Mantis shrimp are aggressive marine crustaceans that possess spiny, raptorial appendages resembling those of the praying mantis, an insect from the order Mantodea. Although these creatures are named shrimp, they are not true shrimp (infraorder Caridea) but distant cousins of them, belonging to the order Stomatopoda (hence also called stomatopods). They are extremely colorful, ranging from brown to brighter, fluorescent shades, and they often flash these colors at potential mates to attract them.
These arthropods are mostly found in the Pacific and the Indian oceans, hiding in burrows and crevices or under rocks. Though primarily solitary, they are territorial and defensive, launching power-packed punches at their enemies using their claws.
They are typically around 3.9 in (10 cm) long, but a few may reach up to 15 in (38 cm) in length.
The largest species, the zebra mantis shrimp (Lysiosquillina maculata), measures 15.7 in (40 cm).
As crustaceans, mantis shrimp have bodies divided into three segmented regions: the head, thorax, and abdomen. A hard carapace covers the rear part of the head and the first four thoracic segments, providing protection. The head also bears the eyes and antennae, which help in detecting prey and the environment.
Mantis shrimp possess eight pairs of legs, with the first five pairs modified into claws for grasping and holding prey. The abdominal legs, or pleopods, are specially adapted for swimming.
The second pair of thoracic limbs, called maxillipeds, are adapted for close-range combat with prey. Depending on the morphology of these limbs, mantis shrimp are mainly categorized as either ‘spearers’ or ‘smashers.’
In spearers, the limbs are spiny, with barbed tips designed for piercing prey. These raptorial limbs resemble the forelegs of praying mantises. In contrast, smashers have heavily mineralized, club-like appendages that help deliver powerful blows, breaking apart their prey. These clubs consist of three distinct subregions: the impact region, the periodic region, and the striated region.
Although rare, some mantis shrimp display limbs that are hatchet-like or are intermediate between the spearing and smashing types (known as spike smashers).
Mantis shrimp have a pair of stalked compound eyes that move independently of each other and in all dimensions, thereby providing an all-round vision of their surroundings. The eyes are controlled by eight eyecup muscles, which can enable movement of up to 70 degrees in all possible axes. Such flexibility allows for an intense scanning of the forms, shapes, and landscapes around the animals.
Each eye can be divided into two flattened hemispheres made of numerous ommatidia, which are clusters of photoreceptor cells. These hemispheres, used primarily for sensing form and motion, are separated by a midband comprising parallel rows of specialized ommatidia. Such segregation divides the eye into three distinct regions, and thus, these animals are thought to have ‘trinocular vision.’
Depending on the species, the number of these ommatidial rows varies from two to six. For instance, in members of the superfamilies Gonodactyloidea, Lysiosquilloidea, and Hemisquilloidea, the midband comprises six ommatidial rows, of which rows 1 to 4 help in processing colors. These four rows have between 12 to 16 types of photoreceptor cells that detect ultraviolet (UV) light. In contrast, rows 5 and 6 sense circular or linear polarized light.
Six mantis shrimp species reportedly detect circularly polarized light by converting it into linear polarized form, an ability not yet documented in any other animal. The species Gonodactylus smithii simultaneously detects both circular and linear polarized light.
Ommatidial Rows
These rows contain eight different classes of visual pigments distributed in three different pigmented layers called tiers, each for sensing different wavelengths. These tiers comprise multiple color filters that allow mantis shrimp to detect a wide range of wavelengths, from deep ultraviolet (300 nm) to far-red (720 nm). In the absence of these filters, the visual pigments can detect only a narrow range of the visual spectrum, around 490 to 550 nm.
Some mantis shrimp, like Haptosquilla trispinosa, can tune the sensitivity of wavelength vision according to their environment, a feature known as spectral tuning.
These shrimps are named after the praying mantis because of the sheer resemblance of their raptorial appendages. In ancient times, mantis shrimp were referred to as ‘sea locusts’ by the Assyrians. In Australia, they are sometimes called ‘prawn killers,’ while the nickname’ thumb splitters’ reflects the painful wounds they can inflict if handled carelessly.
Currently, over 520 species of mantis shrimp are divided into 7 superfamilies and 17 families. All species belong to the broad suborder Unipeltata.
Based on fossil records, the oldest undisputed stem-group mantis shrimp date back to around 359 to 300 million years ago in the Carboniferous Period. These members belonged to the groups Palaeostomatopodea and Archaeostomatopodea, with the latter being more closely related to modern members.
Additionally, members of the group Unipeltata first appeared around 250 million years ago in the Triassic Period.
Most mantis shrimp species are found in the tropical and subtropical waters of the Indo-West Pacific region, specifically in the Indian and Pacific Oceans between Hawaii and East Africa.
They are mostly found hidden in soft sediments, burrows, and coral cavities or crevices. Since they undergo periodic growth, they often switch to new and larger cavities or burrows during development.
Their diet depends on how they hunt their prey. While spearers generally consume soft-bodied animals, like worms, squid, and fish, smashers eat harder creatures, such as snails, clams, and small crustaceans.
Mantis shrimp are typically solitary and territorial animals that barely leave their burrows. When they do, they do so in search of food or to relocate. Depending on the species, they are diurnal (active in the day), nocturnal (active at night), or crepuscular (active at twilight).
They are extremely intelligent and can easily recognize their neighbors, and they frequently interact with them through visual signs and smells.
They move from one place to another using a unique hybrid metachronal propulsion. In this mechanism, the shrimp move their paddle-like pleopods from a posterior to an anterior direction during the power stroke. During the recovery stroke, they draw these limbs back almost synchronously.
As territorial crustaceans, mantis shrimp actively defend their owned spaces and hunt or kill their enemies if needed. They have evolved complex social behaviors to protect their territories from their rivals.
They do not instantly attack when threatened. Instead, they stand on their hind limbs, puff up their chests, and spread their arms to appear large and formidable to their enemies. Such a display is called a meral spread. If the attacker fails to shy away in fright, these shrimp switch to violence.
These animals hurl powerful punches at their enemies at a rate equivalent to a 0.22 caliber bullet when fired. Both spearers and smashers rapidly unfold their claws and swing them at the predator, producing a force of around 1500 N. Smashers, in particular, are quicker at delivering these punches, with speeds of about 51 mph (83 km/h).
As an impact of such heavy punches, vapor-filled bubbles called cavitation bubbles are formed in the water. When these bubbles collapse, they produce remarkable forces on the prey, adding to the hitting force of about 1500 N. Technically, the prey is struck twice: first by the direct impact of the mantis shrimp’s claws and then again by the collapse of the cavitation bubbles created by the rapid movement of the claws. In fact, even if the shrimp fails to launch a successful strike, the shock wave resulting from the blow often stuns the prey.
These animals are long-lived and are found to survive up to 20 years.
Mantis shrimp are gonochoric, having separate male and female sexes, and thus reproduce sexually. Though most of these shrimp mate with multiple partners, some, like the tiger mantis shrimp, are monogamous and maintain a single mating pair for life.
Males typically display courtship behaviors when females are fertile, which usually occurs during specific phases of the tidal cycle. For instance, they sometimes flash the fluorescent markings on their bodies at the females to catch their attention. Once a pair forms, they mate, and the male transfers his sperm to the female. Mantis shrimp can mate up to 30 times in their lifetime.
The females are oviparous and often build burrows to lay their eggs. They also carry the eggs along under their tails till they are ready to hatch. In such cases, the eggs are glued with secretions from the cement gland on the abdomen. Females of some species, like those in the genera Pullosquilla and Nannosquilla, lay two clutches of eggs, one of which is reared by the males. Generally, it is the females that brood and protect the eggs.
On hatching, the offspring often spend up to three months as plankton.
These crustaceans are preyed upon by fish (like tuna), sharks, squid, cuttlefish, and octopus. They are also consumed by humans and are considered a delicacy in Asian countries like Japan and Vietnam.
Since these shrimp are good burrowers, they contribute to the turnover and oxygenation of sediments. Moreover, they are quite sensitive to pollutants and serve as excellent bioindicators of pollution in coral reefs.
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