Mantis shrimp
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.
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 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. These claws can accelerate at a rate comparable to that of a.22 caliber bullet when fired, having around 1500 newtons of force with each swing/attack. 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 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 ; 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 : 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.
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 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 omatidial 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. 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, 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 to far-red 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. 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 is divided into three different pigmented layers, each for different wavelengths. The three tiers in rows 2 and 3 are separated by colour 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, 12 for colour analysis in the different wavelengths 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.
The midband covers only about 5 to 10° of the visual field at any given instant, but like most crustaceans, mantis shrimps' eyes are mounted on stalks. In mantis shrimp, the movement of the stalked eye is unusually free, and can be driven up to 70° in all possible axes of movement 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 hemispheres of the eyes. They can also track moving objects using large, rapid eye movements where the two eyes move independently.