stabilize while swimming as the water flows over the body. It was thought to keep them on course in high-flow, turbulent conditions. However, the carapace has little effect on stability. In fact, it is destabilizing, as it increases drag. Instead, the caudal fin is the stabilizing force and acts as a rudder. Additionally, boxfish have a keel that help them in swimming. A keel is a ridge near the caudal peduncle which helps with stabilization against roll. Imagine without it, the boxfish would be rolling around in the ocean.

The boxfish is found in reefs throughout the Pacific and Indian Ocean as well as in some parts of the Atlantic Ocean. It reaches a maximum length of 45cm.

Defence mechanisms

The yellow boxfish is able to secrete the neurotoxin ‘ostracitoxin’ as mucus when stressed. This is highly toxic but not to worry, it is only dangerous to other fish not to humans. This toxin will break down the red blood cells and clump them together which will obstruct the blood flow in the fish. The toxin does not even have to be ingested / eaten. A fish may be killed by just swimming past and being exposed to this neurotoxin.

The bright yellow color and black spots are a form of warning coloration to any potential predators. In juveniles, the bright warning coloration is brighter. As it ages, the brightness fades and very old boxfish have blue-grey to even black coloration with faded yellow.

Communication

The yellow boxfish is able to make sounds. They are able to produce hums and clicking noises using the muscles of the swim bladder. The spotted box fish is also able to produce sounds The yellow boxfish however can make louder sounds and it has a wider range of click sounds. Males are able to call for a longer period of time than females and juveniles. Probably the purpose of the hum and click sounds are to either communicate with members of the same species or to deter predators.

As a model for engineering

In 2006, Mercedes-Benz unveiled its Bionic concept car which was inspired by the shape of the yellow boxfish. It was assumed that due to the extreme agility with which boxfish move, that their shape was aerodynamic and self-stabilizing. But instead their agility is due to the combination of an aerodynamically unstable body and the manner in which the fish use their fins for movement.

Box girders were also modelled after the yellow boxfish. Box girders are used in bridge construction. Engineers found that yellow boxfish have very sturdy skeletons, and a shape that has low-flow resistance. They used this structure in order to reduce drag of box girders. The box girders are similar to the box-like body shape and they also incorporate the yellow boxfish mouth. The mouth of the yellow boxfish is the structure that is associated with its low-flow resistance.

Engineers used the boxfish carapace and shape to design an airship with higher manoeuvrability, stability, and overall flight.

While you would think that the boxfish’s shape was inefficient in movement, it is quite the opposite. The yellow boxfish is inspiring many engineers to improve steering and movement with its shape.