Scientists in the US have solved one of nature’s little mysteries – how the harlequin mantis shrimp can generate enough force to smash aquarium glass, without doing any significant damage to its pair of “dactyl” clubs. The researchers believe that the secret of the clubs, which are normally used by the shrimp to crack open tough shellfish, lies in how they combine materials with very different properties. Measurements reveal that the clubs have a much higher specific strength and toughness than any synthetic composite material – a finding that the researchers think could lead to stronger materials, including those for use in body armour.

Measuring just 3–18 cm in length, the harlequin mantis shrimp (Odontodactylus scyllarus) can accelerate its clubs to reach speeds in excess of 80 km per hour, allowing it to deliver an instantaneous force of more than 700 N. In addition to this blunt force, air bubbles are trapped between the club and the creature’s shell, which then collapse to create regions of great local stress. This process can be repeated thousands of times without any apparent damage to the club, which is eventually replaced in a moulting process.

To learn more about the shrimp’s prowess, James Weaver of Harvard University, Garrett Milliron of the University of California, Riverside and Ali Miserez of the Nanyang Technological University in Singapore used a variety of different analytical techniques to determine the structure and composition of the club, including electron microscopy, X-ray microtomography, synchrotron X-ray diffraction and energy-dispersive X-ray spectroscopy.

Fragile surface?

One surprising aspect of the club is that its striking face is made of a layer of very hard crystalline calcium-phosphate ceramic material, known as hydroxyapatite, that is about 60 µm thick. On its own, such a material would not be very durable because it would be likely to fracture on impact. Behind this hard surface, however, the team found a much thicker region comprising layers of fibres made from the polysaccharide chitosan – a much more elastic material that is commonly found in the exoskeletons of shrimp.

Each successive layer of fibres is parallel to the surface but offset from its neighbour by a small angle such that the layers are rotated 180° over a distance of about 75 µm. The club is held together at the edges by a third structure made from chitosan fibres. The team believes that it is the layered “helicoidal” structure that gives the club its extreme resistance to fracturing. Any crack propagating through the material, the researchers say, would have to continually change direction – making fracture unlikely.

Cracks change direction

The team also found that the elastic modulus of the club changes as a function of depth – the effect being that some of the impact energy is reflected back towards the surface as the shock propagates into the club. This change in the modulus could also reflect a propagating crack, thus reducing the risk of fracture. The team believes that insights from the shrimp’s formidable club could lead to better body armour based on composites of hard ceramic and elastic organic materials.

This is not the first time that mantis shrimps have been on biophysicists’ radar. The animals are known to have a highly developed visual system and in 2008 researchers showed that two species of the shrimp can detect the circular polarization of light – the first living organisms shown to do so.

The study is described in Science.