Mantis shrimp

Ecology

Around 400 species of mantis shrimp have currently been discovered worldwide; all living species are in the suborder Unipeltata.

These aggressive and typically solitary sea creatures spend most of their time hiding in rock formations or burrowing intricate passageways in the sea bed. They rarely exit their homes except to feed and relocate, and can be active during the day, nocturnal, or active primarily at twilight, depending on the species. Unlike most crustaceans, they sometimes hunt, chase, and kill prey. Although some live in temperate seas, most species live in tropical and subtropical waters in the Indian and Pacific Oceans between eastern Africa and Hawaii.

Claws

The mantis shrimp's second pair of thoracic appendages has been adapted for powerful close-range combat with high modifications. The appendage differences divide mantis shrimp into two main types: those that hunt by impaling their prey with spear-like structures and those that smash prey with a powerful blow from a heavily mineralised appendage type club. A considerable amount of damage can be inflicted after impact with these robust, hammer-like claws. This club is further divided into three sub-regions: the impact region, the periodic region, and the striated region. Mantis shrimp are commonly separated into two distinct groups determined by the type of claw they possess:

Both types strike by rapidly unfolding and swinging their raptorial claws at the prey, and are capable of inflicting serious damage on victims significantly greater in size than themselves. In smashers, these two weapons are employed with blinding quickness, with an acceleration of 10,400 g (102,000 m/s² or 335,000 ft/s²) and speeds of 23 m/s from a standing start. Because they strike so rapidly, they generate air bubbles between the appendage and the striking surface—known as cavitation bubbles. The collapse of these cavitation bubbles produces measurable forces on their prey in addition to the instantaneous forces of 1,500 newtons that are caused by the impact of the appendage against the striking surface, which means that the prey is hit twice by a single strike; first by the claw and then by the collapsing cavitation bubbles that immediately follow. Even if the initial strike misses the prey, the resulting shock wave can be enough to stun or kill.

The impact can also produce sonoluminescence from the collapsing bubble. This will produce a very small amount of light within the collapsing bubble, although the light is too weak and short-lived to be detected without advanced scientific equipment. The light emission probably has no biological significance but is rather a side-effect of the rapid snapping motion. Pistol shrimp produce this effect in a very similar manner.

Smashers use this ability to attack snails, crabs, molluscs and rock oysters, their blunt clubs enabling them to crack the shells of their prey into pieces. Spearers, on the other hand, prefer the meat of softer animals, like fish, which their barbed claws can more easily slice and snag.

Structural and micromechanical characterization

There are several studies that focus on the appendage club structure and characteristics. A study by David Kisalius, a materials scientist at the University of California, Riverside, showed that the mantis shrimp's club is the most electron-dense region of the stomatopod exoskeleton, being up to five times as thick as its adjacent appendages.

It is possible to observe a clear correlation between the degree of mineralization and its corresponding mechanical properties which was observed using nano-indentation (a process of applying small volumes of pressure to test the mechanical properties of materials) and energy-dispersive spectroscopy (EDS) techniques (an x-ray technique used to analyze the chemical characterization of a sample).

Calcium concentration decreases step by step as the further down the appendage while phosphorus concentration decreases following a gradient line. On the other hand, magnesium and carbon concentrations were increased from the impact surface to the inside of the dactyl club. The region with a higher phosphorus content gives the highest hardness and measured values.

Focusing on the club, it can be divided into three mechanical regions:

The appendages are being studied as a micro-scale analogue for new macro-scale material structures.

Eyes

The mantis shrimp has one of the most elaborate visual systems ever discovered. Compared to the three types of colour-receptive cones that humans possess in their eyes, the eyes of a mantis shrimp carry 16 types of colour receptive cones. Furthermore, the shrimp is capable of tuning the sensitivity of its long-wavelength vision to adapt to its environment. This phenomenon, known as “spectral tuning” is species-specific. Cheroske et al. did not observe spectral tuning in N. oerstedii, the species with the most monotonous natural photic environment. In N. bredini, a species with a variety of habitats ranging from 5-10m (although it can be found up to 20m), spectral tuning was observed, but the ability to alter wavelengths of maximum absorbance was not as pronounced as in N. wennerae: a species with much higher ecological/photic habitat diversity.

The midband region of its eye is made up of six rows of specialised ommatidia—a cluster of photoreceptor cells. Four rows carry up to 16 different photoreceptor pigments, 12 for colour sensitivity, others for colour filtering. The vision of the mantis shrimp can perceive both polarised light and multispectral images. Their eyes (mounted on mobile stalks and capable of moving independently of each other) are similarly variably coloured and are considered to be the most complex eyes in the animal kingdom.

Each compound eye is made up of up to ten thousand side-by-side ommatidia. Each eye consists of two flattened hemispheres separated by six parallel rows of specialised ommatidia, collectively called the midband. This divides the eye into three regions. This configuration enables mantis shrimp to see objects with three parts of the same eye. In other words, each eye possesses trinocular vision and therefore depth perception. The upper and lower hemispheres are used primarily for recognition of form and motion, like the eyes of many other crustaceans.

Rows 1–4 of the midband are specialised for colour vision, from ultra-violet to longer wavelengths. Their UV-vision can detect five different frequency bands in the deep ultraviolet. To do this they use two photoreceptors in combination with four different colour filters. They are not currently believed to be sensitive to infrared light. The optical elements in these rows have eight different classes of visual pigments and the rhabdom (area of eye that absorbs light from a single direction) is divided into three different pigmented layers (tiers), each for different wavelengths. The three tiers in rows 2 and 3 are separated by colour filters (intrarhabdomal filters) that can be divided into four distinct classes, two classes in each row. It is organised like a sandwich; a tier, a colour filter of one class, a tier again, a colour filter of another class, and then a last tier. It is these colour filters that allow the mantis shrimp to see with diverse colour vision. Without the filters, the pigments themselves range only a small segment of the visual spectrum: about 490-550nm. Rows 5–6 are also segregated into different tiers, but have only one class of visual pigment (a ninth class) and are specialised for polarisation vision. They can detect different planes of polarised light. A tenth class of visual pigment is found in the upper and lower hemispheres of the eye.

The midband only covers about 5°–10° of the visual field at any given instant, but like most crustaceans, mantis shrimps' eyes are mounted on stalks. In mantis shrimps the movement of the stalked eye is unusually free, and can be driven in all possible axes of movement—up to at least 70°—by eight eyecup muscles divided into six functional groups. By using these muscles to scan the surroundings with the midband, they can add information about forms, shapes and landscape which cannot be detected by the upper and lower hemisphere of the eye. They can also track moving objects using large, rapid eye movements where the two eyes move independently. By combining different techniques, including movements in the same direction, the midband can cover a very wide range of the visual field.

Some species have at least 16 photoreceptor types, which are divided into four classes (their spectral sensitivity is further tuned by colour filters in the retinas), 12 of them for colour analysis in the different wavelengths (including six which are sensitive to ultraviolet light) and four of them for analysing polarised light. By comparison, most humans have only four visual pigments, of which three are dedicated to see colour, and the human lenses block ultraviolet light. The visual information leaving the retina seems to be processed into numerous parallel data streams leading into the central nervous system, greatly reducing the analytical requirements at higher levels.

At least two species have been reported to be able to detect circularly polarised light. Some of their biological quarter-wave plates perform more uniformly over the visual spectrum than any current man-made polarizing optics, and it has been speculated that this could inspire a new type of optical media that would outperform the current generation of Blu-ray disc technology.

The species Gonodactylus smithii is the only organism known to simultaneously detect the four linear and two circular polarization components required to measure all four Stokes parameters, which yield a full description of polarization. It is thus believed to have optimal polarization vision. They are the only animals known for having dynamic polarization vision. This is achieved by rotational eye movements to maximise the polarization contrast between the object in focus and its background. Because each eye moves independently from the other, it creates two separate streams of visual information.

The huge diversity seen in mantis shrimp photoreceptors likely comes from ancient gene duplication events. One interesting consequence of this duplication is the lack of correlation between opsin transcript number and physiologically expressed photoreceptors. One species may have 6 different opsin genes, but only express one spectrally distinct photoreceptor. Over the years certain mantis shrimp species have lost the ancestral phenotype, although some still maintain 16 distinct photoreceptors and 4 light filters. Species that live in a variety of photic environments have high selective pressure for photoreceptor diversity, and maintain ancestral phenotypes better than species that live in murky waters or are primarily nocturnal.

Suggested advantages of visual system

What advantage sensitivity to polarization confers is unclear; however, polarization vision is used by other animals for sexual signaling and secret communication that avoids the attention of predators. This mechanism could provide an evolutionary advantage; it only requires small changes to the cell in the eye and could be easily selected for.

The eyes of mantis shrimp may enable them to recognise different types of coral, prey species (which are often transparent or semi-transparent), or predators, such as barracuda, which have shimmering scales. Alternatively, the manner in which mantis shrimp hunt (very rapid movements of the claws) may require very accurate ranging information, which would require accurate depth perception.

During mating rituals, mantis shrimp actively fluoresce, and the wavelength of this fluorescence matches the wavelengths detected by their eye pigments. Females are only fertile during certain phases of the tidal cycle; the ability to perceive the phase of the moon may therefore help prevent wasted mating efforts. It may also give mantis shrimp information about the size of the tide, which is important to species living in shallow water near the shore.

It has been suggested that the capacity to see UV light enables observation of otherwise hard-to-detect prey on coral reefs.

Research also shows their visual experience of colours is not that different from humans'. The eyes are actually a mechanism that operates at the level of individual cones and makes the brain more efficient. This system allows the visual information to be preprocessed by the eyes instead of the brain, which would otherwise have to be larger to deal with the stream of raw data and thus require more time and energy. While the eyes themselves are complex and not yet fully understood, the principle of the system appears to be simple. It is similar in function to the human eye but works in the opposite manner. In the human brain, the inferior temporal cortex has a huge amount of colour-specific neurons which process visual impulses from the eyes to create colourful experiences. The mantis shrimp instead uses the different types of photoreceptors in its eyes to perform the same function as the human brain neurons, resulting in a hardwired and more efficient system for an animal that requires rapid colour identification. Humans have fewer types of photoreceptors, but more colour-tuned neurons, while mantis shrimps appears to have fewer colour neurons and more classes of photoreceptors.

An October 2014 publication by researchers from the University of Queensland stated that the compound eyes of mantis shrimp are capable of detecting cancer and the activity of neurons, since they are sensitive to detecting polarised light that reflects differently off cancerous and healthy tissue. The study claims that this ability can be replicated through a camera through the use of aluminium nanowires to replicate polarisation-filtering microvilli on top of photodiodes. In February 2016 it was also revealed that the shrimps are using a form of reflector of polarised light not seen in nature or human technology before. It allows the manipulating of light across the structure rather than through its depth, the typical way polarisers work. This allows the structure to be both small and microscopically thin, and still be able to produce big, bright and colourful polarised signals.

Behaviour

Mantis shrimp are long-lived and exhibit complex behaviour, such as ritualised fighting. Some species use fluorescent patterns on their bodies for signalling with their own and maybe even other species, expanding their range of behavioural signals. They can learn and remember well, and are able to recognise individual neighbours with whom they frequently interact. They can recognise them by visual signs and even by individual smell. Many have developed complex social behaviour to defend their space from rivals.

In a lifetime, they can have as many as 20 or 30 breeding episodes. Depending on the species, the eggs can be laid and kept in a burrow, or they can be carried around under the female's tail until they hatch. Also depending on the species, male and female may come together only to mate, or they may bond in monogamous long-term relationships.

In the monogamous species, the mantis shrimp remain with the same partner for up to 20 years. They share the same burrow and may be able to coordinate their activities. Both sexes often take care of the eggs (biparental care). In Pullosquilla and some species in Nannosquilla, the female will lay two clutches of eggs: one that the male tends and one that the female tends. In other species, the female will look after the eggs while the male hunts for both of them. Once the eggs hatch, the offspring may spend up to three months as plankton.

Although stomatopods typically display the standard types of movement seen in true shrimp and lobsters, one species, Nannosquilla decemspinosa, has been observed flipping itself into a crude wheel. The species lives in shallow, sandy areas. At low tides, N. decemspinosa is often stranded by its short rear legs, which are sufficient for movement when the body is supported by water, but not on dry land. The mantis shrimp then performs a forward flip in an attempt to roll towards the next tide pool. N. decemspinosa has been observed to roll repeatedly for 2 metres (6.6 ft), but specimens typically travel less than 1 m (3.3 ft).

Culinary uses

In Japanese cuisine, the mantis shrimp species Oratosquilla oratoria, called shako (蝦蛄), is eaten boiled as a sushi topping, and occasionally, raw as sashimi.

Mantis shrimp are abundant in the coastal regions of south Vietnam, known in Vietnamese as tôm tít or tôm tech. In regions such as Nha Trang, they are called "ban chai", named for its resemblance to a scrub brush. The shrimp can be steamed, boiled, grilled or dried; used with pepper, salt, and lime; fish sauce and tamarind; or fennel.

In Cantonese cuisine, the mantis shrimp is known as "pissing shrimp" (Chinese: 瀨尿蝦; pinyin: lài niào xiā; Jyutping: laaih niu hā) because of their tendency to shoot a jet of water when picked up. After cooking, their flesh is closer to that of lobsters than that of shrimp, and like lobsters, their shells are quite hard and require some pressure to crack. Usually they are deep fried with garlic and chili peppers.

In the Mediterranean countries the mantis shrimp Squilla mantis is a common seafood, especially on the Adriatic coasts (canocchia) and the Gulf of Cádiz (galera).

In the Philippines, the mantis shrimp is known as tatampal, hipong-dapa or alupihang-dagat and is cooked and eaten like any other shrimp.

In Hawaii, some mantis shrimp have grown unusually large in the very dirty water of the Grand Ala Wai Canal in Waikiki, however the usual dangers associated with consuming seafood caught in contaminated waters still applies to mantis shrimp.

Aquaria

Some saltwater aquarists keep stomatopods in captivity. The peacock mantis is especially colourful and desired in the trade.

While some aquarists value mantis shrimp, others consider them harmful pests, because:

The live rock with mantis shrimp burrows are actually considered useful by some in the marine aquarium trade and are often collected. It is not uncommon for a piece of live rock to convey a live mantis shrimp into an aquarium. Once inside the tank, they may feed on fish, and other inhabitants. They are notoriously difficult to catch when established in a well-stocked tank, and there are accounts of them breaking glass tanks. It should be noted that while stomatopods do not eat coral, the smashers can damage it if they wish to make a home within it.

Example species

A large number of the mantis shrimp species were first scientifically described by one carcinologist, Raymond B. Manning, and the collection of stomatopods he amassed is the largest in the world, covering 90% of the known species.