Hippoidea is a superfamily of

decapod The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfish, shrimp, and p ...

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s known as mole crabs or sand crabs.

Ecology

Hippoids are adapted to burrowing into

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

beach A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from Rock (geology), rock, such as sand, gravel, shingle beach, shingle, pebbles, etc., or biological s ...

es, a habit they share with raninid crabs, and the parallel evolution of the two groups is striking. In the family Hippidae, the body is almost

ovoid An oval () is a closed curve in a plane which resembles the outline of an egg. The term is not very specific, but in some areas of mathematics (projective geometry, technical drawing, etc.), it is given a more precise definition, which may inc ...

, the first

pereiopod The anatomy of a decapod consists of 20 body segments grouped into two main body parts: the cephalothorax and the pleon (abdomen). Each segment – often called a somite – may possess one pair of appendages, although in various groups these m ...

s have no

claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or Arthro ...

s, and the

telson The telson () is the hindmost division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment (biology), segment of the arthropod body, or an additional division that is not a true segm ...

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

decapods The Decapoda or decapods, from Ancient Greek δεκάς (''dekás''), meaning "ten", and πούς (''poús''), meaning "foot", is a large order (biology), order of crustaceans within the class Malacostraca, and includes crabs, lobsters, crayfis ...

, sand crabs cannot

walk Walking (also known as ambulation) is one of the main gaits of terrestrial locomotion among legged animals. Walking is typically slower than running and other gaits. Walking is defined as an "inverted pendulum" gait in which the body vaults over ...

; instead, they use their legs to dig into the sand. Members of the family Hippidae beat their

uropod Uropods are posterior appendages found on a wide variety of crustaceans. They typically have functions in locomotion. Definition Uropods are often defined as the appendages of the last body segment of a crustacean. An alternative definition sugge ...

s to swim. Apart from the

polar region The polar regions, also called the frigid zones or polar zones, of Earth are Earth's polar ice caps, the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitu ...

s, hippoids can be found on beaches throughout the world. Larvae of one species have also been found in Antarctic waters, despite the lack of suitable sandy beaches in the Antarctic.

Classification

Alongside

hermit crab Hermit crabs are anomuran Decapoda, decapod crustaceans of the superfamily (taxonomy), superfamily Paguroidea that have adapted to occupy empty scavenged mollusc shells to protect their fragile exoskeletons. There are over 800 species of hermit c ...

s and allies (Paguroidea),

squat lobster Squat lobsters are dorsoventrally flattened crustaceans with long tails held curled beneath the cephalothorax. They are found in the two superfamilies Galatheoidea and Chirostyloidea, which form part of the decapod infraorder Anomura, alongs ...

s and allies ( Galatheoidea) and the hairy stone crab (''Lomis hirta'', Lomisoidea), Hippoidea is one of the four groups that make up the infraorder

Anomura Anomura (sometimes Anomala) is a group of decapod crustaceans, including hermit crabs and others. Although the names of many anomurans include the word ''crab'', all true crabs are in the sister group to the Anomura, the Brachyura (the two groups ...

. Of the four, Hippoidea is thought to be the most basal, with the other three groups being more closely related to each other than to Hippoidea. The

fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

of sand crabs is sparse, but extends back to the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

period. Sand crabs are placed in three families (exclusively fossil taxa are marked †): ; Albuneidae Stimpson, 1858 *'' Albunea'' 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'' Randall, 1840 *'' Lophomastix'' Benedict, 1904 ; Hippidae Latreille, 1825 *'' Emerita'' Scopoli, 1777 *'' Hippa'' Fabricius, 1787 *'' Mastigochirus'' Miers, 1878

References

External links

* {{Taxonbar, from=Q1145440 Anomura Extant Maastrichtian first appearances Arthropod superfamilies