''Eigenmannia vicentespelaea'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of weakly electric knifefish in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Sternopygidae. Native to the São Domingos

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 ...

area in central

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

, it is the only known knifefish to exclusively inhabit caves. Measuring up to long, ''E. vicentespelaea'' can be distinguished from its relatives by its translucent body and reduced or absent eyes. As some individuals retain well-developed eyes, this fish may have colonized caves only recently in

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

ary time.

Taxonomy

Brazilian

ichthyologist Ichthyology is the branch of zoology devoted to the study of fish, including bony fish (Osteichthyes), cartilaginous fish (Chondrichthyes), and jawless fish (Agnatha). According to FishBase, 35,800 species of fish had been described as of March 2 ...

Mauro Luís Triques described ''E. vicentespelaea'' in a 1996 volume of the

scientific journal In academic publishing, a scientific journal is a periodical publication designed to further the progress of science by disseminating new research findings to the scientific community. These journals serve as a platform for researchers, schola ...

''Revue Française de Aquariologie'', from two specimens caught in 1978. He chose 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 ...

''vicentespelaea'' from the São Vicente I cave, the

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...

locality, and ''speleum'',

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 "cave". Based on color pattern and

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

, it is included in the '' E. virescens''

species group In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

with ''E. virescens'', '' E. trilineata'', and a third undescribed species.

Distribution and habitat

The only known troglobitic (exclusively cave-dwelling) member of the

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Gymnotiformes The Gymnotiformes are an order of teleost bony fishes commonly known as Neotropical knifefish or South American knifefish. They have long bodies and swim using undulations of their elongated anal fin. Found almost exclusively in fresh water (the ...

, ''E. vicentespelaea'' is found in the São Vicente I and São Vicente II caves near São Domingos, in the central

Brazilian state The federative units of Brazil () are subnational entities with a certain degree of autonomy (self-government, self-regulation, and self-collection) and endowed with their own government and constitution, which together form the Federative Repu ...

Goiás Goiás () is a Brazilian States of Brazil, state located in the Central-West Region, Brazil, Central-West region. Goiás borders the Federal District (Brazil), Federal District and the states of (from north clockwise) Tocantins, Bahia, Minas Ge ...

. These

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

caves are fed by streams connected to the

Paranã River Paranã (formerly known as São João da Palma) is a municipality in the state of Tocantins in the Northern region of Brazil. See also *List of municipalities in Tocantins This is a list of the municipalities in the States of Brazil, state of T ...

, a

tributary A tributary, or an ''affluent'', is a stream or river that flows into a larger stream (''main stem'' or ''"parent"''), river, or a lake. A tributary does not flow directly into a sea or ocean. Tributaries, and the main stem river into which they ...

of the upper

Tocantins River The Tocantins River ( , Parkatêjê dialect, Parkatêjê: ''Pyti'' ɨˈti is a river in Brazil, the central fluvial artery of the country. In the Tupi language, its name means "toucan's beak" (''Tukã'' for "toucan" and ''Ti'' for "beak"). It ...

in the

Amazon Basin The Amazon basin is the part of South America drained by the Amazon River and its tributary, tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries ...

. Four other knifefish, ''

Archolaemus blax ''Archolaemus blax'' is a species of glass knifefish endemic to Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-larges ...

'', '' Porotergus'' species, ''

Sternarchorhynchus ''Sternarchorhynchus'' is a genus of ghost knifefishes with a long, decurved snout that are found in river basins in tropical South America. Distribution, habitat and conservation Of the 32 recognized species, more than are restricted to the A ...

mesensis'' and '' Sternopygus'' species, have been recorded in the São Vicente II cave together with ''E. vicentespelaea'', but they are not true troglobites and also inhabit aboveground waters.

Description

''Eigenmannia vicentespelaea'' has an elongated, laterally compressed body with curved dorsal and ventral profiles, tapering to a filament without a

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

. The

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

stretches the length of the body, while the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

and

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hi ...

s are absent. The ratio between the head length and the distance from snout tip to anus is greater than in other ''Eigenmannia''. The snout is short, with a small terminal mouth containing small, slender teeth on both jaws. The posterior pair of nares are positioned close to the eyes, which are reduced in size or, in a minority of individuals, externally absent. The

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

openings are small. The body is entirely covered by tiny

cycloid scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scale (zoology), scales, which can also provide effective Underwater camouflage, camouflage through the us ...

s. Living fish are translucent, more so than other ''Eigenmannia'', with 2–3 horizontal stripes. Among the larger members of its genus, ''E. vicentespelaea'' reaches in total length.

Biology and ecology

The variation in the eye size of ''E. vicentespelaea'' suggest that this species may have been isolated in caves for a relatively short time. As far as known, all South American knifefishes are capable of generating an

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

for

electrolocation Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes, such ...

; this

adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...

for activity in low light conditions would have been conducive to them colonizing completely dark cave environments. The relatively larger size attained by ''E. vicentespelaea'' compared to its sister species may indicate a release from

predation Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

or competition once it became cave-dwelling.

Human interactions

''E. vicentespelaea'' is threatened by siltation caused by destruction of

riparian forest A riparian forest or riparian woodland is a forested or wooded area of land adjacent to a body of water such as a river, stream, pond, lake, marshland, estuary, canal, Sink (geography), sink, or reservoir. Due to the broad nature of the definitio ...

s upstream of its cave habitat. The range of this species itself protected by Terra Ronca State Park.

References

{{Taxonbar, from=Q3049356 vicentespelaea Cave fish Knifefish of Brazil Endemic fish of Brazil Taxa named by Mauro Luís Triques Fish described in 1996