Seychellum
Encyclopedia
Seychellum alluaudi is a species of freshwater crab
endemic to the Seychelles
. It lives in rainforest streams on the granitic high islands, where it may be abundant. It grows to a carapace
width of around 50 mm (2 in), and is almost quadrangular in outline. It was described in the East African genus Deckenia
in 1893 and 1894, and separated off as the monotypic genus Seychellum in 1995. It is listed as Least Concern
on the IUCN Red List
, although little is known about its biology. Several hypotheses have been published to explain how the species reached the Seychelles.
in the Seychelles; all the other true crabs
in the islands have marine larvae
. Adult specimens reach a carapace
width of 30–52 mm (1.2–2 ), and a carapace length about 80% of the width. The carapace is almost quadrangular and quite flat, in constract to the more rounded outline in Deckenia
. Its surface is rough, with scattered tubercle
s, and is divided into distinct regions by a series of grooves. Whereas Deckenia has distinctly flattened walking legs, those of Seychellum are normal.
in 1893 by Alphonse Milne-Edwards
and Eugène Louis Bouvier
, as a species in the genus Deckenia
, D. alluaudi; the specific epithet commemorates Charles A. Alluaud
, who had collected the specimens they used. Unaware of that description, Mary J. Rathbun described "Deckenia cristata" in 1894, in a paper published in the Proceedings of the United States National Museum; her description was based on specimens donated to the United States National Museum by William Louis Abbott
who had travelled to the Seychelles in 1890. At the time, the only other species in the genus was D. imitatrix from the coast of East Africa
, and a second species, also from East Africa, was added in 1898. Rathbun synonymised
D. cristata and D. alluaudi herself in 1906. The three species of Deckenia at that time were considered to be sufficiently distinct from other crabs to warrant placement in a separate subfamily (Deckeniinae) or family (Deckeniidae).
In 1995, Peter K. L. Ng, Zdravko Števčić and Gerhard Pretzmann revised the family Deckeniidae, as then circumscribed
, and concluded that "D. alluaudi" could not be accommodated with the other species of Deckenia in the same genus, or even in the same family. They described a new genus, Seychellum, and the species took on its current name. They placed Seychellum in the family Gecarcinucidae
, leaving Deckenia as the only genus in the family Deckeniidae, and restricting its distribution to the African mainland. That taxon is now considered a subfamily of the larger family Potamonautidae
, and Seychellum is again considered a part of it.
, four cannot be assessed in Seychellum alluaudi because of a lack of information. Under the remaining criterion, which measures the species' geographic range
, the species would qualify as Endangered
if there were was a "decline in habitat quality". It is, however, abundant in places, and is present within a protected area, and is therefore listed as "Least Concern
".
in the inner group of the Seychelles
, in the western Indian Ocean
. It is found on the four largest granitic
islands – Mahé
, La Digue
, Silhouette
and Praslin
– and lives in mountain streams that flow through rainforest
.
The closest relatives of Seychellum are the two species of Deckenia
from the African mainland, which together form the subtribe Deckeniina. The bilobed form of the last segment of the mandibular palp, which had been used to argue for a close relationship between Seychellum and Indian crabs of the family Gecarcinucidae
, does not appear to be a reliable phylogenetic character.
The presence of a strictly freshwater species on the Seychelles is hard to explain, biogeographically
. Several possible explanations have been proposed.
The most recent research favours trans-oceanic dispersal, but some uncertainty remains.
Freshwater crab
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...
endemic to the Seychelles
Seychelles
Seychelles , 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....
. It lives in rainforest streams on the granitic high islands, where it may be abundant. It grows to a carapace
Carapace
A carapace is a dorsal 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 underside is called the plastron.-Crustaceans:In crustaceans, the...
width of around 50 mm (2 in), and is almost quadrangular in outline. It was described in the East African genus Deckenia
Deckenia (crab)
Deckenia is a genus of freshwater crabs from East Africa, in the family Potamonautidae, or sometimes in a family of its own, Deckeniidae. The genus was named by Hilgendorf after Karl Klaus von der Decken who collected the first examples during his expeditions to Africa...
in 1893 and 1894, and separated off as the monotypic genus Seychellum in 1995. It is listed as Least Concern
Least Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
on the IUCN Red List
IUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
, although little is known about its biology. Several hypotheses have been published to explain how the species reached the Seychelles.
Description
Seychellum alluaudi is the only truly freshwater crabFreshwater crab
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...
in the Seychelles; all the other true crabs
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
in the islands have marine 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...
. Adult specimens reach a carapace
Carapace
A carapace is a dorsal 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 underside is called the plastron.-Crustaceans:In crustaceans, the...
width of 30–52 mm (1.2–2 ), and a carapace length about 80% of the width. The carapace is almost quadrangular and quite flat, in constract to the more rounded outline in Deckenia
Deckenia (crab)
Deckenia is a genus of freshwater crabs from East Africa, in the family Potamonautidae, or sometimes in a family of its own, Deckeniidae. The genus was named by Hilgendorf after Karl Klaus von der Decken who collected the first examples during his expeditions to Africa...
. Its surface is rough, with scattered tubercle
Tubercle
A tubercle is generally a wart-like projection, but it has slightly different meaning depending on which family of plants or animals it is used to refer to....
s, and is divided into distinct regions by a series of grooves. Whereas Deckenia has distinctly flattened walking legs, those of Seychellum are normal.
Taxonomic history
Seychellum alluaudi was first describedAlpha taxonomy
Alpha taxonomy is the discipline concerned with finding, describing and naming species of living or fossil organisms. This field is supported by institutions holding collections of these organisms, with relevant data, carefully curated: such institutes include natural history museums, herbaria and...
in 1893 by Alphonse Milne-Edwards
Alphonse Milne-Edwards
Alphonse Milne-Edwards was a French mammalologist, ornithologist and carcinologist. He was English in origin, the son of Henri Milne-Edwards and grandson of Bryan Edwards, a Jamaican planter who settled at Bruges .Milne-Edwards obtained a medical degree in 1859 and became assistant to his father...
and Eugène Louis Bouvier
Eugène Louis Bouvier
Eugène Louis Bouvier was a French entomologist and carcinologist. Bouvier was a professor at the Muséum national d'Histoire naturelle...
, as a species in the genus Deckenia
Deckenia (crab)
Deckenia is a genus of freshwater crabs from East Africa, in the family Potamonautidae, or sometimes in a family of its own, Deckeniidae. The genus was named by Hilgendorf after Karl Klaus von der Decken who collected the first examples during his expeditions to Africa...
, D. alluaudi; the specific epithet commemorates Charles A. Alluaud
Charles A. Alluaud
Charles A. Alluaud was a French entomologist.The Alluaud family had owned porcelain factories since the 18th century. His great grandfather had been chairman of the and his grandfather, François Alluaud , was a porcelain manufacturer, archaeologist, and geologist...
, who had collected the specimens they used. Unaware of that description, Mary J. Rathbun described "Deckenia cristata" in 1894, in a paper published in the Proceedings of the United States National Museum; her description was based on specimens donated to the United States National Museum by William Louis Abbott
William Louis Abbott
William Louis Abbott was a wealthy American medical doctor, explorer, ornithologist and field naturalist. He is notable for his prodigious collections of biological specimens and ethnological artefacts from around the world, especially from the Malay Archipelago, as well as for his financial...
who had travelled to the Seychelles in 1890. At the time, the only other species in the genus was D. imitatrix from the coast 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 a second species, also from East Africa, was added in 1898. Rathbun synonymised
Synonym (taxonomy)
In scientific nomenclature, a synonym is a scientific name that is or was used for a taxon of organisms that also goes by a different scientific name. For example, Linnaeus was the first to give a scientific name to the Norway spruce, which he called Pinus abies...
D. cristata and D. alluaudi herself in 1906. The three species of Deckenia at that time were considered to be sufficiently distinct from other crabs to warrant placement in a separate subfamily (Deckeniinae) or family (Deckeniidae).
In 1995, Peter K. L. Ng, Zdravko Števčić and Gerhard Pretzmann revised the family Deckeniidae, as then circumscribed
Circumscription (taxonomy)
In taxonomy, circumscription is the definition of the limits of a taxonomic group of organisms. One goal of taxonomy is to achieve a stable circumscription for every taxonomic group. Achieving stability can be simple or difficult....
, and concluded that "D. alluaudi" could not be accommodated with the other species of Deckenia in the same genus, or even in the same family. They described a new genus, Seychellum, and the species took on its current name. They placed Seychellum in the family Gecarcinucidae
Gecarcinucidae
Gecarcinucidae is a family of freshwater crabs. Some scientists also include the genera placed in the family Parathelphusidae in a larger Gecarcinucidae....
, leaving Deckenia as the only genus in the family Deckeniidae, and restricting its distribution to the African mainland. That taxon is now considered a subfamily of the larger family Potamonautidae
Potamonautidae
Potamonautidae 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...
, and Seychellum is again considered a part of it.
Conservation status
Of the five criteria assessed by the International Union for Conservation of Nature for its IUCN Red ListIUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
, four cannot be assessed in Seychellum alluaudi because of a lack of information. Under the remaining criterion, which measures the species' geographic range
Range (biology)
In biology, the range or distribution of a species is the geographical area within which that species can be found. Within that range, dispersion is variation in local density.The term is often qualified:...
, the species would qualify as Endangered
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...
if there were was a "decline in habitat quality". It is, however, abundant in places, and is present within a protected area, and is therefore listed as "Least Concern
Least Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
".
Distribution and biogeography
Seychellum alluaudi is endemic to the granitic high islandsGranitic Seychelles
The Granitic Seychelles are the islands in the Seychelles which lie in central position on the Seychelles Bank and are composed of granite rock. They make up the majority of the Inner Islands, which in addition include the coral islands along of the rim of the Seychelles Bank, namely Bird Island...
in the inner group of the Seychelles
Seychelles
Seychelles , 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....
, in the western Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
. It is found on the four largest granitic
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
islands – Mahé
Mahé, Seychelles
Mahé is the largest island of the Seychelles, lying in the north east of the nation. The population of Mahé is 80,000. It contains the capital city of Victoria and accommodates 90% of the country's total population...
, La Digue
La Digue
La Digue is the fourth largest inhabited island of the Seychelles, lying east of Praslin and west of Felicite Island. It has a population of about 2,000 people, who mostly live in the west coast villages of La Passe and La Réunion. It has an area of 10 km²...
, Silhouette
Silhouette Island
Silhouette Island lies 20 km northwest of Mahé in the Seychelles. It is the third largest island in the Seychelles. It has an area of 20 km² and has a population of 135, mostly workers on the island. The main settlement is La Passe, where there is a hotel for visitors to Silhouette...
and Praslin
Praslin
Praslin is the second largest island of the Seychelles, lying 44 km north east of Mahé. Praslin has a population of around 6,500 people and comprises two administrative districts; Baie Sainte Anne and Grand' Anse . The main settlements are the Baie Ste Anne, Anse Volbert and Grand' Anse.It was...
– and lives in mountain streams that flow through rainforest
Rainforest
Rainforests are forests characterized by high rainfall, with definitions based on a minimum normal annual rainfall of 1750-2000 mm...
.
The closest relatives of Seychellum are the two species of Deckenia
Deckenia (crab)
Deckenia is a genus of freshwater crabs from East Africa, in the family Potamonautidae, or sometimes in a family of its own, Deckeniidae. The genus was named by Hilgendorf after Karl Klaus von der Decken who collected the first examples during his expeditions to Africa...
from the African mainland, which together form the subtribe Deckeniina. The bilobed form of the last segment of the mandibular palp, which had been used to argue for a close relationship between Seychellum and Indian crabs of the family Gecarcinucidae
Gecarcinucidae
Gecarcinucidae is a family of freshwater crabs. Some scientists also include the genera placed in the family Parathelphusidae in a larger Gecarcinucidae....
, does not appear to be a reliable phylogenetic character.
The presence of a strictly freshwater species on the Seychelles is hard to explain, biogeographically
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
. Several possible explanations have been proposed.
- In 1902, prior to the development of plate tectonicsPlate tectonicsPlate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, Arnold Edward OrtmannArnold Edward OrtmannArnold Edward Ortmann was a naturalist, and zoologist who specialized in malacology.- Biography :Ortmann was born in Magdeburg, Prussia. A student of Ernst Haeckel, he was graduated from the University of Jena in 1885 with a Ph.D. From 1886 on, he worked as an instructor at the University of...
proposed that land bridgeLand bridgeA land bridge, in biogeography, is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonise new lands...
s formerly connected the Seychelles to other land masses. - It has also been suggested that the ancestors of Deckenia and Seychellum lived on a landmass comprising Seychelles and the African mainland, which then separated by continental driftContinental driftContinental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...
; GondwanaGondwanaIn 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,...
is believed to have split up , with the Seychelles separating from the Indian Plate around . An ancient origin would be supported by the absence of freshwater crabs from oceanic islands in the same area, such as MauritiusMauritiusMauritius , officially the Republic of Mauritius is an island nation off the southeast coast of the African continent in the southwest Indian Ocean, about east of Madagascar...
and RéunionRéunionRéunion is a French island with a population of about 800,000 located in the Indian Ocean, east of Madagascar, about south west of Mauritius, the nearest island.Administratively, Réunion is one of the overseas departments of France...
. - Finally, the ancestor of Seychellum may have travelled across the western Indian OceanIndian OceanThe Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
by raftingRaftingRafting or white water rafting is a challenging recreational outdoor activity using an inflatable raft to navigate a river or other bodies of water. This is usually done on white water or different degrees of rough water, in order to thrill and excite the raft passengers. The development of this...
, perhaps at a time when the Seychelles Bank was larger due to lower sea levels. The greater extent of the Seychelles Bank may also explain how the species is now found on four separate islands in the Seychelles archipelago. The time of divergenceDivergenceIn vector calculus, divergence is a vector operator that measures the magnitude of a vector field's source or sink at a given point, in terms of a signed scalar. More technically, the divergence represents the volume density of the outward flux of a vector field from an infinitesimal volume around...
between Seychellum and Deckenia has been estimated independently at and . Seychellum is reported to be more tolerant of salt water than other families of freshwater crabs, which may have allowed it to survive rafting between the African continent and the Seychelles Bank.
The most recent research favours trans-oceanic dispersal, but some uncertainty remains.