Niger stingray
Encyclopedia
The Niger stingray or smooth freshwater stingray, Dasyatis garouaensis, is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of stingray
Stingray
The stingrays are a group of rays, which are cartilaginous fishes related to sharks. They are classified in the suborder Myliobatoidei of the order Myliobatiformes, and consist of eight families: Hexatrygonidae , Plesiobatidae , Urolophidae , Urotrygonidae , Dasyatidae , Potamotrygonidae The...

 in the family
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...

 Dasyatidae, native to rivers in Nigeria
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...

 and Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...

. Attaining a width of 40 cm (15.7 in), this species can be distinguished by its thin, almost circular pectoral fin disk, slightly projecting snout tip, and mostly smooth skin with small or absent dermal denticles. The Niger stingray feeds on aquatic insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

 larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e and is ovoviviparous. The long stinging spine on the tail of this ray can inflict a painful wound. It has been assessed as Vulnerable
Vulnerable species
On 30 January 2010, the IUCN Red List of Threatened Species identified 9694 Vulnerable species, subspecies and varieties, stocks and sub-populations.-References:...

 by the International Union for Conservation of Nature (IUCN), as its numbers are declining in some areas and it faces heavy fishing pressure
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....

 and habitat degradation.

Taxonomy

The original description of the Niger stingray was published by Alfred Stauch and M. Blanc in 1962, in the scientific journal
Scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...

 Bulletin du Muséum National d'Histoire Naturelle (Série 2). The specific epithet garouaensis refers to the city of Garoua
Garoua
Garoua is the capital of the North Province of Cameroon, lying on the Benue River. The city had 235,996 inhabitants at the 2005 Census, and is an important river port.- Overview :...

, Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...

, where the type specimen was caught. Stauch and Blanc assigned this species to Potamotrygon
Potamotrygon
Potamotrygon is a genus of freshwater stingrays native to the rivers of South America. As of July 2006, FishBase lists seventeen distinct species. The type species is P. hystrix....

, which would have made it the only representative of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 outside South America
South America
South 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...

. In 1975, Thomas Thorson and Donald Watson examined new specimens and concluded, on morphological
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....

 and physiological
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...

 grounds, that this species is a member of Dasyatis
Dasyatis
Dasyatis is a genus of stingray. The members of the genus Neotrygon were formerly included in Dasyatis.-Species:...

.

Distribution and habitat

Along with the pincushion ray
Pincushion ray
The pincushion ray or thorny freshwater stingray is a little-known species of stingray in the family Dasyatidae, found in the rivers and lakes of West Africa...

 (Urogymnus ukpam), the Niger stingray is one of the two freshwater stingrays in Africa
Africa
Africa 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...

. This ray is only known from three river systems in Nigeria
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...

 and Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...

: the Benue
Benue River
The Benue River is the major tributary of the Niger River. The river is approximately 1,400 km long and is almost entirely navigable during the summer months...

 and lower Niger River
Niger River
The Niger River is the principal river of western Africa, extending about . Its drainage basin is in area. Its source is in the Guinea Highlands in southeastern Guinea...

, the lower Sanaga River
Sanaga River
The Sanaga River is a river of South Province, Cameroon, Centre Province, Cameroon, and West Province, Cameroon. Its length is 890 kilometers.The Sanaga River forms a boundary between two tropical moist forest ecoregions...

, and the Cross River
Cross River (Nigeria)
Cross River is the main river in southeastern Nigeria and gives its name to Cross River State.It originates in Cameroon, where it takes the name of the Manyu River....

. Records of this species in Lagos Lagoon
Lagos Lagoon
Lagos Lagoon is a lagoon sharing its name with the city of Lagos, Nigeria, the second largest city in Africa, which lies on its south-western side...

 are unconfirmed. The Niger stingray has only been found in fresh water
Fresh Water
Fresh Water is the debut album by Australian rock and blues singer Alison McCallum, released in 1972. Rare for an Australian artist at the time, it came in a gatefold sleeve...

, though the possibility that it may be 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...

 (tolerant of varying salinities
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

) cannot be discounted.

Description

The pectoral fin disk of the Niger stingray is nearly circular, only about 5% longer than it is wide, and contains 122–124 internal rays on either side. The disk is more flattened than that of any other stingray in the region, measuring no more than 11% as thick as wide. The anterior margins of the disk are weakly concave, with the tip of the snout slightly protruding. The eyes are medium-sized and protruding, with small spiracles placed behind. The nares are covered by a flap of skin with a fringed posterior margin, that reaches the small mouth. There are 16–18 tooth rows in the upper jaw and 14–28 tooth rows in the lower jaw. The teeth are small and closely set; those of females are oval and blunt while those of males are triangular with a sharp backward-pointing cusp. A single row of five papilla
Papilla (fish mouth structure)
The papilla, in certain kinds of fish, particularly rays, sharks, and catfish, are small lumps of dermal tissue found in the mouth, where they are "distributed uniformly on the tongue, palate, and pharynx"...

e is found on the floor of the mouth.

The pelvic fins are roughly triangular, and the tail is whip-like and measures twice as long as the disk is wide. One or more stinging spines are positioned between one-sixth and one-fifth of the way along the tail, and a narrow ventral fin fold originates closely behind the spine insertion. The skin is mostly smooth, except for a patch of small, flat dermal denticles at the center of the back. Some rays may lack denticles altogether, while others may have a 2–4 slightly larger denticles along the midline. The coloration is plain brown or gray above, lightening at the disk margin, and white below with the fins outlined in starker white. This species attains a maximum recorded disk width of 40 cm (15.7 in).

Biology and ecology

In common with other freshwater members of its family, the osmoregulatory system
Osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move...

 of the Niger stingray retains substantial amounts of urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

 within the body (albeit much lower than in marine species), as well as the means to concentrate urea. Compared to the Potamotrygon stingrays, which osmoregulate in a similar fashion as freshwater bony fishes and have speciated
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

 dramatically, the Niger stingray and its relatives may be more recent colonizers of fresh water. The ampullae of Lorenzini
Ampullae of Lorenzini
The ampullae of Lorenzini are special sensing organs called electroreceptors, forming a network of jelly-filled pores. They are mostly discussed as being found in cartilaginous fishes ; however, they are also reported to be found in Chondrostei such as Reedfish and sturgeon. Lungfish have also been...

 in this species are smaller and simpler than those of marine stingrays, reflecting the constraints imposed on electroreception
Electroreception
Electroreception is the biological ability to perceive natural electrical stimuli. It has been observed only in aquatic or amphibious animals, since water is a much better conductor than air. Electroreception is used in electrolocation and for electrocommunication.- Overview :Electroreception is...

 by the freshwater environment. The Niger stingray feeds almost exclusively on the aquatic nymphs
Nymph (biology)
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a...

 of mayflies, stoneflies, and caddisflies, rarely also taking true flies
Diptera
Diptera , or true flies, is the order of insects possessing only a single pair of wings on the mesothorax; the metathorax bears a pair of drumstick like structures called the halteres, the remnants of the hind wings. It is a large order, containing an estimated 240,000 species, although under half...

.

The Niger stingray is ovoviviparous; females have a single functional ovary
Ovary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

 on the left side. This species has been estimated to mature at two years of age, with males living up to five years and females seven years. In the Benue River, one examined female 31.5 cm (12.4 in) across was immature, while a male 34.4 cm (13.5 in) across was mature. In the Sanaga River, males mature at under 26.4 cm (10.4 in) across and females at 26–30 cm (10.2–11.8 in) across.

Human interactions

The tail spine of the Niger stingray is potentially injurious to waders; it is known to the Hausa people
Hausa people
The Hausa are one of the largest ethnic groups in West Africa. They are a Sahelian people chiefly located in northern Nigeria and southeastern Niger, but having significant numbers living in regions of Cameroon, Ghana, Cote d'Ivoire, Chad and Sudan...

 as kunaman ruwa or "water scorpion". This species is caught incidentally and sold fresh or smoked for human consumption. Historically it was most common in the Niger-Benue river system, though its numbers there are diminishing; the possible disappearance of this species from its type locality near Garoua was reported to have resulted from drought
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...

. A substantial number can be found in the Sanaga River, though the population trend there is unknown. The International Union for Conservation of Nature (IUCN) has assessed this species as Vulnerable, citing documented population declines and the possible negative effects of overfishing
Overfishing
Overfishing occurs when fishing activities reduce fish stocks below an acceptable level. This can occur in any body of water from a pond to the oceans....

 and habitat degradation. These pressures are likely to increase as the range of this species becomes more heavily populated.
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