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Hippoidea

Hippoidea

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Hippoidea is a superfamily
Taxonomic rank
In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

 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 sand crabs or mole crabs.

Ecology


Hippoids are adapted
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

 to burrowing into sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

y beach
Beach
A beach is a geological landform along the shoreline of an ocean, sea, lake or river. It usually consists of loose particles which are often composed of rock, such as sand, gravel, shingle, pebbles or cobblestones...

es, a habit they share with raninid
Raninidae
Raninidae is a family of unusual crabs named for their frog-like appearance, taken by most scientists to be quite primitive. These animals closely resemble the mole crabs, due to parallel evolution or convergent evolution. In both groups, the claws are modified into tools for digging, and the body...

 crabs, and the parallel evolution
Parallel evolution
Parallel evolution is the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades.-Parallel vs...

 of the two groups is striking. The whole body is almost ovoid, the first pereiopods have no claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...

s, and the telson
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...

 is long, none of which are seen in related groups. Unlike most other decapods
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...

, sand crabs cannot walk
WALK
WALK may refer to:*WALK , a radio station licensed to East Patchogue, New York, United States*WALK-FM, a radio station licensed to Patchogue, New York, United States...

; instead, they use their legs
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...

 to dig into the sand. Members of the family Hippidae
Hippidae
Hippidae is a family of decapod crustaceans, commonly known as "mole crabs" or "sand crabs". They are closely related to the family Albuneidae, with which they are usually joined in the superfamily Hippoidea. The family Hippidae comprises the three genera Emerita, Hippa and Mastigochirus...

 beat their uropods to swim.

Apart from the polar region
Polar region
Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...

s, hippoids can be found on beaches throughout the world. Larvae of one species have also been found in Antarctic waters
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...

, despite the lack of suitable sandy beaches in the Antarctic.

Classification


Alongside hermit crab
Hermit crab
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...

s and allies (Paguroidea), squat lobster
Squat lobster
Squat lobsters are decapod crustaceans of the families Galatheidae, Chirostylidae and Kiwaidae, including the common genera Galathea and Munida. They are not lobsters at all, but are more closely related to porcelain crabs, hermit crabs and then, more distantly, true crabs...

s and allies (Galatheoidea
Galatheoidea
Galatheoidea is a superfamily of decapod crustaceans comprising some squat lobsters and the porcelain crabs. Two other families of squat lobsters, Chirostylidae and Kiwaidae, are not closely related to the members of Galatheoidea, and are now placed in other superfamilies...

) and the hairy stone crab
Hairy stone crab
The hairy stone crab, Lomis hirta, is a crab-like crustacean that lives in the littoral zone of southern Australia from Bunbury, Western Australia to the Bass Strait. It is the only species in its family...

 (Lomis hirta, Lomisoidea), Hippoidea is one of the four groups that make up the infraorder Anomura
Anomura
Anomura is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans includes the word crab, all true crabs are in the sister group to the Anomura, the Brachyura .-Description:The name Anomala reflects the unusual variety of forms in this group;...

. Of the four, Hippoidea is thought to be the most basal
Basal (phylogenetics)
In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

, with the other three groups being more closely related to each other than to Hippoidea.

The fossil record of sand crabs is sparse, but extends back to the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 period. Sand crabs are placed in three families († represents exclusively fossil taxa):
Albuneidae
Albuneidae
Albuneidae is a little-known family of specialized burrowing sand crabs. There are 50 extant species as well as nine fossil species that have been described. Fossil specimens have been described from the Cretaceous, Miocene and Oligocene....

 Stimpson, 1858
  • Albunea
    Albunea (genus)
    Albunea is a genus of mole crab within the Albuneidae. Little is known about this group's biology. A. groeningi is named after Matt Groening, creator of The Simpsons.The genus Albunea contains the following species:...

    Weber, 1795
  • Austrolepidopa Efford & Haig, 1968
  • Harryhausenia Boyko, 2004
  • Italialbunea Boyko, 2002
  • Lepidopa Stimpson, 1858
  • Leucolepidopa Efford, 1969
  • Paralbunea Serène, 1977
  • Paraleucolepidopa Calado, 1996
  • Praealbunea Fraaije, 2002
  • Squillalbunea Boyko, 2002
  • Stemonopa Efford & Haig, 1968
  • Zygopa Holthuis, 1961


Blepharipodidae Boyko, 2002
  • 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...

    Randall, 1840
  • Lophomastix Benedict, 1904

Hippidae
Hippidae
Hippidae is a family of decapod crustaceans, commonly known as "mole crabs" or "sand crabs". They are closely related to the family Albuneidae, with which they are usually joined in the superfamily Hippoidea. The family Hippidae comprises the three genera Emerita, Hippa and Mastigochirus...

 Latreille, 1825
  • Emerita
    Emerita (genus)
    Emerita is a small genus of decapod crustaceans, known as mole crabs. These are small animals that burrow in the sand in the wash zone and use their antennae for filter feeding.-Description:...

    Scopoli, 1777
  • Hippa Fabricius, 1787
  • Mastigochirus Miers, 1878