Mantis shrimp are fascinating creatures with incredible eyesight, and are able to move around at unbelievable speeds. These active hunters range greatly in size from ¾ of an inch to over a foot long, can see much better than sophisticated military imaging software, and are able to strike their prey with the force of a .22 caliber bullet. Mantis shrimp are extremely beautiful and high level predators that exist in coral reefs all over the tropical and sub-tropical waters of the world.

Mantis shrimp, often referred to as stomatopods, play an important role in reef ecosystems by keeping populations of their prey species in check and promoting overall species richness. The burrowing behavior of these benthic crustaceans allows for turnover and oxygenation of sediment keeping reef substrate cleaner and more conducive to life. Mantis shrimp also serve as good bio-indicators because they are so sensitive to environmental pollutants, making them important barometers of coral reef health.

Stomatopods belong to the class Malacostraca, meaning they are related to other members of this class including lobsters, crabs, shrimp, krill, isopods, and amphipods. Stomatopods diverged from these lineages approximately 400 million years ago, so this relationship is quite distant. All mantis shrimp belong to the order Stomatopoda and all living stomatopods are further grouped into the suborder Unipeltata. Stomatopods are aggressive, solitary crustaceans that spend most of their time in a burrow and typically only come out to hunt, relocate, or find a mate; they can be found at a variety of depths from shallow sand beds down to 1500 m. There are over five hundred identified species of mantis shrimp, many of which occur in the Indo-West Pacific and Australia, and they can be divided into two main functional groups: spearers and smashers.

Spearers vs. Smashers

The killing weapons of stomatopods are their claws or raptorial appendages, which can be used either to spear fleshy prey items or smash creatures with hard shells or exoskeletons. The raptorial appendages (rapts for short) of spearers have many spines with barbed tips that are used to catch and stab prey, usually unsuspecting fish. Smashers have clubbed rapts with rudimentary spines that are used to crush their prey, usually crabs, snails, and other mollusks.

"Mantis shrimp, often referred to as stomatopods, play an important role in reef ecosystems by keeping the populations of their prey species in check and promoting overall species richness."

Figure 2: Spearer (top) versus smasher (bottom) raptorial appendages of mantis shrimp (copyright Dr. Roy Caldwell).

The Knock-Out Punch

The raptorial claws of smashers have been studied extensively, and the forces at work that make stomatopods such formidable hunters are amazing. Their strike can reach speeds of 23 m/s with an acceleration of 10,400 g. This striking action is so fast that a camera capable of filming at least 5000 frames per second had to be used to capture the movement. Research by Sheila Patek and Roy Caldwell showed that the strike of a mantis shrimp actually generates two different high-amplitude force peaks that occur within 480 ms of each other. Not only can stomatopods generate impact forces with their rapts ranging from 400-1500 N, this extremely fast movement also causes the formation and collapse of cavitation bubbles. These are responsible for a second force peak of up to 504 N. These collapsing cavitation bubbles are known to emit light and produce heat in the range of several thousand Kelvin as a result of an intense sound wave, such as the snap resulting from the feeding strike of a stomatopod’s raptorial claw. The subject of this study, Odontodactylus scyllarus (Peacock mantis shrimp), which can reach a maximum size of 17 cm (7 inches), “can generate impact forces thousands of times its body weight” (Patek and Caldwell, 2005).

Figure 3: The Peacock mantis shrimp (Odontodactylus scyllarus) can grow up to 7 inches long and has an extremely powerful strike which it uses to crush the shells of its prey (copyright Erwin Kodiat).

The production of this magnitude of force would be impossible for a creature this size without some sophisticated method of energy storage. The hammer-like rapts of these stomatopods have a “latch and spring” mechanism that allows them to generate 470,000 watts of power per kilogram of muscle, a force that can shatter snail shells. There are latch-type structures (sclerites) in the joints that when engaged keep the limb from moving. There is a saddle-shaped structure on top of the appendage that is extremely flexible and is likely important in the reduction of local buckling during the actual striking movement as well as contributing 10% of the stored elastic energy in the limb. The rest of the energy is stored in another structure called the “meral-V” located adjacent to the saddle that is thought to function as a spring, allowing elastic potential energy to be stored without injury or fatigue to the animal. When the latches are disengaged, the smashing appendage is released along with plenty of stored energy and a fresh meal. This collection of specialised joint articulations, elastic structures, and mineralized areas function collectively as a power amplification system giving stomatopods one of the fastest and most powerful feeding strikes yet identified in the animal kingdom. A stomatopod’s strike can be likened to the release of a cross-bow. In both a stomatopod limb and a human arm, muscles are the actual source of the energy, but are not responsible for storing it; instead, mechanical structures like the saddle and meral-V or the cross-bow store this elastic energy, allowing these systems to function to a much higher degree than would be possible with only a muscle contraction.

Figure 4: These illustrations show actual (top) and  cartoon (bottom) drawings of the raptorial appendage of a smashing stomatopod. The saddle/spring is one of the structures that allows mantis shrimp to store an incredible amount of potential energy in their muscles without the claw buckling under pressure (copyright Dr. Sheila Patek.)

"The hammer-like rapts of these stomatopods have a “latch and spring” mechanism that allows them to generate 470,000 watts of power per kilogram of muscle, a force which can shatter snail shells."

Amazing Eyes 

Humans have two types of photoreceptor cells and four visual pigments in their eyes: the rods are useful in dim light for seeing black-and-white images and the cones allow us to see what seems to be a magnificent combination of red, blue, and yellow-green. In comparison, mantis shrimp can have up to sixteen different types of photoreceptors, twelve of which are responsible for colour vision extending into the far-red and ultraviolet ranges, and four that analyze polarized light. Their compound eyes are mounted on stalks that move independently of one another and each eye is composed of up to 10,000 ommatidia (ommatidium — an optical unit composed of photoreceptor cells, a cornea, and an axon to transmit information to the brain). Each eye is divided into three sections giving stomatopods trinocular vision. The upper and lower hemispheres of each eye are responsible for shape recognition and motion and are separated from one another by the midband; the midband is made up of six parallel rows of ommatidia which are specialized for colour vision. Rows one through four are used for color vision ranging from red to ultraviolet and row five and six are specialized for seeing polarized light. Only one species of mantis shrimp, Gonodactylus smithii, is reported to see all six components of polarized light (four linear, two circular) known as Stokes parameters, and consequently has optimal polarization vision.

Figure 5: Close-up of a stomatopod eye showing both hemispheres and the midband. (Copyright Dr. Roy Caldwell).

"Mantis shrimp have the most complex eyes in the animal kingdom. The evolution of such a complex organ is certainly the result of many selective pressures."

There are several theories about why mantis shrimp have the most complex eyes in the animal kingdom. (1) They exhibit cryptic colouration on their bodies that can only be seen through a polarized lens or by another mantis shrimp, indicating that there is some form of intraspecies communication and/or sexual signaling occurring between different stomatopods species. (2) They live in extremely complex environments where prey and predators abound, and they need to be able to quickly identify hard-to-see prey items such as transparent cleaner shrimp or camouflaged crabs and snails. They also have to spot predatory fish with shimmering bodies such as barracuda. (3) Their striking movements are extremely fast and stomatopods require acute depth perception in order to hunt successfully. (4) Many mantis shrimp species actively fluoresce during mating rituals, and their eye pigments are tuned to recognize the precise wavelengths of one anothers’ fluorescence. (5) Visual data has to be analyzed in real time by the eye because the invertebrate brain cannot process that much information in a timely fashion. The evolution of such a complex organ is certainly the result of many selective pressures and cannot be explained by one school of thought.

Development

Some stomatopod species are promiscuous while others are monogamous; in monogamous species, a pair can stay together for as long as twenty years and some exhibit biparental  egg care. Some species lay two clutches of eggs and each parent cares for its own clutch, whereas in other species the male hunts for both of them while the female tends to the eggs. After the eggs hatch, the offspring spend a short amount of time as bottom-dwelling larvae. After they have completed a few molts, they spend one to several months as completely transparent, predatory, planktonic alima larvae before adopting their benthic, burrowing adult lifestyle.  

Figure 6: Planktonic stomatopod larvae, at early (left) and late (right) stages (copyright Image Quest 3d).

Stomatopods in home aquaria  

Mantis shrimp are absolutely fascinating when kept as pets. They are not considered “reef safe” for home aquaria due to their habit of knocking over delicate corals while burrowing and their inclination to eat other much more expensive tank mates. They consequently do well in species tanks where all other inhabitants are considered fair game as a meal. There are rumours of stomatopods breaking aquarium glass, but this likely only occurs when large stomatopods are burrowing in shallow substrate or when they are housed in tanks that are too small for their adult size; it is therefore recommended that they be kept in acrylic tanks to avoid the possibility of breakage.

"The uniqueness and abundance of stomatopods is a testament to the diversity fostered by the complexity of coral reef ecosystems."

The peacock mantis shrimp (O. scyllarus) is a popular aquarium species because of its vibrant colors and curious nature. Neogonodactylus wennerae is also a common home aquarium species because it stays relatively small (5-7 cm) and doesn’t require a large aquarium. Gonodactylus smithii is also commonly kept in home aquaria because it is colourful, interactive and hardy. When considering stomatopods for a home aquarium, it is important to be a responsible hobbyist. Remember that similar to many popular aquarium fish species, mantis shrimp are often collected in a non-sustainable fashion, so specimens must be obtained from people or companies that do not use bleach, dynamite, or any other environmentally harmful practices to collect their animals.

Stomatopods are just another one of the endless stream of fascinating creatures which inhabit reef ecosystems all over the world. The uniqueness and abundance of stomatopods are a testament to the diversity fostered by the complexity of coral reef ecosystems. Coral reefs and their inhabitants need to be protected by way of public education and environmental stewardship so that everyone can understand and experience the beauty of reefs and their incredible creatures for generations to come.

Acknowledgements

The author wishes to thank Dr. Roy Caldwell and Dr. Sheila Patek for their time and expertise in contributing to this article.

References

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Patek, S.N., Nowroozi, B. N., Baio, J. E., Caldwell, R. L., and A. P. Summers. 2007. Linkage mechanics and power amplification of the mantis shrimp's raptorial strike. Journal of Experimental Biology 210:3677-3688.

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