Gymnotus interruptus, a new species of electric fish from the Rio de Contas basin, Bahia, Brazil (Teleostei: Gymnotiformes: Gymnotidae)

A new species of Gymnotus Linnaeus is described from Riacho Cambiriba a clear water stream, tributary of the Rio de Contas basin, Bahia, northeastern Brazil. The new species is a member of the G. carapo species-group, which is characterized by the presence of a transparent patch near the posterior end of the anal fin and two (vs. one) laterosensory pores in the dorso-posterior region of the preopercle bone. Gymnotus interruptus may be diagnosed by a unique combination of morphometric and meristic characters: wider interorbital distance (44.6 – 45.9% of STO); 9 scales above lateral line at midbody; 23 – 28 ventrally oriented lateral line rami; 37 – 40 pored lateral line scales to first ventral ramus of lateral line; shorter snout (29.6 – 30.5% of STO). Gymnotus interruptus, the first record of a gymnotiform fish from the rio de Contas basin, may be easily distinguished from G. carapo or G. bahianus, two congeners also occurring in northeastern Brazil, by its colour pattern, in which the pale interbands, anterior to vertical through first ventral lateral line ramus, are ventrally and/or dorsally fragmented, allowing union of adjacent dark bands.


Introduction
The Gymnotiformes comprises a diversified assemblage of slender electric fishes, with about 170 living species, all endemic to the Neotropical region.They are classified into five families: Gymnotidae, Hypopomidae, Rhamphichthydae, Sternopygidae and Apteronotidae (aLbert & Crampton, 2003b), among which the Gymnotidae comprises 38 species, including the one herein described.Except Electrophorus electricus (Linnaeus, 1758), all species of the latter family are placed in Gymnotus Linnaeus, the most geographically widespread genus of the order, occurring from southern Mexico (15°N) to northern Argentina (36°S), in all major river systems.The main apomorphic character states diagnostic for the genus are the absence of displaced haemal spines at the posterior end of the body cavity, two processes separated by a notch in the anterior portion of mesethmoid bone and the absence of cranial fontanels on the dorsal surface of the head in adults (Campos-da-paz & Costa, 1996;aLbert & Crampton, 2003b).On the basis of derived colour patterns, morphometrics and osteology, three main monophyletic groups have been recognized within Gymnotus (sensu aLbert et al. 2005): the G. cylindricus, G. pantherinus and G. carapo species-groups.The latter group contains 21 living species (including the one described here) and is distributed along the whole South America north of the latitude 36ºS, including the Island of Trinidad.It is mainly characterized by the presence of a transparent patch on the posterior end of the anal fin and two pores on the dorso-posterior corner of the preopercle bone (aLbert et al., 2005).The present paper has as purpose to describe a new species of Gymnotus, belonging to the G. carapo species-group, collected in the Riacho Cambiriba, a tributary of the Rio de Contas basin, northeastern Brazil.

Materials and Methods
All measurements were taken as point-to-point linear distances and recorded to the nearest 0.1 mm with the use of digital calipers.Specimens were fixed in 10% formalin during 14 days and then preserved in 70% ethanol.Drawings were made with a camera lucida connected to a stereoscopic microscope.Morphometric measurements follow mago- LeCCia (1978) and Campos-da-paz (2002).The term "postorbital" is used instead of "postocular".To make possible the construction of the diagnosis through the comparison of obtained data with the data available in literature (e.g.original descriptions of valid species and reference papers as aLbert & Crampton, 2003a) the measurements reported as head length (HL) percentage were also reported as snout-to-opercle length percentage (equivalent to head length of some authors, e.g., aLbert & Crampton, 2003a;maxime et al., 2011), making the data comparable to most data present in the literature.Meristic data were taken following the procedures given by aLbert & Crampton (2003a), except for the number of precaudal vertebrae, which follows Campos-da-paz (2002).Bands are counted from posterior to anterior tips of trunk.Osteological data were obtained from the single specimen cleared and counterstained through a modification of the method of tayLor & Van dyke (1985).Bone nomenclature follows aLbert et al. (2005) andmago-LeCCia (1978).The selection of osteological features, relevant at generic and specific levels, follows Campos-da-paz & Costa (1996) & maxime et al. (2011).Measurements and counts were taken from the left side of specimens.None of the exemplars had significant damage at the caudal appendage.All material examined is deposited in the collection of the Laboratory of Systematics and Evolution of Teleost Fishes, Department of Zoology, Institute of Biology, Federal University of Rio de Janeiro (UFRJ).extremities with similar width, anterior portion tube like and posterior portion with globoid expansion; no ossified basibranchials; first pharyngobranchial absent as independently ossified element; second and third ossified, fourth cartilaginous; epibranchials 1 -4 ossified, fifth cartilaginous; upper pharyngeal tooth plate without ossified connection to fourth epibranchial; mesocoracoid present; scapular foramen absent; coracoid (fig.9) without postero-ventral process and with dorsal process; posttemporal in contact but not co-ossified with supracleithrum; extrascapular absent; knife shaped postcleithrum almost as long as coracoid and with half width of coracoid; four pectoral radials; 33 precaudal vertebrae (including those of Weberian apparatus); neural and haemal spines present; well-developed inter-muscular bones, clustered posteriorly to nape and sparse on dorsal and dorso-lateral back, most of them branched at extremities.

Colouration in life.
Overall colour pattern appears in Fig. 10.Head dark to light brown on region above ventral-most pore of infraorbital series.Ventro-lateral and ventral portions of snout light brown.Ventrolateral and ventral postorbital regions of head with white ground colour with tiny brown chromatophores.Pupils black, sclerotic region white.Highly vascularized regions, like opercle, with pink to red colour than wide; posterior processes of parasphenoid robust, width similar to length; antero-ventral and posterolateral portions of pterosphenoid reaching parasphenoid; cranial fontanels closed; maxilla (fig.6) shaped as inverted "Y", its length approximately one third of half premaxillary length, slightly anteriorly tilted; premaxilla with convex anterior margin; most of sensory canal of mandibular portion autogenous; mesopterygoid with well-developed ascending process reaching horizontal line with median orbitosphenoid; mesopterygoid ascending process simple, robust and longer than maxilla; preopercle with two laterosensory pores on its dorso-posterior portion; opercle sub-triangular; four laminar branchiostegal rays (fig.7), progressively wider from inner pair to outer pair; urohyal similar in size to inner branchiostegal ray, composed of arrowhead shaped ventral plate, anteriorly bifurcated, with dorsal laminar crest as wide as the plate and with similar shape; basihyal (fig.8) equal in length to first ceratobranchial, narrower in its medial portion, both   Colouration in alcohol.Specimens fixed in 10% formalin and preserved for three months in 70% ethanol  this basin is the fourth largest on north-eastern Brazil.
How ever, despite the great area occupied by this important basin, no species of Gymnotus has been recorded to it until this moment (reis et al., 2003).
All specimens of the type series of G. interruptus were collected in the Riacho Cambiriba, a small tributary of rio Gongogi which in turn is a tributary of rio de Contas, consisting of the first gymnotiform fish recorded to the rio de Contas basin.
Other congeners occurring in areas near to the rio de Contas basin are G. bahianus, endemic to rio Almada basin, an isolated small coastal basin of Bahia, and G. carapo which has been recorded from a huge area, including the Amazon and Orinoco basins below c. 500 m elevation, the Island of Trinidad, isolated drainages of Guyanas, and northeastern Brazil Band margins irregularly shaped.In biggest paratype (121 mm TL) ground color gets darker, making bands visualization more difficult.
Habitat and Ecology notes.The specimens were collected with hand nets in a clear water stream which topical width was about 4 m, depth about 1.5 m, with bottom composed of sand, gravel and rocks of variable size (fig.11).Individuals were found hidden among marginal vegetation and roots.The stomach content of smaller paratype showed traces of Nematocera larvae, adult Coleoptera and insect eggs.
Etymology.Named interruptus from the Latin adjective interrupted due to its pale interbands, anterior to vertical through first ventral lateral line ramus, ventrally and/or dorsally fragmented (same as interrupted), allowing union of adjacent dark bands.

Discussion
The rio de Contas basin, located in the Brazilian state of Bahia, has as main components the rivers de Con tas, Gongogi, Brumado, do Antônio, Jacaré, Sin corá, Gavi ão e Jequiezinho.Occupying an area of 55.534 km²,  ( aLbert & Crampton, 2003a).Distinction of G. inter ruptus among the latter congeners is possible, even during field works, by the observation of its colour pattern, in which the pale interbands, anterior to vertical through first ventral lateral line ramus, are ventrally and/or dorsally fragmented, allowing union of adjacent dark bands.Moreover G. interruptus may be distinguished from G. bahianus and G. carapo by having shorter snout ).

Fig. 8 .
Fig. 8. Branchial arches of Gymnotus interruptus (UFRJ 8243) in dorsal view.Inferior right side suppressed to allow better visualization on superior right side.Cartilage presented in dense dotting.

Fig. 11 .
Fig. 11.Riacho Cambiriba, One of the points of view from the environment where the type series of Gymnotus interruptus was collected.

Table 1 .
Morphometric data of Gymnotus interruptus showed as percentage of Total length.

Table 2 .
Morphometric data of Gymnotus interruptus showed as percentage of Head length.