Freshwater crab
Encyclopedia
There are around 1,300 species of freshwater crabs, distributed throughout the tropics and subtropics, divided among eight families
. They show direct development and maternal care of a small number of offspring, in contrast to marine crabs which release thousands of planktonic larvae
. This limits the dispersal abilities of freshwater crabs, so they tend to be endemic to small areas. As a result, a large proportion are threatened with extinction
.
of freshwater crabs, out of a total of 6,700 species of crabs across all environments. The total number of species of freshwater crabs, including undescribed species is thought to be up to 65% higher, potentially up to 2,155 species, although most of the additional species are currently unknown to science. They belong to eight families, each with a limited distribution, although various crabs from other families are also able to tolerate freshwater conditions (euryhaline
) or are secondarily adapted to fresh water. The phylogenetic relationships
between these families is still a matter of debate, and it is therefore unclear how many times the freshwater lifestyle has evolved among the true crabs. The eight families are:
Superfamily Trichodactyloidea
Superfamily Potamoidea
:
Superfamily Gecarcinucoidea
Superfamily Pseudothelphusoidea:
The fossil record of freshwater organisms is typically poor, and so few fossils of freshwater crabs have been found. The oldest is Tanzanonautes tuerkayi, from the Oligocene
of East Africa
, and the evolution of freshwater crabs is likely to post-date the break-up of the supercontinent Gondwana
.
The external morphology
of freshwater crabs varies very little, and so the form of the gonopod (first adbominal appendage, modified for insemination) is of critical importance for classification
. Development
of freshwater crabs is characteristically direct, where the eggs
hatch as juveniles
, with the larval stages
passing within the egg. The broods comprise only a few hundred eggs (compared to hundreds of thousands for marine crabs) each of which is quite large, at a diameter of around 1 mm (0.0393700787401575 in).
The colonisation of fresh water has required crabs to alter their water balance; freshwater crabs can reabsorb salt from their urine
, and have various adaptation
s to reduce the loss of water. In addition to their gill
s, freshwater crabs have a "pseudo-lung" in their gill chamber that allows them to breathe in air. These developments have pre-adapted
freshwater crabs for terrestrial living, although freshwater crabs need to return to water periodically in order to excrete
ammonia
.
s, as well as in tree boles or cave
s. They are primarily nocturnal, emerging to feed at night; most are omnivore
s, although a small number are specialist predators, such as Platyhelphusa armata from Lake Tanganyika
, which feeds almost entirely on snail
s. Some species provide important food sources for various vertebrate
s. A number of freshwater crabs are secondary hosts of flukes
in the genus Paragonimus
, which causes paragonimiasis
in humans.
The majority of species are narrow endemics, occurring in only a small geographical area. This is at least partly attributable to their poor dispersal
abilities, and low fecundity
, and to habitat fragmentation
caused by the world's human population. In West Africa
, species that live in savanna
hs have wider ranges than species from the rainforest
; in East Africa
, species from the mountains have restricted distributions, while lowland species are more widespread.
Every species of freshwater crab described so far has been assessed by the International Union for Conservation of Nature (IUCN); of the species for which data are available, 32% are threatened with extinction
. For instance, all but one of Sri Lanka
's 50 freshwater crab species are endemic to that country, and more than half are critically endangered
.
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
. They show direct development and maternal care of a small number of offspring, in contrast to marine crabs which release thousands of planktonic larvae
Crustacean larvae
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow...
. This limits the dispersal abilities of freshwater crabs, so they tend to be endemic to small areas. As a result, a large proportion are threatened with extinction
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...
.
Systematics
There are more than 1,300 described speciesSpecies description
A species description or type description is a formal description of a newly discovered species, usually in the form of a scientific paper. Its purpose is to give a clear description of a new species of organism and explain how it differs from species which have been described previously, or are...
of freshwater crabs, out of a total of 6,700 species of crabs across all environments. The total number of species of freshwater crabs, including undescribed species is thought to be up to 65% higher, potentially up to 2,155 species, although most of the additional species are currently unknown to science. They belong to eight families, each with a limited distribution, although various crabs from other families are also able to tolerate freshwater conditions (euryhaline
Euryhaline
Euryhaline organisms are able to adapt to a wide range of salinities. An example of a euryhaline fish is the molly which can live in fresh, brackish, or salt water. The European shore crab is an example of a euryhaline invertebrate that can live in salt and brackish water...
) or are secondarily adapted to fresh water. The phylogenetic relationships
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
between these families is still a matter of debate, and it is therefore unclear how many times the freshwater lifestyle has evolved among the true crabs. The eight families are:
Superfamily Trichodactyloidea
- TrichodactylidaeTrichodactylidaeTrichodactylidae is a family of crabs, in its own superfamily, Trichodactyloidea. They are all freshwater animals from Central and South America, including some offshore islands, such as Ilhabela, São Paulo. Only one of the 50 species is known from the fossil record, Sylviocarcinus piriformis from...
(Central AmericaCentral AmericaCentral America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...
and South AmericaSouth AmericaSouth America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
)
Superfamily Potamoidea
Potamoidea
Potamoidea is a superfamily of freshwater crabs, comprising the two families Potamidae and Potamonautidae. Two previously recognised families, Deckeniidae and Platythelphusidae, are now treated as parts of the family Potamonautidae....
:
- PotamidaePotamidaePotamidae is a family of freshwater crabs. Its more than 650 species and nearly 100 genera are divided into two subfamilies — Potaminae around the Mediterranean Sea, on Socotra and eastwards to Northern India, and Potamiscinae in East Asia....
(Mediterranean BasinMediterranean BasinIn biogeography, the Mediterranean Basin refers to the lands around the Mediterranean Sea that have a Mediterranean climate, with mild, rainy winters and hot, dry summers, which supports characteristic Mediterranean forests, woodlands, and scrub vegetation...
and AsiaAsiaAsia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
) - PotamonautidaePotamonautidaePotamonautidae is a family of freshwater crabs endemic to tropical parts of Africa and adjacent islands, including Madagascar, the Seychelles, Zanzibar, Mafia, Pemba, Bioko, São Tomé, Príncipe and Sherbro Island. It comprises 18 extant genera and 138 extant species...
(AfricaAfricaAfrica is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, including MadagascarMadagascarThe Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
) - Deckeniidae (East AfricaEast AfricaEast Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
and SeychellesSeychellesSeychelles , officially the Republic of Seychelles , is an island country spanning an archipelago of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar....
) – also treated as part of Potamonautidae - Platythelphusidae (East Africa) – also treated as part of Potamonautidae
Superfamily Gecarcinucoidea
Gecarcinucoidea
Gecarcinucoidea is a superfamily of freshwater crabs. Its members have been grouped into families in various ways, with some authors recognising families such as "Deckeniidae", "Sundathelphusidae", but only two families are currently recognised: Gecarcinucidae and Parathelphusidae....
- GecarcinucidaeGecarcinucidaeGecarcinucidae is a family of freshwater crabs. Some scientists also include the genera placed in the family Parathelphusidae in a larger Gecarcinucidae....
(Asia) - ParathelphusidaeParathelphusidaeParathelphusidae is a family of freshwater crabs mainly in Southeast Asia. Their closest living relatives are the Gecarcinucidae.The Parathelphusidae inhabit rivers, lakes and rice paddies. Some species, for example from the genus Somanniathelphusa, are locally important as food, particularly in...
(Asia and AustralasiaAustralasiaAustralasia is a region of Oceania comprising Australia, New Zealand, the island of New Guinea, and neighbouring islands in the Pacific Ocean. The term was coined by Charles de Brosses in Histoire des navigations aux terres australes...
)
Superfamily Pseudothelphusoidea:
- PseudothelphusidaePseudothelphusidaePseudothelphusidae is a family of freshwater crabs found chiefly in mountain streams in the Neotropics. They are believed to have originated in the Greater Antilles and then crossed to Central America via a Pliocene land bridge.-Parasitology:...
(Central America and South America)
The fossil record of freshwater organisms is typically poor, and so few fossils of freshwater crabs have been found. The oldest is Tanzanonautes tuerkayi, from the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
of East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
, and the evolution of freshwater crabs is likely to post-date the break-up of the supercontinent Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
.
Description and life cycle
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
of freshwater crabs varies very little, and so the form of the gonopod (first adbominal appendage, modified for insemination) is of critical importance for classification
Biological classification
Biological classification, or scientific classification in biology, is a method to group and categorize organisms by biological type, such as genus or species. Biological classification is part of scientific taxonomy....
. Development
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
of freshwater crabs is characteristically direct, where the eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
hatch as juveniles
Juvenile (organism)
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles sometimes look very different from the adult form, particularly in terms of their colour...
, with the larval stages
Crustacean larvae
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow...
passing within the egg. The broods comprise only a few hundred eggs (compared to hundreds of thousands for marine crabs) each of which is quite large, at a diameter of around 1 mm (0.0393700787401575 in).
The colonisation of fresh water has required crabs to alter their water balance; freshwater crabs can reabsorb salt from their urine
Urine
Urine is a typically sterile liquid by-product of the body that is secreted by the kidneys through a process called urination and excreted through the urethra. Cellular metabolism generates numerous by-products, many rich in nitrogen, that require elimination from the bloodstream...
, and have various adaptation
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....
s to reduce the loss of water. In addition to their gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s, freshwater crabs have a "pseudo-lung" in their gill chamber that allows them to breathe in air. These developments have pre-adapted
Preadaptation
In evolutionary biology, preadaptation describes a situation where a species evolves to use a preexisting structure or trait inherited from an ancestor for a potentially unrelated function...
freshwater crabs for terrestrial living, although freshwater crabs need to return to water periodically in order to excrete
Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
.
Ecology and conservation
Freshwater crabs are found throughout the tropical and sub-tropical regions of the world. They live in a wide range of water bodies, from fast-flowing rivers to swampSwamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...
s, as well as in tree boles or cave
Cave
A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...
s. They are primarily nocturnal, emerging to feed at night; most are omnivore
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
s, although a small number are specialist predators, such as Platyhelphusa armata from Lake Tanganyika
Lake Tanganyika
Lake Tanganyika is an African Great Lake. It is estimated to be the second largest freshwater lake in the world by volume, and the second deepest, after Lake Baikal in Siberia; it is also the world's longest freshwater lake...
, which feeds almost entirely on snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
s. Some species provide important food sources for various vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s. A number of freshwater crabs are secondary hosts of flukes
Trematoda
Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes".-Taxonomy and biodiversity:...
in the genus Paragonimus
Paragonimus
Paragonimus is an important genus of flatworms, or platyhelminths, that includes Paragonimus westermani, an infectious lung fluke endemic to Asia....
, which causes paragonimiasis
Paragonimiasis
Paragonimiasis is a food-borne parasitic infection caused by the lung fluke, most commonly Paragonimus westermani. It infects an estimated 22 million people worldwide. It is particularly common in East Asia. More than 30 species of trematodes of the genus Paragonimus have been reported which...
in humans.
The majority of species are narrow endemics, occurring in only a small geographical area. This is at least partly attributable to their poor dispersal
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
abilities, and low fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...
, and to habitat fragmentation
Habitat fragmentation
Habitat fragmentation as the name implies, describes the emergence of discontinuities in an organism's preferred environment , causing population fragmentation...
caused by the world's human population. In West Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...
, species that live in savanna
Savanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...
hs have wider ranges than species from the rainforest
Rainforest
Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm...
; in East Africa
East Africa
East Africa or Eastern Africa is the easterly region of the African continent, variably defined by geography or geopolitics. In the UN scheme of geographic regions, 19 territories constitute Eastern Africa:...
, species from the mountains have restricted distributions, while lowland species are more widespread.
Every species of freshwater crab described so far has been assessed by the International Union for Conservation of Nature (IUCN); of the species for which data are available, 32% are threatened with extinction
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...
. For instance, all but one of Sri Lanka
Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...
's 50 freshwater crab species are endemic to that country, and more than half are critically endangered
Critically endangered
Version 2010.3 of the IUCN Red List of Threatened Species identified 3744 Critically Endangered species, subspecies and varieties, stocks and subpopulations.Critically Endangered by kingdom:*1993 Animalia*2 Fungi*1745 Plantae*4 Protista-References:...
.