Alpheus is the most diverse shrimp genus. Some of their defining characteristics are orbital hoods and an oversized claw which they use for defense and killing prey. When this claw is snapped shut it shoots out water which creates a low pressure air bubble behind it. The bubble reaches temperatures of 5000 degrees Kelvin (half of the heat of the sun)(4). When the bubble pops it makes a loud sound and produces a flash of light. The shockwave from the pop stuns or kills the shrimp’s prey. The shrimp have developed orbital hoods to protect their eyes from the shockwave. However, this orbital hood leads to poor vision. In order to make up for this shortcoming some shrimp have developed a symbiotic relationship with gobies. The gobies protect the shrimp from predators by looking around for predators. When a goby detects a predator it moves its tail in a way that warns the shrimp. The shrimp feels the movements with its antennae and they both go hide. The shrimp and goby live in close proximity.
Pistol Shrimp live in “crevices of coral rubble, under intertidal rocks, in self-bored tunnels in basaltic rocks, associated with sea anemones, among seagrass roots, in sponge canals, coralline algae, in excavated burrows in mangrove mud, in burrows shared with partner gobies, in tunnels of echiuran worms, in burrows in silty sediments of the deep sea, or in freshwater, in burrows made under river banks.” (1)
Evolution and Systematics
The pistol shrimp has evolved to have orbital hoods, which is basically permanently shut eyelids. These orbital hoods protect the shrimp’s eyes from the shock wave of the explosion of the bubble. The shock wave is strong enough to stun or kill the shrimp’s prey, and it could cause serious damage to the eyes of the shrimp. However, these orbital hoods lead to reduced vision. Some shrimp have developed a symbiotic relationship with gobies to overcome this obstacle.
A specialized claw of the pistol shrimp creates a localized pressure wave by bubble collapse.
"This process is known as cavitation, and the energy generated by it almost defies belief; the collapsing gas bubbles create explosive plasma balls that, for an instant, reach 5000 C - almost the temperature of the Sun." (Downer 2002)
Learn more about this functional adaptation.
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:1226
Specimens with Barcodes:744
Species With Barcodes:216
Alpheidae is a family of caridean snapping shrimp characterized by having asymmetrical claws, the larger of which is typically capable of producing a loud snapping sound. Other common names of these species include pistol shrimp or alpheid shrimp.
The family is diverse and worldwide in distribution, consisting of about 600 species within 38 or more genera. The two most prominent genera are Alpheus and Synalpheus, with species numbering well over 250 and 100, respectively. Most snapping shrimp dig burrows and are common inhabitants of coral reefs, submerged seagrass flats and oyster reefs. While most genera and species are found in tropical and temperate coastal and marine waters, Betaeus inhabits cold seas and Potamalpheops is found only in freshwater caves.
The snapping shrimp grows to only 1–2 inches (3–5 cm) long. It is distinctive for its remarkably disproportionate large claw, larger than half the shrimp's body. The claw can be on either arm of the body, and unlike most shrimp claws does not have typical pincers at the end. Rather, it has a pistol-like feature made of two parts. A joint allows the "hammer" part to move backward into a right-angled position. When released, it snaps into the other part of the claw, emitting an enormously powerful wave of bubbles capable of stunning larger fish and breaking small glass jars.
Some pistol shrimp species share burrows with goby fishes in a symbiotic relationship. The burrow is built and tended by the pistol shrimp, and the goby provides protection by watching out for danger. When both are out of the burrow, the shrimp maintains contact with the goby using its antennae. The goby, having the better vision, alerts the shrimp of danger using a characteristic tail movement, and then both retreat into the safety of the shared burrow. So far this association has been observed in species that inhabit coral reef habitats.
Social behavior has been discovered in the genus Synalpheus. The species Synalpheus regalis lives inside sponges in colonies that can number over 300 members. All of them are the offspring of a single large female, the queen, and possibly a single male. The offspring are divided into workers who care for the young and predominantly male soldiers who protect the colony with their huge claws.
Pistol shrimp have also been noted for their ability to reverse claws. When the snapping claw is lost the missing limb will regenerate into a smaller claw and the original smaller appendage will grow into a new snapping claw. Laboratory research has shown that severing the nerve of the snapping claw induces the conversion of the smaller limb into a second snapping claw. This phenomenon of claw symmetry in snapping shrimp has only been documented once in nature.
The snapping shrimp competes with much larger animals such as the sperm whale and beluga whale for the title of loudest animal in the sea. The animal snaps a specialized claw shut to create a cavitation bubble that generates acoustic pressures of up to 80 kPa at a distance of 4 cm from the claw. As it extends out from the claw, the bubble reaches speeds of 60 miles per hour (97 km/h) and releases a sound reaching 218 decibels. The pressure is strong enough to kill small fish. It corresponds to a zero to peak pressure level of 218 decibels relative to one micropascal (dB re 1 μPa), equivalent to a zero to peak source level of 190 dB re 1 μPa at the standard reference distance of 1 m. Au and Banks measured peak to peak source levels between 185 and 190 dB re 1 μPa at 1 m, depending on the size of the claw. Similar values are reported by Ferguson and Cleary. The duration of the click is less than 1 millisecond.
The snap can also produce sonoluminescence from the collapsing cavitation bubble. As it collapses, the cavitation bubble reaches temperatures of over 5,000 K (4,700 °C). In comparison, the surface temperature of the sun is estimated to be around 5,800 K (5,500 °C). The light is of lower intensity than the light produced by typical sonoluminescence and is not visible to the naked eye. It is most likely a by-product of the shock wave with no biological significance. However, it was the first known instance of an animal producing light by this effect. It has subsequently been discovered that another group of crustaceans, the mantis shrimp, contains species whose club-like forelimbs can strike so quickly and with such force as to induce sonoluminescent cavitation bubbles upon impact.
The snapping is used for hunting (hence the alternative name "pistol shrimp"), as well as for communication. When feeding, the shrimp usually lies in an obscured spot, such as a burrow. The shrimp then extends its antennae outwards to determine if any fish are passing by. Once it feels movement, the shrimp inches out of its hiding place, pulls back its claw, and releases a "shot" which stuns the prey; the shrimp then pulls it to the burrow and feeds.
- Acanthanas Anker, Poddoubtchenko & Jeng, 2006
- Alpheopsis Coutière, 1896
- Alpheus Fabricius, 1798
- Amphibetaeus Coutière, 1896
- Arete Stimpson, 1860
- Aretopsis De Man, 1910
- Athanas Leach, 1814
- Athanopsis Coutière, 1897
- Automate De Man, 1888
- Bannereus Bruce, 1988
- Batella Holthuis, 1955
- Bermudacaris Anker & Iliffe, 2000
- Betaeopsis Yaldwyn, 1971
- Betaeus Dana, 1852
- Coronalpheus Wicksten, 1999
- Coutieralpheus Anker & Felder, 2005
- Deioneus Dworschak, Anker & Abed-Navandi, 2000
- Fenneralpheus Felder & Manning, 1986
- Harperalpheus Felder & Anker, 2007
- Jengalpheops Anker & Dworschak, 2007
- Leptalpheus Williams, 1965
- Leptathanas De Grave & Anker, 2008
- Leslibetaeus Anker, Poddoubtchenko & Wehrtmann, 2006
- Metabetaeus Borradaile, 1899
- Metalpheus Coutière, 1908
- Mohocaris Holthuis, 1973
- Nennalpheus Banner & Banner, 1981
- Notalpheus G. Méndez & Wicksten, 1982
- Orygmalpheus De Grave & Anker, 2000
- Parabetaeus Coutière, 1896
- Pomagnathus Chace, 1937
- Potamalpheops Powell, 1979
- Prionalpheus Banner & Banner, 1960
- Pseudalpheopsis Anker, 2007
- Pseudathanas Bruce, 1983
- Pterocaris Heller, 1862
- Racilius Paul’son, 1875
- Richalpheus Anker & Jeng, 2006
- Rugathanas Anker & Jeng, 2007
- Salmoneus Holthuis, 1955
- Stenalpheops Miya, 1997
- Synalpheus Bate, 1888
- Thuylamea Nguyên, 2001
- Vexillipar Chace, 1988
- Yagerocaris Kensley, 1988
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