Emerita (genus)
Encyclopedia
Emerita is a small genus of decapod
Decapoda
The decapods or Decapoda are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with...

 crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, known as mole crabs. These are small animals that burrow in the sand in the wash zone
Swash
Swash, in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...

 and use their antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

 for filter feeding.

Description

Emerita has a barrel
Barrel
A barrel or cask is a hollow cylindrical container, traditionally made of vertical wooden staves and bound by wooden or metal hoops. Traditionally, the barrel was a standard size of measure referring to a set capacity or weight of a given commodity. A small barrel is called a keg.For example, a...

-shaped body. It has a tough exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...

 and can hold its appendages close to the body, allowing it to roll in the tidal currents and waves. It has feathery antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....

, which are used to filter plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 and detritus
Detritus
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...

 from the swash
Swash
Swash, in geography, is a turbulent layer of water that washes up on the beach after an incoming wave has broken. The swash action can move beach material up and down on the beach, which results in the cross-shore sediment exchange. The time-scale of swash motion varies from seconds to minutes...

.

Males are typically smaller than females, and in some species, such as Emerita rathbunae, the minute males live attached to the legs of the female. Females are around 8–37 mm (0.31496062992126–1.5 ) in 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...

 length, depending on the species, while males vary from a similar size to females in E. austroafricana, down to 2.5 mm (0.0984251968503937 in) carapace length in E. rathbunae and E. talpoida.

Distribution

The genus as a whole has a broad distribution in tropical and subtropical regions. Most individual species, however, are restricted to smaller areas, and their ranges
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...

 rarely overlap. The genus is common on both coasts of the United States and along the Atlantic coast of Africa; the related genus Hippa is found in 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...

.

Species

Ten species are recognised :
  • Emerita analoga
    Emerita analoga
    Emerita analoga, the Pacific sand crab or Pacific mole crab, is a species of small, sand-burrowing decapod crustacean found living in the sand along the temperate western coasts of America. It is found on exposed sandy beaches in the swash region of the intertidal zone.-Description:The Pacific sand...

    (Stimpson, 1857) – western North America and western South America
  • Emerita austroafricana Schmitt, 1937 – south-eastern Africa and Madagascar
  • Emerita benedicti Schmitt, 1935 – Gulf of Mexico
  • Emerita brasiliensis Schmitt, 1935 – southeastern Brazil
  • Emerita emeritus (Linnaeus, 1767) – South Asia and South East Asia
  • Emerita holthuisi Sankolli, 1965 – western India, Persian Gulf and Red Sea
  • Emerita karachiensis Niazi & Haque, 1974 – Pakistan
  • Emerita portoricensis Schmitt, 1935 – Caribbean Sea
  • Emerita rathbunae Schmitt, 1935 – western Central America
  • Emerita talpoida (Say, 1817) – eastern North America


It had been widely thought that the Old World
Old World
The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....

 species formed a monophyletic group, as did the New World
New World
The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...

 species. The use of molecular phylogenetics has shown, however, that Emerita analoga
Emerita analoga
Emerita analoga, the Pacific sand crab or Pacific mole crab, is a species of small, sand-burrowing decapod crustacean found living in the sand along the temperate western coasts of America. It is found on exposed sandy beaches in the swash region of the intertidal zone.-Description:The Pacific sand...

, a species living along the Atlantic coast of North America, is more closely related to Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...

n species than it is to other New World species.

Taxonomy

The genus Emerita was erected by Giovanni Antonio Scopoli
Giovanni Antonio Scopoli
Giovanni Antonio Scopoli was an Italian physician and naturalist.-Biography:...

 in his 1777 work Introductio ad Historiam Naturalem. The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...

 is Cancer emeritus (now Emerita emeritus), because at one time it was the only species in the genus. Other genera with the same name have been rejected for nomenclatural purposes; these were published by Laurens Theodorus Gronovius
Laurens Theodorus Gronovius
Laurens Theodorus Gronovius , also known as Laurentius Theodorus Gronovius or as Laurens Theodore Gronow, was a Dutch naturalist who was born in Leiden...

 (1764) and Friedrich Christian Meuschen
Friedrich Christian Meuschen
Friedrich Christian Meuschen was a German diplomat and conchologist born in Hanau. He was the son of theologian Johann Gerhard Meuschen ....

 (1778 and 1781).

Ecology and behaviour

Emerita is adept at burrowing, and is capable of burying itself completely in 1.5 seconds. Unlike mud shrimp
Thalassinidea
Thalassinidea is an infraorder of decapod crustaceans that live in burrows in muddy bottoms of the world's oceans. In Australian English, the littoral thalassinidean Trypaea australiensis is referred to as the yabby , frequently used as bait for estuarine fishing; elsewhere, however, they are...

, Emerita burrows tail-first into the sand, using the pereiopods to scrape the sand from underneath the body. During this action, 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...

 is pressed into the sand as anchorage for the digging limbs. The digging requires the sand to be fluidised by wave action, and Emerita must bury itself in the correct orientation before the wave has passed in order to be safe from predators.

As the tide
Tide
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....

 changes, Emerita changes its position on the beach; most individuals stay in the zone of breaking waves. This may be detected by the physical characteristics of the sand. As the tide falls, the sand is allowed to settle; when Emerita detects this, it uses the temporary liquefaction from a breaking wave to emerge from its burrow, and is carried down the beach by the wave action. Longshore drift
Longshore drift
Longshore drift consists of the transportation of sediments along a coast at an angle to the shoreline, which is dependent on prevailing wind direction, swash and backwash. This process occurs in the littoral zone, and in or within close proximity to the surf zone...

 may also drag Emerita laterally along a beach.

The main predators of Emerita are fish; in the eastern Pacific Ocean, the barred surfperch
Barred surfperch
The barred surfperch occurs from northern California to southern Baja California. Like other surfperches, it gives birth to live young. The diet of the barred surfperch is made up predominantly of sand crabs .- Fishery :...

 (Amphistichus argenteus) is particularly important. Seabird
Seabird
Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...

s also eat Emerita, but do not appear to target the aggregations of mole crabs. Carcasses
Carrion
Carrion refers to the carcass of a dead animal. Carrion is an important food source for large carnivores and omnivores in most ecosystems. Examples of carrion-eaters include vultures, hawks, eagles, hyenas, Virginia Opossum, Tasmanian Devils, coyotes, Komodo dragons, and burying beetles...

 of Emerita provide an important food source for the closely related scavenger
Scavenger
Scavenging is both a carnivorous and herbivorous feeding behavior in which individual scavengers search out dead animal and dead plant biomass on which to feed. The eating of carrion from the same species is referred to as cannibalism. Scavengers play an important role in the ecosystem by...

 Blepharipoda
Blepharipoda
Blepharipoda is a genus of mole crabs, containing the following species:*Blepharipoda doelloi Schmitt, 1942*Blepharipoda liberata Shen, 1949*Blepharipoda occidentalis Randall, 1840*Blepharipoda spinosa...

.

Life cycle

Emerita has a short life span, perhaps no more than 2–3 years, and can reproduce in its first year of life. The eggs are bright orange, and hatch into larvae
Crustacean larvae
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow...

, which may live as plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 for more than 4 months and can be carried long distances by ocean currents. The number of zoeal stages varies between species from 6 to 11.
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