''Rhamphichthys'' (''Rhamphos ='' Greek for beak and ''Ichthys'' = Greek for fish) is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of fish that includes the South American sand knifefish. These fish are eel shaped (or anguiform) with a distinct beak like snout which gave them their name. Like most other knifefish ''Rhamphichthys'' species have electrical organs that help them live in the murky waters of South America. Currently there are 10 recognized species of ''Rhamphichthys'', although many changes have been made in their taxonomy since their original discovery.

Species

There have been multiple name changes within the genus, included here are currently accepted species, previous groupings that have been collapsed into single species, and genus changes. * '' Rhamphichthys apurensis'' Fernández-Yépez, 1968 **''Gymnorhamphichthys apurensis'' Fernández-Yépez, 1968 * '' Rhamphichthys atlanticus'' Triques, 1999 * '' Rhamphichthys drepanium'' Triques, 1999 * '' Rhamphichthys hahni'' Meinken, 1937 **''Sternarchorhamphus hahni'' Triques, 1999 * ''

Rhamphichthys heleios ''Rhamphichthys'' (''Rhamphos ='' Greek for beak and ''Ichthys'' = Greek for fish) is a genus of fish that includes the South American sand knifefish. These fish are eel shaped (or anguiform) with a distinct beak like snout which gave them thei ...

'' T. P. Carvalho & J. S. Albert, 2015Carvalho, T.P. & Albert, J.S. (2015): A New Species of ''Rhamphichthys'' (Gymnotiformes: Rhamphichthyidae) from the Amazon Basin. ''Copeia, 103 (1): 34-41.'' * '' Rhamphichthys lineatus'' Castelnau, 1855 * '' Rhamphichthys longior'' Triques, 1999 * ''

Rhamphichthys marmoratus ''Rhamphichthys'' (''Rhamphos ='' Greek for beak and ''Ichthys'' = Greek for fish) is a genus of fish that includes the South American sand knifefish. These fish are eel shaped (or anguiform) with a distinct beak like snout which gave them thei ...

'' Castelnau, 1855 * '' Rhamphichthys pantherinus'' Castelnau, 1855 * '' Rhamphichthys rostratus''

Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...

, 1766 **''Rhamphichthys blochii'' Kaup, 1856 **''Gymnotus longirostrus'' Lecepède, 1800 **''Phamphichthys reinhardti Kaup, 1856'' **''Gymnotus rostratus''

, 1766 **''Rhamphichthys schneider'' Kaup, 1856 **''Rhamphichthys schomburgki'' Kaup, 1856 Rhamphichthys rostratus

Range and Habitat

All species within ''Rhamphichthys'' can be found in the major river systems of

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 ...

, notably the

Amazon Basin The Amazon basin is the part of South America drained by the Amazon River and its 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 of Bolivi ...

Orinoco River The Orinoco () is one of the longest rivers in South America at . Its drainage basin, sometimes known as the Orinoquia, covers , with 76.3 percent of it in Venezuela and the remainder in Colombia. It is the fourth largest river in the wor ...

Rio de la Plata Rio or Río is the Portuguese, Spanish, Italian, and Maltese word for "river". When spoken on its own, the word often means Rio de Janeiro, a major city in Brazil. Rio or Río may also refer to: Geography Brazil * Rio de Janeiro * Rio do Sul, a ...

, and the

Paraná River The Paraná River ( es, Río Paraná, links=no , pt, Rio Paraná, gn, Ysyry Parana) is a river in south-central South America, running through Brazil, Paraguay, and Argentina for some ."Parana River". Encyclopædia Britannica. Encyclopædia Br ...

. They have been found in pools created during the rainy season then become isolated as the waters recede. They prefer to stay near the bottom of soft bottom rivers near steep banks with lots of vegetation. These waters are usually extremely murky and full of silt deposits which makes sight difficult, perfect for organisms that can detect their surroundings with weak electric signals.

Characteristics and Biology

The sand knifefishes are characterized by their elongated bill-like snout and elongated eel-like body. On average they will reach between 26.5 and 100 cm in length. They have a long

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

that starts just behind their small

pectoral fins Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

and end almost at the end of the body. Their

caudal Caudal may refer to: Anatomy * Caudal (anatomical term) (from Latin ''cauda''; tail), used to describe how close something is to the trailing end of an organism * Caudal artery, the portion of the dorsal aorta of a vertebrate that passes into the ...

or tail fin is either highly reduced or missing (depending on the species). All species lack both

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 * Dorsal co ...

and

pelvic fins Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

but do have a dorsal ridge down the center of their back. They also have very small eyes, not relying on sight to find food or detect other organisms. Coloration varies between species, but all seem to have highly mottles coloration with a lighter base color on the belly (or venter) and darker colors on the back (or dorsum). The spots seem to be larger on the back and get smaller as they move down to the underside. Some individuals seem to have a blue tint to their anal fin, but it is not yet understood if that is characteristic of a species or not. Internally they have highly reduced

gill rakers Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch (gill arch) and are involved with suspension feeding tiny prey. They are not to be confused with the gill filaments that compose the fleshy part of the ...

, a large stomach, and an anteriorly positioned anus which lies directly under the

Not much is known about their behavior and biology. They are opportunistic predators feeding predominately on invertebrates such as insects and crustaceans dug up from the sandy river bottoms, but seem to eat small fish as well. They seem to keep to a relatively small home range and are solitary except while breeding. They use their electrical organs to identify prey, competitors, mates, and possible predators. It is thought they mate during the dry season when waters are warm and low.

Evolution

Within the order

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 ...

there are five families: ''Rhamphichthys'' along with the genera ''

Steatogenys ''Steatogenys'' is a genus of gymnotiform knifefishes found in the Amazon, Orinoco and Essequibo river basins in tropical South America. The widespread and common ''S. elegans'' is found in a wide range of habitats, from the shallow essentiall ...

'', Hypopygus, ''

Gymnorhamphichthys ''Gymnorhamphichthys'' is a genus of South American sand knifefishes found in the Amazon, Araguaia, Orinoco and Río de la Plata basins, as well as rivers in the Guianas. They inhabit both small streams and large rivers, but usually over a san ...

'' and Iracema, lie within the family

Rhamphichthyidae Sand knifefish are freshwater electric fish of the family Rhamphichthyidae, from freshwater habitats in South America. Just like most part of the members of the Gymnotiformes group, they also have elongated and compressed bodies and electric o ...

which is sister to Hypopmidae. These two families are the second youngest within the Order. The youngest being Apteronotidae. It is interesting to note that there are fewer described species in this order with respect to the number of Families described. This is likely due to how little we know about the group as a whole, they are hard to find and study, so it is likely that there are far more species than we currently know about. It is also possible that because of their specialized nature they have not been driven to differentiate further to alter competition loads. Please note the phylogeny in this image is incorrect: all recent phyoogenetic and phylogenomic studies report the following interfamily relationships: Gymnotidae ((Hypopomidae, Rhamphichthyidae) (Sternopygidae, Apteronotidae)). See Tagliacollo, V. A., Bernt, M. J., Craig, J. M., Oliveira, C., & Albert, J. S. (2016). Model-based total evidence phylogeny of Neotropical electric knifefishes (Teleostei, Gymnotiformes). ''Molecular phylogenetics and evolution'', ''95'', 20-33.

Weakly Electric Ability

''Rhamphichthys'', like other Gymnotiformes fishes, contain an electric organ that takes up the back third if the body. This electric organ pulses at a constant frequency and allows the fish to 'visualize' its surroundings by forming an electric field around the body. Any disturbance to that electric field indicates an object that it close y. Electroreceptors an the body allows the fish to indicate the type of disturbance which allows them to indicate that type of object is nearby. There are different types of electrical pulses that the fish can give off and each has a different function. This is necessary because their electric fields can be disturbed by other fish using their own electrical field, causing 'noise'. This makes it difficult for individuals to orient themselves. By using different types of pulses noise is reduced. Different species emit at different frequency ranges and individuals within a species emit at their own unique frequency. These differences in frequency allow individuals to identify other individuals, not just distinguish between species. Different types of pulses * Orientation - constant low-level output for orientation in the water to avoid obstacles * Prey location - Short pulses that work similar to sonar where disturbance responses identify location of prey items * JAR (jamming avoidance response) - Defense strategy where a strong pulse is emitted to interrupt sensory systems of potential predators or threats. * Gradual Frequency falls - short identifying pulses used to communicate with same species individuals or 'neighbors' for territory identification or finding mates.

References

Rhamphichthyidae Fish of South America Freshwater fish genera Taxa named by Johannes Peter Müller Taxa named by Franz Hermann Troschel {{Gymnotiformes-stub