Mantis shrimp
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Mantis shrimp are marine crustaceans belonging to the order Stomatopoda, one part of the class Malacostraca, the largest class of crustaceans. They are neither shrimps nor mantids, but receive their name purely from the physical resemblance to both the terrestrial praying mantis and the shrimp. They may grow to a length of 20–30 cm, although most species are considerably smaller, and their carapace covers only the rear part of the head and the first three segments of the thorax. Mantis shrimp appear in a variety of colours, from rather dull browns to stunning neon.
Called "sea locusts" by ancient Assyrians, and now sometimes referred to as "thumb splitters" by modern divers - because of the relative ease the creature has in mutilating small appendages - mantis shrimp sport powerful claws, formed like jackknives, that they use to attack and kill prey by spearing, stunning or dismemberment. Some pet mantis shrimp have managed to break through their double-paned aquarium glass with a single strike from this weapon.
Ecology
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 either wait for prey to chance upon them or, unlike most crustaceans, actually hunt, chase and kill living prey. They rarely exit their homes except to feed and relocate, and can be diurnal, nocturnal or crepuscular, depending on the species. Most species live in tropical and subtropical seas (Indian and Pacific Oceans between eastern Africa and Hawaii), although some live in temperate seas.
- Nha Trang, Vietnam
- Mediterranean Sea
- Ala Wai Canal, Hawaii
- Kiribati
- Christmas Island
- Chesapeake Bay
Classification and the claw
Around 400 species of mantis shrimp have currently been described worldwide, which are commonly separated into two distinct groups determined by the manner of claws they possess:- spearers are armed with spiny appendages topped with barbed tips, used to stab and snag prey and some have a blunt, calcified club on the elbow
- smashers, on the other hand, possess a much more developed club and a more rudimentary spear (which is nevertheless quite sharp); the club is used to bludgeon and smash their meals apart. The inner aspect of the dactyl (the terminal portion of the appendage) can also possess a sharp edge, with which the animal can cut prey while it swims. Any aquarium store that has a resident mantis shrimp of this type can put on a blood-sport show with a feeder goldfish and a hungry shrimp.
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.
A mantis shrimp, housed at the Sea Life Centre, Norfolk, England, shattered quarter-inch-thick glass at its enclosure in 1998. The 4-inch shrimp was relocated to a private tank, and nicknamed Tyson.
The eyes
"It's been blowing us away for years how complex the stomatopod visual system is. There's no question this is the most complex eye in the animal kingdom. It has the same capability in the ultraviolet alone that we have in normal light."
-Roy Caldwell, UC Berkeley
Mantis shrimp are the only animals with hyperspectral colour vision. Their eyes - both mounted on mobile stalks and constantly moving about (apparently independently of one another) - are similarly variably coloured, and are considered to be the most complex eyes in the animal kingdom. They permit both serial and parallel analysis of visual stimuli. The compound eye is made up from up to 10,000 separate elements and are of the apposition type. Each eye consists of two flattened hemispheres separated by six parallel rows of highly specialised ommatidia called the midband. This divides the eye into three regions: the dorsal and ventral hemispheres and the midband. This is a design which makes it possible for them to see objects with three different parts of the same eye, in other words; each individual eye possesses trinocular vision and depth perception. The upper and lower hemispheres are used primarily for recognition of forms and motion, not color vision, like the eyes of so many other crustaceans.
It is the six-rowed midband that is the really unique part of the three regions. Rows 1-4 are specialised for colour vision, from ultra-violet to infra-red. The optical elements in these rows have 8 different classes of visual pigments and the rhabdom is divided into 3 different pigmented layers (tiers), each adapted 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. Rows 5-6 are segregated into different tiers too, have only one class of visual pigment (a ninth class), are specialised for polarisation vision and can see different planes of polarised light; perhaps they are even able to detect circularly polarised light. A tenth class of visual pigment is found in the dorsal and ventral hemispheres of the eye.
The midband only covers a small area on about 5-10° of the visual field at any given instant, but as mentioned, the eyes are mounted on stalks. This is also the case for other groups of crustaceans, but in mantis shrimps the movement of the stalked eye is unusually free, and can be driven in all possible axes, up to at least 70°, of movement by eight individual eyecup muscles divided into six functional groups. By using these muscles for scanning the surroundings with the midband, they can add information about forms, shapes and landscape which can not 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 are moving independently. By combining different techniques, as those mentioned and saccadic movements, the midband can cover a very wide range of the visual field in front of the mantis shrimp.
Some species have at least sixteen different photo-receptor types who are divided into four classes (their spectral sensitivity is further tuned by colour filters in the retinas), twelve of them for colour analysis in the different wavelengths (including four which are sensitive to ultraviolet light) and four of them for analysing polarised light. By comparison, humans have only three visual pigments. 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.
Possible rationales for such excellent vision have included:
- The ability to see smaller prey species, which are often transparent or semitransparent. But mantis shrimp as an order eat a wide variety of species and sizes.
- The ability to evade predators such as barracuda, which have shimmering scales. But their primary defense is a burrow or reef habitat.
- The need for accurate ranging to targets, necessitated by the short, powerful stroke of the claws. This explains the multiform depth perception, but not hyperspectral capability.
- The need to see fluorescent displays of other mantis shrimp, for mating rituals or territorial disputes. Shortly after the discovery of active fluorescence, the wavelengths were measured to coincide with receptor sensitivities.
- The need to see lunar cycles underwater, as the females of many species are only fertile at certain points. Failure to correctly interpret a courtship ritual could lead to loss of a burrow, and possibly up to and including death.
- The ability to distinguish reef corals and flora, much as insects distinguish flowers. However, animals with a far greater dependence on coral do not have such developed sight.
- The small, invertebrate brain cannot handle the incoming data as a single stream from each eye. It offloads signal processing to the eye, at the mechanical level.
- The ability to live at a range of depths, with variable illumination levels and filtered color values.
- The ability to see lunar cycles, to predict tidal depths. But mantis shrimp range from coastal tidepools, to open-water depths of 40 meters, where tides are largely irrelevant.
Behaviour
Mantis shrimp appear to be highly intelligent, are long-lived and exhibit complex behaviour, such as ritualised fighting. Scientists have discovered that some species use fluorescent patterns on their bodies for signaling 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 a 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 carried around under the female's tail until they hatch. Also depending on the species, male and female come together only to mate or 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 there are reasons to suspect that these pairs can coordinate their activities. Both sexes are often taking 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 till three months as plankton.
Most stomatopods display the standard locomotion types as 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 when the body is supported by water. The mantis shrimp then performs a forward flip, in attempts to roll towards the next tidepool. Caldwell once observed an N. decemspinosa to roll repeatedly for a total of six feet.
Cookery
In Japanese cuisine, the mantis shrimp is eaten as sashimi and as a sushi topping, and is called shako (蝦蛄).In Cantonese cuisine, the mantis shrimp is a popular dish known as "pissing shrimp" (濑尿虾, Mandarin pinyin: lài niào xiā, modern Cantonese: laaih niuh hā) due to its tendency to urinate when cooked. Because of this, mantis shrimp are speared to induce them to evacuate their bowels prior to being introduced into the cookpot. 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.
There are health concerns associated with consuming mantis shrimp, as the shrimp is known to dwell in contaminated waters.
External links
- [Crustacea.net Stomapoda pages]
- [The Lurker's Guide to Stomatopods - mantis shrimp]
- [Mantis shrimp - colourful and aggressive]
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