''Cancrocaeca xenomorpha'' is a species of

troglobitic A troglobite (or, formally, troglobiont) is an animal species, or population of a species, strictly bound to underground habitats, such as caves. These are separate from species that mainly live in above-ground habitats but are also able to live u ...

(cave-dwelling)

freshwater crab Around 1,300 species of freshwater crabs are distributed throughout the tropics and subtropics, divided among eight family (biology), families. They show direct development and maternal care of a small number of offspring, in contrast to marine c ...

from

Sulawesi Sulawesi ( ), also known as Celebes ( ), is an island in Indonesia. One of the four Greater Sunda Islands, and the List of islands by area, world's 11th-largest island, it is situated east of Borneo, west of the Maluku Islands, and south of Min ...

, the only species in the

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

genus ''Cancrocaeca''. It has been described as the world's "most highly cave-adapted species of crab".

Description

''Cancrocaeca xenomorpha'' is a small crab with long legs. It has no eyes or even vestiges of

eyestalk In anatomy, an eyestalk (sometimes spelled eye stalk and also known as an ommatophore) is a protrusion that extends an eye away from the body, giving the eye a better field of view. It is a common feature in nature and frequently appears in fict ...

s, and has only vague depressions where the orbits of the eyes might be expected; this is the greatest degree of ocular degeneration of any crab. The

carapace A carapace is a dorsal (upper) 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 unde ...

is rounded in outline, without any

rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit ** podium * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ...

, and is slightly wider than it is long. The five pairs of legs are very long, at up to 3.7 times the width of the carapace in length. Females are slightly larger than males, at up to across the carapace, compared to for males. Females also appear to be a darker colour, but only because of the presence of mud particles on the carapace; both sexes are naturally unpigmented.

Distribution

''Cancrocaeca'' is known only from

fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...

in three

karst Karst () is a topography formed from the dissolution of soluble carbonate rocks such as limestone and Dolomite (rock), dolomite. It is characterized by features like poljes above and drainage systems with sinkholes and caves underground. Ther ...

ic caves in

Maros Regency Maros Regency (, ) is a regency of South Sulawesi province of Indonesia. It covers an area of 1,619.12 sq.km, and had a population of 319,002 at the 2010 CensusBiro Pusat Statistik, Jakarta, 2011. and 391,774 at the Census of 2020.Badan Pusat Stat ...

(

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

). At the time of its discovery, no

members of the family Hymenosomatidae were known, although

Danièle Guinot Danièle Guinot (born 1933) is a French biologist, an emeritus professor at the Muséum national d'histoire naturelle in France, known for her research on crabs. Biography Guinot was born in eastern France and educated at the University of Montpe ...

had predicted in 1988 that the family was a likely candidate to produce troglobites. Since that time, further troglobitic species have been discovered in the same family; although they resemble ''Cancrocaeca'' superficially, this is thought to result from

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

, and it is thought likely that ''Cancrocaeca'' arose recently from marine ancestors.

Biology

The biology of ''Cancrocaeca'' is almost entirely unknown. The original collections were often found in association with pieces of

driftwood Driftwood is a wood that has been washed onto a shore or beach of a sea, lake, or river by the action of winds, tides or waves. It is part of beach wrack. In some waterfront areas, driftwood is a major nuisance. However, the driftwood provides ...

washed into the caves from outside, and were found in water at about . The

eggs An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...

are around in diameter, requiring

gonopore A gonopore, sometimes called a gonadopore, is a genital pore in many invertebrates. Hexapods, including insects, have a single common gonopore, except mayflies, which have a pair of gonopores. More specifically, in the unmodified female, it is ...

s that are 15%–25% as wide as the sternum. Because its eggs are large, it is thought that the

larval A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

development may be abbreviated, and that the species may even show direct development, hatching from the eggs as juvenile crabs, rather than as larvae.

Taxonomic history

''Cancrocaeca'' was discovered in a cave in southern Sulawesi in 1989, as part of an expedition by the ' ("Pyrenean Speleological Association"); the expedition collected a number of crabs and sent them to Peter K. L. Ng of the

National University of Singapore The National University of Singapore (NUS) is a national university, national Public university, public research university in Singapore. It was officially established in 1980 by the merging of the University of Singapore and Nanyang University ...

for identification. Among a large number of '' Parathelphusa'' specimens, he discovered a single male of ''Cancrocaeca''. A subsequent expedition in 1990 provided further specimens, including females. The species was described, in a new genus, in a 1991 publication in the ''

Raffles Bulletin of Zoology ''The Raffles Bulletin of Zoology'' is a peer-reviewed open-access scientific journal published by the Lee Kong Chian Natural History Museum at the National University of Singapore. Overview It covers the taxonomy, ecology, and conservation of ...

''. The genus name ''Cancrocaeca'' is said to be derived from ', the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

for "crab", and ', Latin for "blind". The

specific epithet In Taxonomy (biology), taxonomy, binomial nomenclature ("two-term naming system"), also called binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin gramm ...

''xenomorpha'' is said to be derived from

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

''xenos'' for "strange" and Greek ''morphos'' for "morphology", because of the crab's unusual features. In ancient Greek, ''morphos'' is not attested, while ''morphē'' (μορφή) means "shape" or "form".Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon. revised and augmented throughout by Sir Henry Stuart Jones. with the assistance of. Roderick McKenzie.'' Oxford: Clarendon Press.

References

{{Taxonbar, from=Q5031534 Crabs Freshwater crustaceans of Asia Cave crustaceans Crustaceans of Indonesia Monotypic decapod genera