Ghost crab

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Ghost crabs
Ocypode-ceratophthalma-horned-ghost-crab-krabi-thailand.jpg
Horned ghost crab ( Ocypode ceratophthalma ) in Krabi, Thailand
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Family: Ocypodidae
Subfamily: Ocypodinae
Rafinesque, 1815
Type genus
Ocypode
Weber, 1795
Genera

Ghost crabs are semiterrestrial crabs of the subfamily Ocypodinae. They are common shore crabs in tropical and subtropical regions throughout the world, inhabiting deep burrows in the intertidal zone. They are generalist scavengers and predators of small animals. The name "ghost crab" derives from their nocturnality and their generally pale coloration. [1] [2] They are also sometimes called sand crabs, though the name refers to various other crabs that do not belong to the subfamily.

Contents

Characteristics of the subfamily include one claw being larger than the other, thick and elongated eyestalks, and a box-like body. The differences in claw sizes, however, are not as marked as in male fiddler crabs. The subfamily includes 22 species in two genera.

Taxonomy

Ocypodinae is one of two subfamilies in the family Ocypodidae, the other being the fiddler crab subfamily, Ucinae. Both subfamilies have members in which one of the claw-bearing legs (the chelipeds) is much larger than the other. However, only male fiddler crabs exhibit this, while both male and female ghost crabs have unequally sized claws. The difference is also much more pronounced among fiddler crab males. The fiddler crabs' carapaces are broadened at the front, while the carapaces of ghost crabs are more or less box-like. Lastly, the eyes of ghost crabs have large and elongated eyestalks, with the corneas occupying the entire lower part, while in fiddler crabs the eyestalks are long and thin, with the corneas small and located at the tip. [3] [4]

Ocypodinae was previously regarded as monotypic, with only one genus, Ocypode , classified under it. In 2013, though, Katsushi Sakai and Michael Türkay reclassified the gulf ghost crab into a separate genus, Hoplocypode based on the differences of their gonopods (appendages modified into copulatory organs) from that of the rest of the members of the genus Ocypode . [5]

Genera

The subfamily Ocypodinae currently contains 22 species in these two genera: [5]

Description

Exophthalmy in the painted ghost crab (Ocypode gaudichaudii) Ghost crab - Ocypode gaudichaudii.jpg
Exophthalmy in the painted ghost crab (Ocypode gaudichaudii)

Most ghost crabs have pale-colored bodies that blend in well with the sand, [3] though they are capable of gradually changing body coloration to match their environments and the time of day. [6] [7] Some species are brightly colored, such as Ocypode gaudichaudii and Ocypode ryderi . [2] [5]

Ghost crabs have elongated and swollen eyestalks with very large corneas on the bottom half. Their carapaces are deep and box-like, squarish when viewed from the top with straight or slightly curving sides. The regions of the carapace are usually not clearly defined. The "whip" of their antennules (antennular flagella) are small or rudimentary. They fold back into the body diagonally or almost vertically. The plate between them (the interantennular septum) is broad. The third pair of mouth appendages (maxillipeds) completely cover the mouth opening. A small orifice with edges thickly fringed with hair is found between the bases of the second and third pairs of walking legs. [8]

Stridulating (sound-producing) ridge on the larger claw of Ocypode brevicornis Ocypode brevicornis stridulating ridge.jpg
Stridulating (sound-producing) ridge on the larger claw of Ocypode brevicornis

Ghost crab species can be most reliably identified by means of the area where they were recovered, the presence of "horns" (styles) on their eyestalks (exophthalmy), the pattern of stridulating (sound-producing) ridges on the inside surface of the palms of their larger claws, and the shape of the gonopods in males. [5]

Exophthalmy is exhibited by seven species in the subfamily: Ocypode brevicornis , Ocypode ceratophthalma , Ocypode gaudichaudii , Ocypode macrocera , Ocypode mortoni , Ocypode rotundata , and Ocypode saratan . All of them are found in the Indo-Pacific region with more or less limited ranges, with the exception of Ocypode ceratophthalma, which is found widely. Ocypode cursor , found in the Atlantic Ocean and the Mediterranean Sea, also possesses a tuft of bristles at the end of its eyestalks. [5]

Stridulating ridges also differ from species to species, with some displaying rows of tubercles, others displaying rows of smaller ridges (striae), or a combination of both. These are very important in identifying species as they are displayed by both adults and juveniles (though they may be absent in newly regenerated claws). They are used by ghost crabs for communication. [5]

Both exophthalmy and stridulating ridges, however, can not be reliably used to determine phylogenetic relationships between different species of ghost crabs. Sakai & Türkay (2013) regarded the shape of the gonopods as more suited for this. As gonopods are important in sexual reproduction, they are less likely to evolve randomly in response to the environment. Ghost crabs of the genus Hoplocypode can be distinguished from those in Ocypode by examining their gonopods. In the former, the first gonopod has a complex, hoof-shaped tip, while in the latter they are simple and curved. [5]

Ecology

Ocypode madagascariensis digging a burrow Ocypode madagascariensis.jpg
Ocypode madagascariensis digging a burrow

Ghost crabs dig deep burrows near the intertidal zone of open sandy beaches. The burrows are usually composed of a long shaft with a chamber at the end, occasionally with a second entrance shaft. [1] They are semi-terrestrial and breathe oxygen from the air through moistened gills. They must periodically wet their gills with seawater, [1] [9] usually by taking water from moist sand or by running into the surf and letting the waves wash over them. However, they can only remain under water for a limited amount of time, as they will drown. [10] [11]

Ghost crabs are generalists, scavenging carrion and debris, as well as preying on small animals, including sea turtle eggs and hatchlings, clams, and other crabs. [12] They are predominantly nocturnal. They remain in their burrows during the hottest part of the day, and throughout the coldest part of the winter. [1]

Ghost crabs are swift runners, darting away at the slightest sign of danger. They either head back to their burrows or into the sea to escape intruders. [10] [11] [13] The gaits of ghost crabs alter as their speed increases. Observations on O. ceratophthalma show it can walk indefinitely using all four pairs of walking legs, occasionally alternating which side leads. At higher speeds, the fourth pair of legs is raised off the ground, and at the highest speeds, the crab runs, using only the first and second pairs of walking legs. [14]

Ghost crabs utilize a varied animal acoustic communication system. They can create different sounds by striking the ground with their claws, rubbing their claws together to make a rasping sound, rubbing their legs to make a bubbly noise, and rubbing the teeth inside their stomachs to make a growling sound. The lateral teeth of the gastric mill possess a series of comb-like structures that rub against the median tooth to produce stimulation with dominant frequencies below 2 kHz. [15]

Ghost crabs also have the ability to change colors to match their surroundings by adjusting the concentration and dispersal of pigments within their chromatophores. [6] They can even match the specific colors of the grains of sand in their habitats. [16] However, unlike metachrosis (which is a rapid change of colors), ghost crabs are only capable of morphological color change, which occurs over a longer span of time. [6]

In a study in 1964, individuals of O. ceratophthalma in Hawaii from two populations (one from a black-sand beach and the other from a white-sand beach) were observed to markedly differ in pigmentation although they remain morphologically indistinguishable. Specimens taken from the black-sand beach were much darker than the specimens taken from the white-sand beach, exhibiting nearly 12 times as many black chromatophores. When the black-sand specimens were subsequently exposed to a white background, they were observed to gradually become lighter over the course of about a month. The opposite happened to white-sand specimens when placed in black backgrounds. [6]

A horned ghost crab (Ocypode ceratophthalma) preying on a loggerhead hatchling in Gnaraloo, Western Australia: Ghost crabs are one of the chief causes of egg and hatchling mortality in sea turtles. 110127 Running ghost crab O ceratophtalma prey Loggerhead hatchling Gnaraloo Bay Rookery.JPG
A horned ghost crab ( Ocypode ceratophthalma ) preying on a loggerhead hatchling in Gnaraloo, Western Australia: Ghost crabs are one of the chief causes of egg and hatchling mortality in sea turtles.

In a 2013 study in Singapore, Ocypode ceratophthalma was also discovered to change color in response to the time of day. In a span of 24 hours, they were observed to alternate between lighter and darker coloration, being lightest at midday and darkest at night. However, when placed in a dark background, the crabs showed no significant changes in coloration. This suggests, at least for short-term color changes, they follow a day-night cycle to determine body coloration; basing it on their biological clocks rather than on the brightness or darkness of their environments. The researchers believe relying on the time of day rather than ambient light is more advantageous for survival in ghost crabs. It lets ghost crabs avoid changing color when in temporary shadow (for example within their burrows) and thus still remain inconspicuous when they are once again illuminated by daylight. However, this behavior is only exhibited by juveniles. [7] [16]

Ghost crabs lay their eggs in the sea, which develop into planktonic marine larvae. [1]

Distribution

Ghost crabs dominate sandy shores in tropical and subtropical regions of the world, replacing the sandhoppers that predominate in cooler areas. [1] Three species are found in the Atlantic Ocean and the Mediterranean Sea, and two occur in the eastern Pacific coast of the Americas. The rest of the species are found in the western Pacific and the Indian Ocean to the tip of southern Africa. [5]

Conservation

Ghost crabs are negatively affected by human activity on sandy beaches, such as sand trampling by foot traffic, the building of seawalls, or the presence of inorganic pollutants. Due to their worldwide distribution and the ease by which their burrows can be surveyed, ghost crab burrows are regarded as valuable ecological indicators for quickly assessing the impact of human disturbance on beach habitats. [20] [21] [22]

See also

Related Research Articles

<i>Ocypode</i> Genus of tropical and subtropical crustaceans

Ocypode is a genus of ghost crabs found in the sandy shores of tropical and subtropical regions throughout the world. They have a box-like body, thick and elongated eyestalks, and one claw is larger than the other in both males and females. They inhabit deep burrows in the intertidal zone. They are primarily nocturnal, and are generalist scavengers and predators of small animals. The genus contains 21 species.

<i>Leptuca pugilator</i> Species of crab

Leptuca pugilator, the sand fiddler crab, Atlantic sand fiddler crab, or Calico fiddler, is a species of fiddler crab that is found from Massachusetts to the Gulf of Mexico. It lives in burrows in coastal and estuarine mud-flats, and can be extremely abundant. It can be differentiated from the morphologically similar Minuca pugnax and Minuca minax by the smoothness of the inside of its claws. One claw is larger than the other, and can be much larger than the crab's body, at up to 41 mm (1.6 in) long.

<i>Minuca pugnax</i> Species of crab

Minuca pugnax, commonly known as the Atlantic marsh fiddler crab, is a species of fiddler crab that lives on north-western shores of the Atlantic Ocean.

<span class="mw-page-title-main">Atlantic ghost crab</span> Species of crustacean

The Atlantic ghost crab, Ocypode quadrata, is a species of ghost crab. It is a common species along the Atlantic coast of the United States, where it is the only species of ghost crab; its range of distribution extends from its northernmost reach on beaches in Westport, Massachusetts, south along the coasts of the tropical Western Atlantic Ocean to the beach of Barra do Chui, in Rio Grande do Sul in southern Brazil.

<i>Ocypode ceratophthalmus</i> Species of crab

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 the coast of East Africa to the Philippines and from Japan to the Great Barrier Reef. They also occur in the Pacific Islands to as far east as Polynesia and Clipperton Island. As their common name implies, O. ceratophthalmus possess eyestalks extending beyond the eyes into long points, which are longer in adults, and shorter in juveniles. The crabs have a box-shaped body, 6–8 centimetres (2.4–3.1 in) across the carapace, with a darker markings towards the rear in the shape of an H. The outer edges of the eye-sheaths are also sharp and broadly triangular and distinctly pointing sideways in larger individuals. O. ceratophthalmus can run at speeds of up to 2.1 metres per second (6.9 ft/s).

<i>Ocypode ryderi</i> Species of crab

Ocypode ryderi, also known as the pink ghost crab, is a species of ghost crab found on the east coast of Africa from the Eastern Cape Region to Kenya.

<i>Mictyris longicarpus</i> Species of crab

Mictyris longicarpus, the light-blue soldier crab, 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 (1 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.

<i>Ocypode gaudichaudii</i> Species of crab

Ocypode gaudichaudii, also known as the painted ghost crab or cart driver crab, is a species of crab found on Pacific beaches from El Salvador to Chile as well as on the Galápagos Islands. The species was first described by Henri Milne-Edwards and Hippolyte Lucas in 1843.

<span class="mw-page-title-main">Ocypode cursor</span> Species of crab

Ocypode cursor, the tufted ghost crab, is a species of ghost crab found on sandy beaches along the coasts of the eastern Atlantic Ocean and eastern Mediterranean Sea.

<span class="mw-page-title-main">Gulf ghost crab</span> Species of crab

The gulf ghost crab, Hoplocypode occidentalis, is a species of ghost crabs native to the Pacific coast of the Americas, from the Gulf of California to Colombia. It is the only species in the genus Hoplocypode. Gulf ghost crabs are medium-sized, reaching a maximum overall body diameter of 6 in (15 cm). They are one of only two ghost crab species found in the eastern Pacific. However, gulf ghost crabs can easily be distinguished from painted ghost crabs by the absence of "horns" on their eyes.

<span class="mw-page-title-main">African ghost crab</span> Species of crab

Ocypode africana, commonly known as the African ghost crab, is a species of ghost crabs native to the eastern Atlantic coast of western Africa, from Mauritania to Namibia. They are medium-sized ghost crabs reaching carapace width of 3.4 cm (1.3 in). They can vary in coloration from pinkish to dark grey. They are one of only two ghost crab species found in the eastern Atlantic. However, African ghost crabs can easily be distinguished from tufted ghost crabs by the absence of long tufts of hair on the tip of their eyestalks.

<i>Ocypode brevicornis</i> Species of crab

Ocypode brevicornis is a species of ghost crab native to the Indian Ocean, from the Gulf of Oman to the Nicobar Islands. They are relatively large ghost crabs with a somewhat trapezoidal body. The carapace reaches a length of 41 mm (1.6 in) and a width of 50 mm (2.0 in). They are a mottled brown to yellow in coloration. Like other ghost crabs, one of their claws is much larger than the other. Their eyestalks are large and elongated, tipped with prolongations at the tip known as styles. They are common inhabitants of open sandy beaches, living in burrows in the intertidal zone.

<span class="mw-page-title-main">Golden ghost crab</span> Species of crustacean

Ocypode convexa, commonly known as the golden ghost crab, or alternatively the western ghost crab or yellow ghost crab, is a species of ghost crabs endemic to the coast of Western Australia, from Broome to Perth. They are relatively large ghost crabs, with a carapace growing up to 45 mm (1.8 in) long and 52 mm (2.0 in) wide. They are easily recognisable by their golden yellow colouration. Like other ghost crabs they have box-like bodies with unequally sized claws. They also have large eyestalks with the cornea occupying most of the bottom part.

<i>Ocypode fabricii</i> Species of crab

Ocypode fabricii is a species of ghost crabs endemic to the coast of northern and western Australia, from Darwin to Shark Bay. They are medium-sized ghost crabs with a squarish body. The carapace reaches a length of 38 mm (1.5 in) and a width of 40 mm (1.6 in). Like other ghost crabs, one of their claws is much larger than the other. They live in burrows in the intertidal zones of the muddy to sandy beaches of mangrove forests.

<i>Ocypode macrocera</i> Species of crustacean

Ocypode macrocera, the red ghost crab, is a species of crab in the family Ocypodidae. It is a medium-sized species of Ocypode found in Pakistan, India, Sri Lanka, and Myanmar. In adult males, the outer edges of the eye orbits are tooth-shaped and project forward. The palm of the claws are densely covered with tubercles on the upper surface, with distinctively serrated upper and lower margins. The stridulating ridge has 35 to 56 tubercles with striae. The tip of the smaller claw is characteristically blunt. Their eyestalks possess styles.

<i>Ocypode madagascariensis</i> Species of crustacean

Ocypode madagascariensis is a medium-sized species of Ocypode found in Madagascar and the southeastern coast of Africa. Its carapace is covered densely with rough tubercles on the upper surface and wider than it is long. The outer edges of the eye orbits are broadly triangular and project forward. Their stridulating ridge has 20 to 30 tubercles.

Ocypode mortoni is a small-sized species of Ocypode found in South China and southern Japan. It is closely related to O. stimpsoni which is also found in the same regions, but can be distinguished by the presence of styles on their eyestalks. They also share the same range with O. ceratophthalmus, but O. mortoni have 35 to 71 striae on their stridulating ridges and their smaller claws have broadly rounded to flattened tips.

<i>Ocypode rotundata</i> Species of crustacean

Ocypode rotundata, also known as the rounded ghost crab, is a large-sized species of Ocypode found in the southern coast of the Arabian Peninsula to northern India, including the Persian Gulf. They are very similar to O. saratan but can be distinguished by having 10 to 15 irregularly spaced elongated tubercles with striae on their stridulating ridges and a thumb-like palp on their gonopods. Their eyestalks possess styles.

Ocypode saratan, also known as the Red Sea ghost crab, is a medium to large-sized species of Ocypode found in the Red Sea. Very common species in the Red Sea, and one of only two species known from there. Also known from Madagascar through Indian Ocean to the Western Pacific. Their eyestalks possess styles.

Ocypode stimpsoni is a small-sized species of Ocypode found in China, the Korean Peninsula, and Japan. They closely resemble O. mortoni but can easily be distinguished because they lack styles on their eyestalks.

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[1]

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