Ghost crabs are semiterrestrial crabs of the subfamily Ocypodinae, from Ancient Greek ὠκύς (okús), meaning "swift", and πούς (poús), meaning "foot". 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. They are also sometimes called sand crabs, though the name refers to various other crabs that do not belong to the subfamily.

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.

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.

Genera

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

thumb|left|[[stridulation|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. 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.

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. They are predominantly nocturnal. They remain in their burrows during the hottest part of the day, and throughout the coldest part of the winter. 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.

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.

Ghost crabs also have the ability to change colors to match their surroundings by adjusting the concentration and dispersal of pigments within their chromatophores. 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.

thumb|A horned ghost crab ([[Ocypode ceratophthalma|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. 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.

See also

  • Heloecius - the semaphore crab

References

  • Taylor, Jennifer R. A., et al. “Growling from the Gut: Co-Option of the Gastric Mill for Acoustic Communication in Ghost Crabs.” Proceedings of the Royal Society B: Biological Sciences, vol. 286, no. 1910, 2019, p. 20191161., doi:10.1098/rspb.2019.1161.
  • Video of Ocypode ceratophthalma eating a moon crab in Saint John's Island, Singapore