''Iguanodectes gracilis'' is a small species of freshwater fish from

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

. It is known solely from the Rio Negro river system, which restricts its range only to

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

. The name "gracilis" is in reference to a slender body shape, alongside the diminutive nature of ''I. gracilis'' - it is the smallest member of the genus ''Iguanodectes''. Granted species status in 1993, ''I. gracilis'' is one of the more recent additions to the genus ''Iguanodectes''. The publication that described it also described congeners ''I. variatus'' and ''I. polylepis'', both of which are generally more well-studied.

Description

''Iguanodectes gracilis'' is a small species, reaching a maximum of 4.6 cm (1.8 in)

standard length Fish measurement is the measuring of individual fish and various parts of their anatomies. These data are used in many areas of ichthyology, including taxonomy and fisheries biology. Overall length * Standard length (SL) is the length of a fish m ...

(SL). As with all

Iguanodectinae Iguanodectinae is a subfamily of small freshwater fish in the family Iguanodectidae. They are most prominently found in the Amazon river basin and its major tributaries, but they are also known from the Tocantins, Orinoco, and Paraguay rivers. ...

, ''I. gracilis'' has a compressed, elongate body, comparable to the general shape of a

smelt Smelt may refer to: * Smelting, chemical process * The common name of various fish: ** Smelt (fish), a family of small fish, Osmeridae ** Australian smelt in the family Retropinnidae and species ''Retropinna semoni'' ** Big-scale sand smelt ''At ...

minnow Minnow is the common name for a number of species of small freshwater fish, belonging to several genera of the families Cyprinidae and Leuciscidae. They are also known in Ireland as pinkeens. Smaller fish in the subfamily Leusciscidae are co ...

. When compared to congeners, however, it is slightly more slender, and is the smallest of the genus. (The largest is ''

Iguanodectes variatus ''Iguanodectes variatus'' is a small freshwater fish, described from the rivers of South America in 1993. It has a minor presence in the aquarium trade, but lacks the popularity that would make excessive wild capture a legitimate threat. That b ...

'', which reaches 10.3 cm (4.1 in) SL.) ''Iguanodectes gracilis'' bears the greatest resemblance to congener '' I. adujai'', with generally similar coloration aside from the fins; this includes a prominent lateral stripe on a yellowish base body color. (In all ''Iguanodectes'', the

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

is complete.) The fins are largely hyaline, lacking any marking aside from dark pigmentation in the median caudal rays and some scattered chromatophores (color cells) on the base of the anal fin. This is unusual in ''Iguanodectes'', and can be used to tell it apart from several congeners.Géry, J. 1993.
Description de trois espèces nouvelles du genre Iguanodectes (Pisces, Characiformes, Characidae), avec quelques données récentes sur les autres espèces.
Revue française d'Aquariologie Herpétologie 19(4): 97-106. All members of the

subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxonomic rank, next below family but more inclusive than genus. Standard nomenclature rules end subfamily botanical names with "-oideae", and zoologi ...

, including ''Iguanodectes'', are equipped with multicuspid teeth. This is a feature shared with sister clade ''

Bryconops ''Bryconops'' is a genus of freshwater fish in the family Iguanodectidae from South America. It consists of small fish, all under half-a-foot long, with slender bodies and silvery scales, though there is some mild color variation. Several specie ...

''. ''Iguanodectes'' as a genus is also generally shallow-chested, lacking the pectoral keel seen in sister genus ''

Piabucus ''Piabucus'' is a genus of freshwater tetras in the family Iguanodectidae. All three species are found in South America, largely the Amazon and its major tributaries. None of them are longer than half a foot long, with the largest reaching a max ...

'', but ''I. gracilis'' specifically is somewhat deep-bodied for an iguanodectid."THE FISH FAMILIES". Field Guide to the Fishes of the Amazon, Orinoco, and Guianas, edited by Peter van der Sleen and James S. Albert, Princeton: Princeton University Press, 2017, pp. 95-96. https://doi.org/10.1515/9781400888801-009

Taxonomy

''Iguanodectes gracilis'' has retained its original name since designation in 1993. However, it went from a member of family

Characidae Characidae, the characids or characins is a family of freshwater subtropical and tropical fish, belonging to the order Characiformes. The name "characins" is the historical one, but scientists today tend to prefer "characids" to reflect their st ...

to family

Iguanodectidae Iguanodectidae is a family of freshwater fish in the order Characiformes that lives in South America. It is home to the subfamily Iguanodectinae (Eigenmann, 1909) and the monotypic ''Bryconops'' clade. Several species in the family, such as the ...

upon research in 2011 that prompted this change for the genera ''Iguanodectes'', ''

'', and ''

''. ''Iguanodectes'' and ''Piabucus'' make up subfamily Iguanodectinae, while ''Bryconops'' makes up a monotypic clade. ''Iguanodectes gracilis'' was first described in the same study that named ''

''. (This was a simultaneous publication of the smallest and largest members of the genus, respectively.) This same paper also named ''

Iguanodectes polylepis ''Iguanodectes polylepis'' is a species of freshwater fish from the rivers of South America. Its range is restricted to Brazil, where it inhabits the Madeira and Purus river basins. It is one of the larger members of the genus, but remains gen ...

''.

Etymology

The specific epithet "gracilis" means "slender" (compare the word "graceful"). This refers to the slender shape and small size of ''I. gracilis''. The generic name "iguanodectes" likely comes from "iguana", the lizard, and "dectes", meaning "bite" or "tooth". (The meaning was not made clear in the original text.)

Distribution and habitat

''Iguanodectes gracilis'' is known from the Rio Negro basin. This is the only habitat from which it has been cited, and it is suspected to be restricted to the region. It has largely been observed in tributaries to the river main. The Rio Negro itself is a

blackwater river A blackwater river is a type of river with a slow-moving channel flowing through forested swamps or wetlands. As vegetation decays, tannins leach into the water, making a transparent, acidic water that is darkly stained, resembling black te ...

, which is a slow-moving, acidic river rich in microbial activity. Said activity breaks down abundant leaf litter provided by thick

riparian A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks ar ...

vegetation, which releases humic substances into the water, staining it brown. Microbial activity, therefore, consumes much of the available dissolved oxygen in blackwater rivers. ''I. gracilis'' is not negatively affected by the resultant low oxygen levels.

Diet and ecology

Species-specific research on the diet and environmental interaction of ''I. gracilis'' is lacking. Patterns seen in the rest of the genus indicate that ''I. gracilis'' is most likely herbivorous, perhaps with some omnivorous traits like ''I. geisleri'' and ''I. adujai''. One known instance of

sympatry In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...

is with ''Potamorrhaphis guianensis'', the freshwater needlefish.

Presence and behavior in aquaria

There is little knowledge of ''I. gracilis'' in captivity. Nonetheless, it has been approved by the Brazilian government for export from the country. It is furthermore acknowledged by MERCOSUR Common Nomenclature (MCN), which is a system responsible for maintaining records of known names for various exports.

Conservation status

''Iguanodectes gracilis'' has not been evaluated by the

IUCN The International Union for Conservation of Nature (IUCN; officially International Union for Conservation of Nature and Natural Resources) is an international organization working in the field of nature conservation and sustainable use of natu ...

. Nonetheless, it inhabits

wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...

s in the Rio Negro basin that are under pressure from

anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows: * Human im ...

sources, like illegal mining and infrastructure development. Such wetlands are known for their ecological fragility, and

wetland conservation Wetland conservation is aimed at protecting and preserving areas where water exists at or near the Earth's surface, such as swamps, marshes and bogs. Wetlands cover at least six per cent of the Earth and have become a focal issue for conservation ...

efforts are frequent.

References

{{Taxonbar, from=Q6403613 Fish described in 1993 Taxa named by Jacques Géry Characiformes Fish of South America