Mantis shrimp are carnivorous marine crustaceans of the order Stomatopoda (). Stomatopods branched off from other members of the class Malacostraca around 400 million years ago, with more than 520 extant species of mantis shrimp known. All living species are in the suborder Unipeltata, which arose around 250 million years ago. They are among the most important predators in many shallow, tropical and subtropical marine habitats. Despite being common in their habitats, they are poorly understood, as many species spend most of their lives sheltering in burrows and holes.
Dubbed "sea locusts" by ancient Assyrians, "prawn killers" in Australia, and now sometimes referred to as "thumb splitters" due to their ability to inflict painful wounds if handled incautiously, mantis shrimp possess powerful raptorial appendages that are used to attack and kill prey either by spearing, stunning, or dismembering; the shape of these appendages are often used to classify them into groups: extant mantis shrimp either have appendages which form heavily mineralized "clubs" that can strike with great power, or they have sharp, grasping forelimbs used to swiftly seize prey (similar to those of mantises, hence their common name).
Description
Mantis shrimp typically grow to around in length, while a few species such as the zebra mantis shrimp can reach up to . A mantis shrimp's carapace covers only the rear part of the head and the first four segments of the thorax. Mantis shrimp widely range in colour, with some species mostly being shades of brown, while others have multiple contrasting, vivid colours. Claws The mantis shrimp's second pair of thoracic appendages is adapted for powerful close-range combat. 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 club-like appendage. A considerable amount of damage can be inflicted after impact with these robust, hammer-like claws. This club is further divided into three subregions: the impact region, the periodic region, and the striated region. Mantis shrimp are commonly separated into distinct groups (most are categorized as either spearers or smashers but there are some outliers) as determined by the type of claws they possess:
Spearers are armed with spiny appendages - the spines having barbed tips - used to stab and snag prey. These raptorial appendages resemble those of mantises, hence the common name of these crustaceans. This is the type found in most mantis shrimp. Smashers possess a much more developed club and a more rudimentary spear (which is nevertheless quite sharp and still used in fights between their own kind); the club is used to bludgeon and smash their prey apart. The inner aspect of the terminal portion of the appendage can also possess a sharp edge, used to cut prey while the mantis shrimp swims. This is found in the families Gonodactylidae, Odontodactylidae, Protosquillidae, and Takuidae. Spike smashers (hammers or primitive smashers): An unspecialized form, found only in the basal family Hemisquillidae. The last segment lacks spines except at the tip, so it is not as effective at spearing but can also be used for smashing. Hatchet: An unusual, highly derived appendage that only a few species have. This body plan is largely unresearched.
Both types strike by rapidly unfolding and swinging their raptorial claws at the prey, and can inflict serious damage on victims significantly greater in size than themselves. In Odontodactylus scyllarus, 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 from a standing start. Because they strike so rapidly, they generate vapor-filled bubbles in the water 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 collapsing bubbles also produce light, which is termed sonoluminescence.
The species Gonodactylus smithii has the fastest strikes reported for any species of mantis shrimp species, with a speed of 30.6 m/s (110 km/h; 68 mph) and an acceleration of 1.5×105 m/s2. The species O. scyllarus achieves an acceleration of 1×105 m/s2. By comparison, a Beretta 92S with a 127 mm barrel, 9 mm caliber bullets and a muzzle speed of 335 m/s (1206 km/h; 749 mph) achieves an acceleration of 4.41×105 m/s2.
The power amplifier spring mechanism mediated by a latch has the limitation that the time taken for the strikes being executed is shorter than the nerve conduction time of the mantis shrimp; consequently, the strike cannot be controlled or directed whilst it is being executed, meaning that the trajectory of the strike must be prepared either before or after it is delivered.
Smashers use this ability to attack crabs, snails, rock oysters, and other molluscs, their blunt clubs enabling them to crack the shells of their prey into pieces. Spearers, however, prefer the meat of softer animals, such as fish and cephalopods, which their barbed claws can more easily slice and snag.
The appendages are being studied as a microscale analogue for new macroscale material structures.
Eyes
The eyes of the mantis shrimp are mounted on mobile stalks and their eyes can move independently of each other. The extreme mobility allows them to be rotated in all three dimensions, yet the position of their eyes has shown to have little effect on the perception of their surroundings. Mantis shrimp are thought to have the most complex eyes in the animal kingdom and the most complex front-end for any visual system ever discovered.
Each compound eye is made up of tens of thousands of ommatidia, clusters of photoreceptor cells. Each eye consists of two flattened hemispheres separated by parallel rows of specialised ommatidia, collectively called the midband. The number of Ommatidiumomatidial]] rows in the midband ranges from two to six. This divides the eye into three regions. This configuration enables mantis shrimp to see objects that are near the mid-plane of an eye with three parts of the same eye (as can be seen in some photos showing three pseudopupils in one eye). In other words, each eye possesses trinocular vision, and therefore depth perception, for objects near its mid-plane. The upper and lower hemispheres are used primarily for recognition of form and motion, like the eyes of many other crustaceans.
Compared with the four types of photoreceptor cell that humans possess in their eyes, the eyes of a mantis shrimp have between 12 and 16 types of photoreceptor cells. Furthermore, some of these stomatopods can tune the sensitivity of their long wavelength colour vision to adapt to their environment. This phenomenon, called "spectral tuning", is species-specific. Cheroske et al. did not observe spectral tuning in Neogonodactylus oerstedii, the species with the most monotonous natural photic environment. In N. bredini, a species with a variety of habitats ranging from a depth of 5 to 10 m (although it can be found down to 20 m below the surface), 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 diversity of spectral tuning in Stomatopoda is also hypothesised to be directly linked to mutations in the retinal binding pocket of the opsin.
The huge diversity seen in mantis shrimp photoreceptors likely comes from ancient gene duplication events. One consequence of this duplication is the lack of correlation between opsin transcript number and physiologically expressed photoreceptors. One species may have six different opsin genes, but only express one spectrally distinct photoreceptor. Over the years, some mantis shrimp species have lost the ancestral phenotype, although some still maintain 16 distinct photoreceptors and four 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.
Mantis shrimp can perceive wavelengths of light ranging from deep ultraviolet (300 nm) to far-red (720 nm) and polarised light. In mantis shrimp in the superfamilies Gonodactyloidea, Lysiosquilloidea, and Hemisquilloidea, the midband is made up of six ommatidial rows. Rows 1 to 4 process colours, while rows 5 and 6 detect circularly or linearly polarised light. Twelve types of photoreceptor cells are in rows 1 to 4, four of which detect ultraviolet light. Despite the impressive range of wavelengths that mantis shrimp have the ability to see, they do not have the ability to discriminate wavelengths less than 25 nm apart. It is suggested that not discriminating between closely positioned wavelengths allows these organisms to make determinations of its surroundings with little processing delay. Having little delay in evaluating surroundings is important for mantis shrimp, since they are territorial and frequently in combat. However, some mantis shrimp have been found capable of distinguishing between high-saturation and low-saturation colors.
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.Mantis shrimp wear tinted shades to see UV light . Latimes.com (2014-07-05). Retrieved on 2015-10-21. They are currently believed to be insensitive 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, with two classes in each row. Each consists of a tier, a colour filter of one class, another tier, a colour filter of another class, and then a last tier. These colour filters allow the mantis shrimp to see with diverse colour vision. Without the filters, the pigments themselves make up only a small segment of the visual spectrum, from about 490 to 550 nm. Rows 5 and 6 are also segregated into different tiers, but have only one class of visual pigment, the ninth class, and are specialised for polarisation vision. A tenth class of visual pigment is found in the upper and lower hemispheres of the eye.
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 for colour analysis in the different wavelengths (including six which are sensitive to ultraviolet light) and four for analysing polarised light. By comparison, most humans have only four visual pigments, of which three are dedicated to seeing colour, and human lenses block ultraviolet light. The visual information leaving the retina seems to be processed into numerous parallel data streams leading into the brain, greatly reducing the analytical requirements at higher levels.