Figure 1: Pictured is the brightly coloured peacock mantis shrimp (Odonto s) Taken from: https://www.tes.com/lessons/x12PkPUDnaxDVQ/model-peacock-mantis-shrimp
Figure 1: Pictured is the brightly coloured peacock mantis shrimp (Odontodactylus scyllarus) Image available here.

Mantis shrimp (Stomatopoda) are well renowned for their extremely fast predatory strikes on prey. In other animals, these powerful manoeuvres are limited by slow muscle contractions however, stomatopods have developed mechanisms to overcome this restriction, none more so than the Peacock Mantic Shrimp (Odontodactylus scyllarus). O. scyllarus (Fig. 1) is a highly radiant stomatopod that is commonly found in crevices of corals and rocks on the ocean floor (3-40m) of the warmer waters (22-28°C) of the Indian and Pacific Oceans.

The punch line

Figure 2: The mechanism behind Odontodactylus scyllarus strike. The "saddle" modelled as a spring (orange) in a diagram of the arm of O. scyllarus, storing energy. The top diagram shows pre-strike, the muscle (red) compresses the "saddle", while a click mechanism is engaged. The bottom diagram shows the strike occurring, as the latch is released enabling the "saddle" to extend. The circles on both and top and bottom diagrams represent the rotational movement of the meral-v.
Figure 2: The mechanism behind Odontodactylus scyllarus strike. The saddle in a diagram of the arm of O. scyllarus, stores energy. Left diagram shows pre-strike, the extensor muscle compresses the saddle, while a click mechanism (latch) is engaged. Right diagram shows the strike occurring, as the latch is released enabling the saddle to extend. Image available here

O. scyllarus has the capability of generating the fastest recorded feeding strike in the animal kingdom. Its arms are hinged with club-like attachments on the ends, releasing  the same amount of force as a 22 calibre bullet, which is impossible to see with the naked eye. It is the fastest recorded move in the ocean that muscles alone could never achieve. To achieve such force, O. scyllarus feeding appendages use a saddle-shaped part of the exoskeleton (Fig. 2) , that acts like a spring. During the strike, elastic energy is stored in here on the dorsal side of the merus of all stomatopod species. This mechanism is similar to that of a hyperbolic paraboloid spring, usually used in engineering and architecture: their curvature reduces the chance of breakage by distributing stress across a 3-dimensional surface. This is the first biological hyperbolic-paraboloid spring to be identified.

In order to produce such an extreme movement in water, a substantial sum of energy has to be released in a short period of time. The power of the ‘punch’ is increased by a ‘click mechanism’ that was discovered by biologist Malcolm Burrows in the 1960’s, which prevents the movement of the club until muscle contraction is at its maximum by use of a latch system. The click mechanism is used for a short period of time before the animal wishes to strike prey. When the latch is released, the previously stored energy is delivered over a shorter period of time than the muscle contraction. A system of pivots at the leg joint further increases the extreme speed of the punch (Fig. 2). A saddle-shaped spring is required in the movement of these animals because muscle tendons and fibres do not store enough energy. In a typical strike, 4.7 x 105 watts per kilogram are required at minimum; which cannot be achieved by the use of only muscles.

Figure 3: Image shows a cavitation bubble produced by the peacock mantis shrimp (OS). The bubble is produced at a strike at 23m/s (with forces of about 8,000 G), which is fast enough to rip water apart and create a vacuum (cavitation bubble). Image taken from: http://www.nature.com/nature/journal/v428/n6985/full/428819a.html
Figure 3: Image shows a cavitation bubble produced by the peacock mantis shrimp (Odontodactylus scyllarus). The bubble is produced at a strike at 23m/s (with forces of about 8,000 G), which is fast enough to rip water apart and create a vacuum (cavitation bubble). Image available here.

The strike of O. scyllarus causes cavitation (Fig. 3). Cavitation forms from vapour bubbles that collapse and yield considerable energy in fluids of low pressure, producing heat, light and sound. Cavitation in O. scyllarus produces a loud popping sound and can be sufficient enough to break boat propellers and similar hard surfaces. Although their clubs are highly mineralised, the surface may become damaged over time, thus O. scyllarus have to moult every few months to combat this.

Eyes

As well as being able to pack a punch, they also have the most complex eyes found in the animal kingdom. 12 colour-sensitive cones and millions of photoreceptor cells enable O. scyllarus to detect 10 times more colour than the human eye (that contains only 3 colour-sensitive cones). The eyes are specialised; they are made up of two flattened hemispheres that are separated by six rows of ommatidia (midband) dividing the eye into three regions, giving the mantis shrimp trinocular vision and by extension, depth perception. Rows 1-4 of the midband are used for visualising the ultraviolet part of the spectrum and rows 5-6 are specialised for polarisation vision, to scrutinise the angle at which light waves are travelling; allowing them to accurately pin-point prey (gastropods, crustaceans, molluscs), which is needed when you have a punch that can reach 50mph in 3 milliseconds.

Anthropogenic uses

Mantis shrimp have to capability to detect where the ocean is polarised and where it is not, due to the polarising abilities of their eyes. Some take this one step further and create their own polarised surface on a part of their body. This polarisation is used as a code between mantis shrimp and can be used territoriality (they can tell if another mantis shrimp has claimed a burrow by the way the light is hitting its body). Recently (November, 2016), a group of scientists have been able to use the ability of these mantis shrimp to diagnose injuries and disease by reverse engineering a mantis shrimp’s eye. Some cancers can be revealed by using polarised light. This just goes to show that although the peacock mantis shrimp has the ability to destroy, it may also be the key to saving millions of lives.

 

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