Mictyris longicarpus
Encyclopedia
Mictyris longicarpus is a species
of crab
that lives on sandy beaches from the Bay of Bengal
to Australia
; with other members of the genus Mictyris
, it is "one of the most loved crabs in Australia". Adults are 25 mm (0.984251968503937 in) across, white, with blue on their backs, and hold their claws vertically. They feed on detritus in the sand, leaving rounded pellets of discarded sand behind them. The males may form into large "armies" which traverse the beach at low tide, before the crabs dig into the sand to wait for the next low tide.
is powder blue
, with the rest of the body being white except for purple patches on the joints of the legs. The chela
e (claws) are slim and curve downwards, and are held vertically in front of the crab. Given the crab's upright posture, the eyestalk
s are short. The body is up to 25 millimetre (0.984251968503937 in) across, or "about the size of a cherry".
and the Bay of Bengal
to New Caledonia
and Australia
, reaching as far south as Perth, Western Australia
in the west, and around the coast of Queensland
and New South Wales
to Wilsons Promontory
, Victoria.
, and any small organisms in the sand, such as diatom
s, gastropod eggs or nematode
s.
Predators of adult M. longicarpus include Threskiornis spinicollis
(the straw-necked ibis), Todiramphus chloris (the mangrove kingfisher), Egretta alba
(the white crane), Tetractenos hamiltoni
(the common toadfish) and Metopograpsus messor
(a grapsid crab
). Juveniles are also attacked by the ghost crab Ocypode ceratophthalmus
and the sand snail Conuber sordidus
.
. The first sign that a crab may emerge is the development of "hummocks" which appear on the surface of the sand and increase in size over a period of 10–30 minutes. The number of crabs which emerge is influenced by temperature
, wind
and rain
fall, with the different sexes responding differently, such that one day, nearly all the emerged crabs will be male, while the next day, there may be a mixture of males and females. Emergence of a population from the sand may take up to an hour, or be completed in five minutes, with the adults generally appearing before the juveniles
. Upon emergence, the crab performs "the most aerobatic grooming mechanism recorded from the Brachyura"; in less than a second, the crab falls onto its back, thus removing any sand it has accumulated on the carapace
, and then flips upright again in a "half somersault".
Initially, the crabs feed only tentatively, and within 15 minutes of emergence, they begin the "trek", where large numbers of crabs walk simultaneously towards the water in an almost straight line. Mictyris are among the few crabs adapted to walking forwards, rather than sideways. Juveniles only proceed about 50 yards towards the water, and feed at that level of the beach.
Having reached a suitable moist area, the crabs begin to feed rapidly, working transversely across the beach as they do so. Feeding comprises raising scoops of sand to the mouthparts, with inedible material accumulating at the base of the third maxillipeds, and drop off the crabs as round pellets. Feeding may last 1–2.5 hours, with the crabs spending less and less time feeding as they aggregate into armies. The armies are generally composed solely of males, with the largest individuals at the front, probably because their longer legs mean they walk faster. The army as a whole progresses at a speed of 10 yards per minute (0.34 mph or 0.1519936 m/s), continuing for 0.5–2 hours.
Eventually, the army breaks up and the individual crabs travel up the shore, and dig themselves into the sand in a unique corkscrew
motion. The crabs dig down with the legs on one side of their body, while the legs on the other side walk backwards. They then leave this burrow, and dig another. During this period, encounters between pairs of adult males result in both males adopting the threat display (rearing up onto the last one of two pairs of legs, and stretching the other limbs out as wide as possible), after which the loser – generally the smaller crab – backs down. Eventually, the crabs remain in one of the burrows and await the next falling tide.
Michael Tweedie
considered crabs of the genera Mictyris
and Scopimera to show types of behaviour also seen in human society. While Scopimera were caricature
s of the middle class
, Mictyris were "cheerful bohemians, living crowded together and out-doing in unrepressed and irresponsible behaviour even those human communities which aspire most strenuously towards this ideal".
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
that lives on sandy beaches from the Bay of Bengal
Bay of Bengal
The Bay of Bengal , the largest bay in the world, forms the northeastern part of the Indian Ocean. It resembles a triangle in shape, and is bordered mostly by the Eastern Coast of India, southern coast of Bangladesh and Sri Lanka to the west and Burma and the Andaman and Nicobar Islands to the...
to Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
; with other members of the genus Mictyris
Mictyris
Mictyris is a genus of brightly coloured crabs, placed in its own taxonomical family, the Mictyridae. It inhabits the central Indo-West Pacific region. These crabs will congregate on mud flats or beaches in groups of a few thousand, and filter sand or mud for microscopic organisms...
, it is "one of the most loved crabs in Australia". Adults are 25 mm (0.984251968503937 in) across, white, with blue on their backs, and hold their claws vertically. They feed on detritus in the sand, leaving rounded pellets of discarded sand behind them. The males may form into large "armies" which traverse the beach at low tide, before the crabs dig into the sand to wait for the next low tide.
Description
Mictyris longicarpus is nearly spherical, with an upright body. Its carapaceCarapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
is powder blue
Powder blue
Powder blue may refer to two different colors.The paler variant is often associated with powder snow.-Powder blue :The web color powder blue is shown on the right....
, with the rest of the body being white except for purple patches on the joints of the legs. The chela
Chela
Chela has two main meanings. One derivation comes from Hindi from Sanskrit , meaning "slave" or "servant". In English, the word means a religious student or disciple...
e (claws) are slim and curve downwards, and are held vertically in front of the crab. Given the crab's upright posture, the eyestalk
Eyestalk
In anatomy, an eyestalk is a protrusion that extends the eye away from the body, giving the eye a better field of view than if it were unextended. It is common in nature and in fiction....
s are short. The body is up to 25 millimetre (0.984251968503937 in) across, or "about the size of a cherry".
Distribution
M. longicarpus is found from SingaporeSingapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...
and the Bay of Bengal
Bay of Bengal
The Bay of Bengal , the largest bay in the world, forms the northeastern part of the Indian Ocean. It resembles a triangle in shape, and is bordered mostly by the Eastern Coast of India, southern coast of Bangladesh and Sri Lanka to the west and Burma and the Andaman and Nicobar Islands to the...
to New Caledonia
New Caledonia
New Caledonia is a special collectivity of France located in the southwest Pacific Ocean, east of Australia and about from Metropolitan France. The archipelago, part of the Melanesia subregion, includes the main island of Grande Terre, the Loyalty Islands, the Belep archipelago, the Isle of...
and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, reaching as far south as Perth, Western Australia
Perth, Western Australia
Perth is the capital and largest city of the Australian state of Western Australia and the fourth most populous city in Australia. The Perth metropolitan area has an estimated population of almost 1,700,000....
in the west, and around the coast of Queensland
Queensland
Queensland is a state of Australia, occupying the north-eastern section of the mainland continent. It is bordered by the Northern Territory, South Australia and New South Wales to the west, south-west and south respectively. To the east, Queensland is bordered by the Coral Sea and Pacific Ocean...
and New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...
to Wilsons Promontory
Wilsons Promontory
Wilsons Promontory is a peninsula that forms the southernmost part of the Australian mainland and is located at . South Point at is the southernmost tip of Wilsons Promontory and hence of mainland Australia...
, Victoria.
Ecology
Examination of the gut contents of M. longicarpus showed that the crabs mostly feed on detritusDetritus
Detritus is a biological term used to describe dead or waste organic material.Detritus may also refer to:* Detritus , a geological term used to describe the particles of rock produced by weathering...
, and any small organisms in the sand, such as diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s, gastropod eggs or nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
s.
Predators of adult M. longicarpus include Threskiornis spinicollis
Straw-necked Ibis
The Straw-necked Ibis is a bird of the ibis and spoonbill family Threskiornithidae. It can be found throughout Australia, New Guinea, and parts of Indonesia. Adults have distinctive straw-like feathers on their neck....
(the straw-necked ibis), Todiramphus chloris (the mangrove kingfisher), Egretta alba
Great Egret
The Great Egret , also known as the Great White Egret or Common Egret, White Heron, or Great White Heron, is a large, widely-distributed egret. Distributed across most of the tropical and warmer temperate regions of the world, in southern Europe it is rather localized...
(the white crane), Tetractenos hamiltoni
Common toadfish
The common toadfish or common toado or toadfish or toado is a species of fish in the Tetraodontidae family of order Tetraodontiformes, found along Australia's eastern coast, from northern Queensland to Flinders Island, and around Lord Howe Island, as well as in New...
(the common toadfish) and Metopograpsus messor
Metopograpsus messor
Metopograpsus messor is a species of crab that lives in mangroves from East Africa to Fiji.-Description:It grows up to wide. The carapace and legs are mottled brownish green, while the claws are brownish red.-Distribution:The distribution of M...
(a grapsid crab
Grapsidae
Grapsidae is a family of crabs known variously as marsh crabs, shore crabs or talon crabs. It is not confirmed that the family forms a monophyletic group and some taxa may belong in other families...
). Juveniles are also attacked by the ghost crab Ocypode ceratophthalmus
Ocypode ceratophthalmus
Ocypode ceratophthalmus, the horned ghost crab or horn-eyed ghost crab, is a species of ghost crab. It lives in the Indo-Pacific region from East Africa to the Philippines and the Great Barrier Reef. O...
and the sand snail Conuber sordidus
Conuber sordidus
Conuber sordidus is a species of predatory sea snail, a marine gastropod mollusk family Naticidae, the moon snails.-Distribution:East coast of Australia, from Victoria to Queensland...
.
Behaviour
Mictyris longicarpus spends much of the time buried in the sand. They emerge to the surface a few hours before low tide, although some individuals may remain submerged for the entire tidal cycleTide
Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
. The first sign that a crab may emerge is the development of "hummocks" which appear on the surface of the sand and increase in size over a period of 10–30 minutes. The number of crabs which emerge is influenced by temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
and rain
Rain
Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...
fall, with the different sexes responding differently, such that one day, nearly all the emerged crabs will be male, while the next day, there may be a mixture of males and females. Emergence of a population from the sand may take up to an hour, or be completed in five minutes, with the adults generally appearing before the juveniles
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
. Upon emergence, the crab performs "the most aerobatic grooming mechanism recorded from the Brachyura"; in less than a second, the crab falls onto its back, thus removing any sand it has accumulated on the carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
, and then flips upright again in a "half somersault".
Initially, the crabs feed only tentatively, and within 15 minutes of emergence, they begin the "trek", where large numbers of crabs walk simultaneously towards the water in an almost straight line. Mictyris are among the few crabs adapted to walking forwards, rather than sideways. Juveniles only proceed about 50 yards towards the water, and feed at that level of the beach.
Eventually, the army breaks up and the individual crabs travel up the shore, and dig themselves into the sand in a unique corkscrew
Corkscrew
A corkscrew is a kitchen tool for drawing stopping corks from wine bottles. Generally, a corkscrew consists of a pointed metallic helix attached to a handle. The user grips the handle and screws the metal point into the cork, until the helix is firmly embedded, then a vertical pull on the...
motion. The crabs dig down with the legs on one side of their body, while the legs on the other side walk backwards. They then leave this burrow, and dig another. During this period, encounters between pairs of adult males result in both males adopting the threat display (rearing up onto the last one of two pairs of legs, and stretching the other limbs out as wide as possible), after which the loser – generally the smaller crab – backs down. Eventually, the crabs remain in one of the burrows and await the next falling tide.
Michael Tweedie
Michael Tweedie
Michael Wilmer Forbes Tweedie was a naturalist and archaeologist working in South East Asia, who was Director of the Raffles Museum in Singapore....
considered crabs of the genera Mictyris
Mictyris
Mictyris is a genus of brightly coloured crabs, placed in its own taxonomical family, the Mictyridae. It inhabits the central Indo-West Pacific region. These crabs will congregate on mud flats or beaches in groups of a few thousand, and filter sand or mud for microscopic organisms...
and Scopimera to show types of behaviour also seen in human society. While Scopimera were caricature
Caricature
A caricature is a portrait that exaggerates or distorts the essence of a person or thing to create an easily identifiable visual likeness. In literature, a caricature is a description of a person using exaggeration of some characteristics and oversimplification of others.Caricatures can be...
s of the middle class
Middle class
The middle class is any class of people in the middle of a societal hierarchy. In Weberian socio-economic terms, the middle class is the broad group of people in contemporary society who fall socio-economically between the working class and upper class....
, Mictyris were "cheerful bohemians, living crowded together and out-doing in unrepressed and irresponsible behaviour even those human communities which aspire most strenuously towards this ideal".
External links
- Video of soldier crabs on Yahoo! VideoYahoo! VideoYahoo! Video is a video sharing website on which users could upload and share videos. The service is owned and created by Yahoo!. Yahoo! Video began as an internet-wide video search engine and added the ability to upload and share video clips in June 2006...
- Video of Conuber sordidus hunting Mictyris longicarpus on YouTubeYouTubeYouTube is a video-sharing website, created by three former PayPal employees in February 2005, on which users can upload, view and share videos....