Brachyhypopomus Mago-Leccia, 1994

Crampton, William G. R., Santana, Carlos D. de, Waddell, Joseph C. & Lovejoy, Nathan R., 2016, A taxonomic revision of the Neotropical electric fish genus Brachyhypopomus (Ostariophysi: Gymnotiformes: Hypopomidae), with descriptions of 15 new species, Neotropical Ichthyology (e 150146) 14 (4), pp. 639-790 : 646-664

Brachyhypopomus Mago-Leccia, 1994

Brachyhypopomus Mago-Leccia,1994:47 (descriptionanddiagnosis for genus). -Alves-Gomes et al., 1995: 307-309, figs. 6-8, 312- 314, figs. 9-10 (position in phylogeny of the Gymnotiformes ). - Sullivan, 1997 (diagnosis for genus, position in phylogeny of the Rhamphichthyoidea ). -Albert & Campos-da-Paz, 1998: 423, fig. 2, 426, 436, table 1, 439, table 2 (diagnosis for genus, position in phylogeny of the Gymnotiformes ). - Albert, 2001: 12, table 2, 56, table 3, 68-69 (diagnosis for genus, position in phylogeny of the Gymnotiformes ). -Albert & Crampton, 2003: 494 (catalog of hypopomids). - Carvalho, 2013: 78 (diagnosis for genus, position in phylogeny of the Rhamphichthyoidea ). - Fernandes et al., 2014: 98, fig. 1 (position in phylogeny of the Rhamphichthyoidea ). -Maldonado-Ocampo et al., 2014: 6, 8, fig. 6 (position in phylogeny of the Rhamphichthyoidea ). - Tagliacollo et al., 2016: 10 (position in phylogeny of the Gymnotiformes ). - Crampton et al., 2016: 1-66, table 1, 3-4, figs. 1-20 (phylogeny, biogeography and ecology of Brachyhypopomus ).

Odontohypopomus (subgenus of Brachyhypopomus ) Sullivan et al., 2013: 6 (erected to contain B. bennetti and B. sullivani ).

Type species. Rhamphichthys brevirostris Steindachner (1868a) . Type by original designation (Mago-Leccia, 1994: 47).

Diagnosis. Brachyhypopomus is diagnosed unambiguously from all other Gymnotiformes , including all other rhamphichthyoid species, by a single unreversed shared and derived character: the presence of a disk-like ossification in the anterior portion of the palatoquadrate cartilage in adult specimens. See Sullivan et al. (2013: 7, fig. 1) for a photograph of the ossified palatoquadrate cartilage in a cleared and stained specimen of B. walteri . The disk-like ossification of the palatoquadrate cartilage is present in all species of Brachyhypopomus , but in some specimens is relatively hard to discern due to poor uptake of stain. Likewise, in many immature specimens the ossification is incomplete. This may explain why Sullivan (1997) did not observe this character in some species of Brachyhypopomus ( B. brevirostris , B. bullocki , B. bombilla , B. regani , B. sullivani ), where we did. Moreover, because the ossification of the palatoquadrate cartilage is visible only in cleared and stained specimens, this character is inadequate for determining whether whole specimens belong to the genus Brachyhypopomus . The following combination of characters that do not require invasive techniques serves to delimit members of the genus from all other gymnotiforms.

Brachyhypopomus is diagnosed from the Apteronotidae by the absence of a caudal fin and dorsal electroreceptive filament and from the Gymnotidae and Sternopygidae by the absence of complete rows of teeth on the premaxilla in adults. Brachyhypopomus is diagnosed from the Rhamphichthyidae sensu Maldonado-Ocampo et al. (2014) (which includes Hypopygus and Steatogenys in the tribe Steatogeni ) either by the short snout, 18.7-32.6% of HL, vs. 40.0-68.6% in Gymnorhamphichthys , 53.8-55.4% in Iracema , and 49.8-61.6% in Rhamphichthys (Carvalho & Albert, 2011; Carvalho et al., 2011; Géry & Vu, 1964; Nijssen et al., 1976; Schwassmann, 1976; Schwassmann, 1989; Triques, 1999), or by the absence of a paired accessory electric organ in the mental or humeral region, vs. presence in the Steatogeni (de Santana & Crampton, 2010).

Within the Hypopomidae sensu Maldonado-Ocampo et al. (2014) , Brachyhypopomus is diagnosed from Akawaio by the presence of three or four pectoral proximal radials, vs. five in Akawaio (Maldonado-Ocampo et al., 2014) . Brachyhypopomus is diagnosed from Hypopomus by the shorter snout, 18.7-32.6% of HL, vs. 33.3-40.1% of HL in adult H. artedi (based on measurements taken from 30 specimens of H. artedi from diverse localities in the Guyana Shield). Brachyhypopomus is diagnosed from Racenisia by the presence of the antorbital + infraorbital, and the preopercular cephalic lateral line canal bones, vs. complete absence in Racenisia ; these canal bones are clearly visible in alcohol preserved specimens.

Brachyhypopomus cannot be diagnosed unambiguously from Microsternarchus or from Procerusternarchus on the basis of external characters alone. However, with the exception of B. benjamini and B. provenzanoi , species of Brachyhypopomus are diagnosed from Microsternarchus by the presence of scales on the middorsal portion of the anterior third of the body, vs. absence in M. bilineatus and in M. brevis ( Fernandes et al., 2015) .

Previous generic diagnoses. Brachyhypopomus was assigned to the new family Hypopomidae sensu Mago-Leccia (1978) based on the following combination of characters: absence of teeth in both oral jaws; relatively short snout; nares tubular in shape and well-separated, the anterior nares at the corner of the upper lip; 3-4 pectoral-fin radials; supraorbital canal embedded with frontal bone; posttemporal and supracleithrum not fused; fixed anus position; and anal-fin origin posterior to the pectoral fin base. Two of these characters are now known to apply variably to members of Brachyhypopomus : two species, B. bennetti and B. walteri , possess premaxillary teeth in adults ( Sullivan et al., 2013); and the supraorbital canal is independent from the frontal bone in B. flavipomus and B. verdii . Albert (2001) provided a revised diagnosis for the Hypopomidae , but many of the characters fail to accommodate recent increases in the species diversity of Brachyhypopomus (including many of the species described herein).A new formal generic diagnosis for the family – one which accommodates the recentlydescribed genera Akawaio and Procerusternarchus , and one which excludes the Steatogeni , following Maldonado-Ocampo et al. (2014) – is necessary, but is beyond the scope of this paper.

Mago-Leccia (1994: 47) diagnosed six species of Brachyhypopomus from Hypopomus (but not from other genera) by “their short snout and included mouths, absence of mesocoracoid bridge, short and crescent-shaped maxillary bone” (Mago-Leccia, 1994: 165, figs. 65d,f), and “posterior nares closer to eyes.” Of these characters, only “short snout” is able to diagnose all 28 species of Brachyhypopomus recognized herein from Hypopomus . Albert and Campos-da-Paz (1998: 426) and Albert (2001: 68) diagnosed the (as-then) six valid species of Brachyhypopomus (and seven undescribed species) from other Gymnotiformes based on four characters others than those listed previously by Mago-Leccia (1994): 1. “Premaxilla gracile with a curved anterior margin and forming a distinct angle with the maxilla in lateral view (… except B. beebei ; also present in Steatogenys …)”; 2. “Dentary gracile”; 3. “Body cavity with 16 - 17 PCV [precaudal vertebrae]”; 4. “Single transitional vertebrae (sic).” However, each of these four characters fails to unambiguously diagnose all 28 species of Brachyhypopomus recognized herein from other gymnotiforms.

Sullivan (1997) proposed a single character diagnostic character for the genus, “medial portion of upper jaw (premaxillary portion) forms distinct angle with sides of upper jaw (maxillary portion) in lateral and frontal view” (not present in Microsternarchus and Hypopomus ), but acknowledged that this character also occurs in Steatogenys and some sternopygid genera. Sullivan et al. (2013) revisited this character and considered it to be a “single possible synapomorphy” for Brachyhypopomus – contrasting with the condition in most other Rhamphichthyoidea , in which the “maxilla is straight to slightly curved, and the medial and upper portions of the upper jaw form a continuous curve with little to no inflection point, viewed externally”. We found that the curvature of the area of the jaws in which the premaxilla and maxilla join is highly variable, and is an unreliable means to distinguish individuals of all 28 species of Brachyhypopomus from individuals of Microsternarchus bilineatus (and from specimens of additional undescribed species of Microsternarchus ). For example, Brachyhypopomus batesi , B. benjamini¸ and B. provenzanoi are indistinguishable from Microsternarchus in jaw structure, viewed externally.

Description. Members of the genus exhibit relatively little variation in body and head shape, and are generally delimited by a combination of morphometric and meristic characters. For brevity, we describe here external characters common to all congeners rather than repeating these in each of the species descriptions and redescriptions presented below.

Maximum known body size 461 mm TL (see redescription of B. brevirostris ). Body elongate and laterally compressed (body width as percentage of body depth 36.4- 82.1% and 18.9-51.7% at anal-fin origin and anal-fin middle respectively), reaching maximum width and depth in posterior region of body cavity and tapering gently posteriorly; short to very long caudal filament located posterior to the anal-fin terminus (4.2-54.5% TL; 7.4-83.1% LEA in intact specimens) – comprising mostly electrocytes, vertebral centra or a cartilaginous rod-like structure, and associated blood vessels and nerves ( Albert, 2001). Greatest body depth and width located in area of body cavity or slightly posterior to body cavity, lateral sides of body more or less straight in dorsal view posterior to abdominal cavity. Dorsal profile of body straight or nearly straight in lateral view.

Single hypaxial electric organ (EO) extending just posterior to the urogenital orifice to the end (or near end) of the caudal filament; 1-6 bilateral horizontal (length way) columns of electrocytes along its length, 3-6 columns at anal-fin terminus, and 2-6 columns at a midpoint between anal-fin terminus and end of caudal filament. Paired subdermal accessory EO of unknown function in the opercular region are known from three species: B. bombilla , B. menezesi , and B. regani .

Anal fin elongate (71.6-94.0% LEA) with origin located in anterior portion of body cavity, posterior to cleithrum of pectoral girdle; 143 to 293 rays (branched and unbranched combined). Each anal-fin ray connected via a ball-and-socket joint to a proximal anal-fin pterygiophore, and articulated in circular movements by the pinnalis analis muscles ( Ellis, 1913). Anal-fin pterygiophores shorter than hemal spines at midbody.

Pectoral fin short to moderate (longest pectoral ray 3.2- 8.5% LEA), broad and distally rounded or pointed, with 10-21 rays (branched and unbranched combined). Dorsal, adipose, and pelvic fins, and pelvic girdle absent, in common with all gymnotiforms. Caudal fin absent in common with all Rhamphichthyoidea , Sternopygidae , and Gymnotidae -although see de Santana et al. (2013) for comments on the caudal skeleton of Electrophorus .

Body cavity restricted to no more than 2.5 head-lengths posterior to occiput, with viscera rotated so that the anus and urogenital orifice are positioned below the operculum, near the isthmus. Anus and urogenital orifice in close juxtaposition. Urogenital papilla developed into elevated tube in breeding males and females. Transparent and thinwalled non-vascularized and non-physostomous swim bladder, comprising a smaller anterior chamber, and a larger posterior chamber ( Ellis, 1913: 189, fig. 32). Body cavity short, with 15-26 precaudal vertebrae (including up to 3 transitional vertebrae). 1-3 slender displaced hemal spines embedded in hypaxial musculature posterior to body cavity, sometimes forked with bifurcated portions facing anteriorly.

Scales cycloid, with irregular ovoid shape; smaller dorsally and ventrally and larger laterally, especially in posterior region of body; partially or completely covered with skin and distributed over all of post-cranial portion of body except fins (exceptions are the absence of scales along the middorsal region of anterior third of body and over the anal-fin pterygiophores in B. benjamini and B. provenzanoi ). 4-9 rows of lateral line scales above lateral line to dorsal midline at midbody. Anterior-most perforated lateral line scale located near vertical through pectoral-fin origin. Lateral line continuous in most species anterior to anal-fin terminus, but not continuing to end of caudal filament; usually terminating in the middle portion of the caudal filament. Dorsal branch of posterior lateral line nerve separate from recurrent ramus of anteroventral lateral line nerve ( Albert, 2001: character 117 therein). No fleshy dorsal electroreceptive organ (as in the Apteronotidae ). Body variably pigmented with dark bands and mottled patterns of stellate chromatophores and subcutaneous pigment, but never translucent as in several sternopygid taxa, notably many Eigenmannia and Rhabdolichops species (Crampton, 2007). Chromatophores contract at night resulting in a striking pale appearance of most Brachyhypopomus under artificial illumination; even those that are very darkly pigmented during the day. Ampullary electroreceptors in rosettes, with highest density on snout and around mouth. Schreckstoff club cells and observable fright response absent ( Fink & Fink, 1981: character 117 therein). Head widest in opercular region and deepest in occipital region. Eye small to relatively large in size (5.9-20.6% HL), laterally positioned on dorsal half of head, visible in dorsal view, and completely covered by thin translucent epidermis (orbital margin not free). Mouth small and terminal or slightly to moderately sub-terminal; closed lips meet ventral to a horizontal through ventral margin of eye. 3-7 small, needle-like premaxillary teeth present in all examined small juveniles (<50 mm TL) but absent in adults except in B. bennetti and B. walteri ( Sullivan et al., 2013) . Dentary edentate in adults, although small conical dentary teeth present in all examined small juveniles (<50 mm TL). Maxilla edentate. Premaxillary and maxillary portions of upper jaw gently convex anteriorly, joining to form a slight concave angle or moderate to acute sigmoidal profile to upper jaw. Anterior and posterior nares present; both either ellipsoid or circular in shape. Anterior nares located at upper lip. Posterior nares located near eye (1.1-10.4% HL from anterior margin of orbit). Dorsal and ventral profile of head from almost straight to strongly convex. Snout short (18.7-32.6% HL) and rounded. Cephalic lateral line canals and pores complete in adults, with pores conspicuous (de Santana & Crampton, 2011: 1104, fig. 2) – with the following two exceptions: i. The antorbital is absent in B. bennetti and B. walteri ; ii. The branch of the infraorbital canal over the frontal is absent in B. alberti , B. arrayae , B. belindae , B. hamiltoni , and B. verdii . Branchial opening restricted to a short vertical aperture at posterior margin of opercle, branchial membranes joined at isthmus. 25-63 gill filaments on first arch. Sexual dimorphism of cranial morphology absent in all species in contrast to its presence in many apteronotid taxa (Albert & Crampton, 2009; de Santana & Vari, 2010; Fernandes et al., 2002) and in the sternopygid genus Archolaemus ( Vari et al., 2012) .

Functional biology and ecology. Specializations associated with the Electrogenic and Electrosensory system: As with other Gymnotiformes , Brachyhypopomus species exhibit a suite of morphological specializations associated with active electroreception and nocturnal activity. The eyes are reduced in size and retinal projections to the brain are much reduced ( Lázár et al., 1987). A culteriform (knife-shaped) body plan facilitates maximum dipole separation of the fish as a dipole source (and therefore maximizes field strength per unit body mass, Stoddard et al., 1999), and optimizes use of the body surface as a relatively rigid electroreceptive array. Locomotion by undulation of the elongated anal-fin permits backwards-forwards electroreceptive rostral probing (‘scan swimming’ sensu Julian et al., 2003 ) (Albert & Crampton, 2005; Lannoo & Lannoo, 1993; Lissmann, 1961; Nanjappa et al., 2000; Stoddard et al., 1999) (in addition to anal-fin locomotion, Brachyhypopomus also swim in rapid bursts by anguilliform movement of the body when startled, Ellis, 1913). Body rigidity is augmented by ossified intermuscular bones ( Schlesinger, 1910). Most of the post-cranial body and caudal filament is specialized for electrogenesis, with the body cavity confined to an area extending no more than 2.5 head-lengths posterior to the occiput ( Ellis, 1913: 189, fig. 32).

Electric Organs: The hypaxial electric organ (EO) comprises myogenic electrocytes (derived from muscle cells). Studies of B. gauderio demonstrate that the EO develops from the ventral hypaxial musculature in larvae, and is retained through development ( Franchina, 1997). Initially the EO comprises cylindrical, overlapping stalk-less electrocytes, but these later shorten along the rostrocaudal axis, separate into vertically aligned “rows”, and form stalklike caudal projections onto which the spinal electromotor neurons terminate ( Bass, 1986: 16, fig. 1; Franchina, 1997: 115-117, figs. 7-10). EODs are first generated six days after hatching, at 7 mm TL in B. gauderio ( Franchina, 1997) , and at 6-8 mm in B. beebei ( Westby, 1988) . Beyond the late larval stage (TL> 37 mm), the EO of B. gauderio extends parallel to the body axis along the entire ventrolateral margin, from near the gill isthmus to the tip of the caudal filament (although in some species the distal portions of the caudal filament are sometimes free of electrocytes, especially in breeding males; see descriptions and redescriptions herein).

The adult EO of Brachyhypopomus comprises 3-6 bilateral longitudinal (horizontal) columns of cylindrical or box-shaped electrocytes bound by connective sheaths ( Bass, 1986; Bennett, 1961; 1971a; Franchina, 1997; Hopkins et al., 1990; Stoddard et al., 1999). An opercular accessory electric organ distinct from the main hypaxial EO was first reported by Sullivan (1997) for B. regani (listed as B. electropomus ), and has also been discussed by Crampton & Albert (2006) and Carvalho (2013). Caputi (1999) and Caputi et al. (1998) review how the synchronized activity of electrocytes in the EO of B. gauderio – combined with the filtering properties of the skin and tissue of the fish, and the load of the surrounding water – determine in ensemble the spatiotemporally complex “near field” discharge that sums to the head-to-tail recorded EOD recorded in the far field.

Impedance matching: Impedance matching of electric organ anatomy to narrow ranges of conductivity has been noted in several species ( Crampton, 1998a; Hopkins, 1999). Brachyhypopomus bullocki and B. brevirostris , which are restricted to low conductivity systems (<30 µScm-1) exhibit a predominantly serial arrangement of electrocytes in the caudal portion of the organ (longer caudal filaments comprising relatively few horizontal bilateral columns of electrocytes). In contrast, B. bennetti , B. diazi , and B. occidentalis (and also B. palenque , see description herein), which are restricted to high conductivity systems (>60 µScm-1), exhibit a parallel arrangement of electrocytes (short caudal filaments with more electrocyte columns). The possibility that conductivity may serve as a barrier to the dispersal of some species, and consequently play a role in reproductive isolation, has been suggested by Crampton (1998a; 2011) and Hopkins (1999).

Electric Organ Discharges: Crampton & Albert (2006) reviewed the diversity of head-to-tail recorded EODs in Brachyhypopomus . Of 18 species for which EOD data have been presented, 12 generate biphasic or nearly biphasic EODs with durations varying from ca. 0.5-5 ms. One species, B. bennetti , generates a monophasic EOD. Five species generate relatively short (0.5 - 1.5 ms), more complex triphasic or tetraphasic EODs ( Crampton & Albert, 2006). Monophasic larval EODs have been documented in B. beebei , B. brevirostris , B. gauderio , and B. occidentalis , and this is presumed to be the case in other congeners, as in all other gymnotiforms ( Crampton & Albert, 2006). The EOD pulse rate of Brachyhypopomus varies from ca. 2 to 110 Hz, and typically increases from a lower resting day-time rate to a higher nocturnal active rate ( Crampton & Albert, 2006). Assad et al. (1999), and Stoddard et al. (1999) map heterogeneity of the near-field recorded EOD of B. beebei , B. gauderio , and B. walteri .

There is a rich and rapidly growing literature on the behavior, neuroethology, physiology, energetics, and hormonal basis of EOD generation and EOD plasticity in Brachyhypopomus – focused primarily on the model species B. gauderio , which is reviewed in part by Assad et al. (1999, 1998), Stoddard (2006), Gavassa et al. (2013), Markham (2013), Silva et al. (2013), and Salazar et al. (2013).

Electroreceptors: In common with all other gymnotiforms, Brachyhypopomus possess a cutaneous array of ampullary and tuberous electroreceptors. Ampullary electroreceptors are tuned to low frequencies and facilitate passive electroreception of weak, lowfrequency bioelectric fields. Tuberous electroreceptors are typically tuned to higher frequencies and facilitate both the active electrolocation of objects, and the passive detection of electric fields from other fish – an important component of electrocommunication ( Hopkins, 2005; Hopkins et al., 1997). The morphology of ampullary and tuberous electroreceptors in Brachyhypopomus , their distribution over the body surface, and their directional sensitivity are among the best known of all gymnotiforms, having been described by Bullock et al. (1961), Hagiwara et al. (1962), Szamier & Wachtel (1970), Bennett (1971b), Szabo (1974), Yager & Hopkins (1993), and McKibben et al. (1993). Tuberous electroreceptor frequency responses in the genus, showing a correspondence of electroreceptor tuning with peak frequency characteristics of the EOD, are described by Bastian (1976; 1977) and Hopkins & Heiligenberg (1978).

Groove-like depigmented epidermal canals leading to tuberous electroreceptors, and which therefore may play a role in active electroreception are known from all members of the genus. Sullivan (1997) noted for B. diazi and B. occidentalis that “Microscopic examination and histological sections of these canals show them to be narrow tubes overlying one or more scales with an external pore anteriorly and a tuberous electroreceptor at the posterior end (J. Sullivan and C. Hopkins unpubl.).” The epidermal canals of Brachyhypopomus vary in abundance between species (see species descriptions and redescriptions herein) but are mostly located in the posterior half of the body – where they are restricted to nearby and either side of the lateral line, either side of the dorsal midline, and approximately midway between the lateral line and dorsal midline. In B. diazi the epidermal canals cover a larger area of the body than in congeners, including much of the dorsal and ventral flank of the anterior half of the body. Groove-like epidermal canals are also observable in all other hypopomid genera sensu Maldonado-Ocampo (2014) , including Akawaio , Hypopomus , Procerusternarchus , and (in considerably lower densities), in Microsternarchus and Racenisia .

Auditory sensory apparatus: In addition to providing buoyancy, the swim bladder of Brachyhypopomus is involved in the ostariophysan hearing system, which involves a connection from the anterior chamber of the swim bladder to the stato-acoustic organ of the inner ear via the Weberian ossicles encased by the first four vertebrae. A pseudotympanum, a window-like thinning of the hypaxial muscles lateral to the anterior portion of the swim bladder is present, and comprises three distinct hiatuses in the obliquus inferioris and obliquus superioris muscles ( Dutra et al., 2015). The pseudotympanum may enhance sound detection by acting as a window through which sound waves reach the anterior chamber of the swim bladder ( Dutra et al., 2015).

Adaptations to hypoxia: The influence of dissolved oxygen on the distributions of Brachyhypopomus is discussed by Crampton (1998b; 2011; 2008) and Crampton & Albert (2006). Species that occur in seasonally or perennially dysoxic habitats such as whitewater floodplains and terra firme swamps are adapted to survive protracted periods of hypoxia (<1 mgl-1) or complete anoxia, while those that occur in permanently normoxic habitats, such as terra firme streams, are not ( Crampton, 1998a,b). Some species are able to tolerate hypoxia by undertaking aerial gill respiration – either by periodically aspirating air bubbles into their gill chambers, or by opening their mouths at the surface meniscus to expose the gill lamellae to air ( Crampton, 1998b; Hopkins, 1991). Carter & Beadle (1931) reported an expanded gill chamber, unusually long gill lamellae, and greatly expanded secondary folds of the gill lamellae in B. “brevirostris ” (probably B. gauderio ) from the Paraguayan Chaco, and also concluded that the air bladder and gill chamber epithelium play no significant role in air breathing. Crampton et al. (2008) reported that species endemic to seasonally anoxic whitewater floodplain habitats exhibit significantly larger gills than species endemic to permanently normoxic terra firme stream systems. Many species of Brachyhypopomus also exhibit a reduction of activity and EOD pulse rate in response to declining oxygen levels ( Crampton, 1998b).

Regeneration: Brachyhypopomus species, as in other Rhamphichthyoidea and Sternopygidae , are able to regenerate the entire post-coelomic portion of the body following damage from predators, with the vertebrae replaced by a rigid cartilaginous rod ( Albert, 2001; Albert & Crampton, 2005; Ellis, 1913; Sullivan et al., 2013). The proportion of individuals with caudal filament damage in natural populations has been estimated for three species: B. brevirostris – 8% ( Ellis, 1913); B. draco – 6.7%, with 22.1% exhibiting regeneration from earlier damage ( Cognato et al., 2007); B. occidentalis – varying among populations from 12% to 46% ( Dunlap et al., 2016). Hopkins et al. (1990) and Sullivan et al. (2013) documented how damage to portions of the body posterior to the body cavity can influence the EOD waveform.

Reproductive anatomy: In B. gauderio , the males develop a paired, lobular testis of the unrestricted spermatogonial category, and females develop a paired saccular cystovary. In both sexes the gonads are positioned ventrally, and expand posteriorly until they reach the posterior wall of the abdominal wall ( Quintana et al., 2004). Gonadal morphology appears to be similar in other species. França et al. (2007) and Giora & Burns (2011) describe the ultrastructure of spermatozoa in Brachyhypopomus .

Reproductive biology and life history: Detailed accounts of the reproductive ecology and life history of Brachyhypopomus are available for B. occidentalis from Panama ( Hagedorn, 1986; 1988), and for three species from southern subtropical systems: B. bombilla (Giora et al., 2011) , B. draco ( Schaan et al., 2009) , and B. gauderio ( Giora et al., 2014; Miranda et al., 2008; Quintana et al., 2004; Silva et al., 2007; Silva et al., 2002; Silva et al., 2008) (and see species descriptions herein). The reproductive ecology of Amazon and Orinoco species is largely undocumented, although Alves-Gomes (1997) noted spawning preceding seasonal flooding in the rio Branco system of Roraima, Brazil, and Crampton (1996a) observed spawning coincident with the rising-water period and high water period in species from Amazonian floodplains.

At extreme southern latitudes Brachyhypopomus exhibit several adaptations to the austral winter – including gonadal quiescence, a general reduction in activity and feeding, and an accompanying reduction of the EOD pulse-rate (Giora et al., 2011; 2014; Schaan et al., 2009; Silva et al., 2002). Some species of Brachyhypopomus appear to live for only one year, with mortality following a terminal reproductive effort – including B. bennetti ( Crampton, 1996a) (listed therein as ‘ B. sp. 3’), B. gauderio ( Silva et al., 2003) , B. occidentalis ( Hagedorn, 1988) , and B. pinnicaudatus (Kirschbaum & Schugardt, 2002) . Fractional spawning has been observed in B. bombilla (Giora et al., 2011) , B. brevirostris ( Kirschbaum et al., 2008) , B. draco ( Schaan et al., 2009) , B. gauderio ( Giora et al., 2014; Miranda et al., 2008), B. occidentalis ( Hagedorn, 1988) , and B. pinnicaudatus ( Kirschbaum et al., 2008) . Kirschbaum & Schugardt (2002) reported egg diameters of approximately 1.7 mm in B. pinnicaudatus , with hatching on day three after spawning, and exogenous feeding beginning on day eight.

Neither nesting nor parental care are known for the genus, including species bred and observed carefully in captivity (pers. comm. P. K. Stoddard, Florida International University, USA, for B. gauderio ; F. Kirschbaum, Humboldt University, Germany, for B. brevirostris and B. pinnicaudatus ). Westby (1988) reported small aggregations of larval Brachyhypopomus beebei in a coastal stream of French Guiana but these were apparently not attended by a parent. However, Giora et al. (2014) reported aggregations of larval B. gauderio near single mature males.

Cytogenetics. Chromosomal rearrangements in gymnotiforms are suspected to play a significant role in reproductive isolation ( Milhomem et al., 2008;

Nagamachi et al., 2010). Nonetheless, despite many descriptive cytogenetic studies of gymnotiform fishes (Oliveira et al., 2009), Brachyhypopomus remains poorly studied – with karyotypes available for only four species. Brachyhypopomus brevirostris exhibits a diploid number of 36, fundamental number (FN) of 42, and karyotypic formula 4m /2sm/8st/22a (Almeida-Toledo, 1987, cited in Cardoso et al., 2011). Brachyhypopomus gauderio exhibits a diploid number of 42 in females (all acrocentric), and 41 for males (40 acrocentric, 1 metacentric), a FN of 42, and a karyotypic formula of 1m /42a in females and 1m /41a in males (Almeida-Toledo et al., 2000; Mendes et al., 2012). Brachyhypopomus pinnicaudatus exhibits a diploid number of 42 in females (all acrocentric) and 41 in males (40 acrocentric, 1 metacentric), and a FN of 42, as is the case in B. gauderio , and exhibits a karyotypic formula of 42st-a in females and 1m-sm/40st-a in males ( Cardoso et al., 2015). Brachyhypopomus flavipomus (listed as B. n. sp. FLAV) exhibits a diploid number of 44 in females and 43 in male and a karyotypic formula of 44st-a in females and 1m-sm/42st-a in males ( Cardoso et al., 2015). Cardoso et al. (2011) highlighted the conservancy of the diploid number in the Steatogeni and Rhamphichthys (invariably 2n = 50), in contrast to its variability among Brachyhypopomus spp. (2n = 36, 41-42, or 43-44) and Hypopomus artedi (2n = 38), and noted that this pattern is consistent with the phylogenetic scheme of Alves-Gomes et al. (1995) (and also Carvalho, 2013; Maldonado-Ocampo et al., 2014; Tagliacollo et al., 2016), in which the Steatogeni is placed within the Rhamphichthyidae .

Three of the four Brachyhypopomus with cytogenetic analyses exhibit morphologically differentiated sex chromosomes. Brachyhypopomus flavipomus ( Cardoso et al., 2015) , B. gauderio (Almeida-Toledo et al., 2000; Mendes et al., 2012), and B. pinnicaudatus ( Cardoso et al., 2015) all exhibit an unusual multiple sex chromosome system of the X 1 X 1 X 2 X 2: X 1 X 2 Y type. In contrast, B. brevirostris is chromosomally homomorphic, and sex determination mechanisms are unknown in this species ( Cardoso et al., 2011).

Taxonomic remarks. Here we provide comments on the early assignment of species now listed as Brachyhypopomus to the genus Hypopomus . Previous to the description of B. brevirostris (as Rhamphichthys brevirostris , see Introduction), the genus Rhamphichthys was erected by Kaup (1856) to include two relatively short-snouted species – R. artedi (now Hypopomus artedi ) and R. mulleri – now recognized as a junior synonym of Hypopomus artedi (Albert & Crampton, 2003) , and also five long-snouted species – now recognized as Rhamphichthys spp.

In a brief address to the Academy of Natural Sciences of Philadelphia, Gill (1864) proposed a new genus Hypopomus to accommodate the relatively short-snouted Rhamphichthys mulleri , and separate this from the long-snouted species now assigned to the genus Rhamphichthys . Nonetheless, Gill’s proposal was overlooked until 1903. For instance, Günther’s (1870) catalog of fishes in the British Museum instead divided the genus Rhamphichthys into two subgenera. The first subgenus, Rhamphichthys (Brachyrhamphichthys) , diagnosed by “snout not tubiform; vent behind the eyes; anal fin commencing below the pectorals”, corresponds to the modern genera Hypopomus and Brachyhypopomus (at the time this comprised H. artedi and its now junior synonym H. mulleri , and B. brevirostris ). The second subgenus Rhamphichthys (Rhamphichthys) , diagnosed by “snout produced into a tube; vent below or in advance of the eyes; anal fin commencing at the throat”, corresponded to the modern genus Rhamphichthys . With respect to Brachyhypopomus brevirostris , Günther’s (1870) scheme was followed by Steindachner (1880) and Eigenmann & Eigenmann (1891), and later modified by Eigenmann (1894), who listed Rhamphichthys (Brachyrhamphichthys) brevirostris as simply Brachyrhamphichthys brevirostris .

The first authors to abandon Günther’s (1870) scheme for short-snouted Rhamphichthyoidea , and instead adopt Gill’s (1864) proposed genus Hypopomus , which takes taxonomic priority, were Eigenmann & Kennedy (1903) for “ Hypopomus brevirostris ” (referring in fact to B. gauderio , see synonymy section for B. gauderio ), and later Eigenmann & Ward (1905) for B. brevirostris . Only in 1914, with Regan’s description of Brachyhypopomus occidentalis was a second species added to Hypopomus sensu Gill (1864) . Thereafter, the name Hypopomus was used consistently until Mago-Leccia (1994) divided Gill’s Hypopomus into Hypopomus and Brachyhypopomus (see Introduction).

Keys to species. Here we exclude the following regions represented by only one species: Caribbean drainages of northern Venezuela ( B. diazi ); rio São Francisco ( B. menezesi ); Atlantic and Pacific drainages of southern Costa Rica and Panama to Darién, and Maracaibo, Magdalena, Sinú and Atrato ( B. occidentalis ); Pacific drainages of Ecuador ( B. palenque ).

Key to the species of Brachyhypopomus occurring in the río Orinoco, Essequibo River, coastal drainages of the Guianas, and the rio Negro, including the rio Branco

1A. Absence of accessory electric organ (AEO) over opercular region ............................................................................. 2

1B. Presence of AEO over opercular region......... B. regani (lower and middle Orinoco, upper Essequibo, some coastal drainages of Guianas)

2A. Presence of prominent pale uninterrupted middorsal stripe from occipital region to base of caudal filament ................3

2B.Absence of pale prominent pale uninterrupted middorsal stripe from occipital region to base of caudal filament.....4

3A. Anal-fin rays 214-230...................................... B. beebei

(upper and lower Orinoco, Essequibo, coastal drainages of the Guianas, Negro including Branco).

3B. Anal-fin rays 176-196........................ B. pinnicaudatus (small drainages of coastal French Guiana)

4A. Presence of scales in entire middorsal region .............. 5

4B. Absence of scales in middorsal region of anterior third of body................................................... B. provenzanoi (upper Orinoco, upper Negro)

5A. Presence of dark suborbital stripe ................................ 6

5B.Absence of dark suborbital stripe.................................. 7

6A.Presence of horizontal dark-pigmented band along entire anal-fin base, over distal anal-fin pterygiophores............ ....................... B. hendersoni (lower Negro, Essequibo)

6B. Absence of horizontal dark-pigmented band along anal-fin base............................................................. B. walteri (middle and lower Negro, Essequibo).

7A. Absence of series of diffuse horizontal dash-like dark markings along lateral line in posterior third of body, anterior to anal-fin terminus .......................................... 8

7B. Presence of series of diffuse horizontal dash-like dark markings along lateral line in posterior third of body, anterior to anal-fin terminus ........................ B. hamiltoni

(middle Negro)

8A. Anal-fin rays 143-184................................................... 9

8B.Anal-fin rays 190-293.................................................. 10

9A. Presence of prominent dark flecks on flank ................... ............ B. sullivani (upper, middle and lower rio Negro, Essequibo, central Orinoco)

9B. Absence of prominent dark flecks on flank ...... B. batesi (upper Negro)

10A. Absence of prominent wide brown saddles on flank, or saddles if present pale and restricted to dorsal region, not extending undisrupted across lateral line .................... 11

10B.Presence of prominent wide brown saddles on flanks, which extend undisrupted across lateral line................... .......... B. brevirostris (upper, central, and lower Orinoco, Essequibo, and coastal drainages of the Guianas).

11A. Orbital diameter 14.2-18.5% HL............... B. bullocki (upper, middle and lower Orinoco, Negro including upper Branco, upper and middle Essequibo)

11B. Orbital diameter 9.6-13.0% HL........................ B. diazi (middle Orinoco in Llanos region, lower Orinoco)

Key to the species of Brachyhypopomus occurring in the upper Amazon, central Amazon, lower Amazon, and upper Madeira

1A. Absence of accessory electric organ (AEO) over opercular region............................................................. 2

1B. Presence of AEO over opercular region....................... 3

2A. Presence of prominent pale uninterrupted middorsal stripe from occipital region to base of caudal filament............... 4

2B.Absence of pale prominent pale uninterrupted middorsal stripe from occipital region to base of caudal filament..... 8

3A. Dorsal surface dorsal surface speckled with small brown chromatophores on light brown background; opercular accessory electric organ overlaid with dense scattering

of chromatophores............. B. bombilla (upper Madeira)

3B. Dorsal surface with large dark blotches on pale

background; opercular accessory electric organ overlaid

with depigmented skin ..................................... B. regani

(upper, central, and lower Amazon, lower Madeira, upper

Purus, Tapajós, Araguaia , lower Tocantins)

4A. Precaudal vertebrae 24-26............................................ 5

4B. Precaudal vertebrae 18-23 ............................................ 6

5A. Bilateral columns of electrocytes at anal-fin terminus

3 ............................................. B. verdii (upper Amazon)

5B.Bilateral columns of electrocytes at anal-fin terminus

4-5 ................................... B. belindae (central Amazon)

6 A. Anal-fin rays 175-212................................................... 7

6B. Anal-fin rays 214-230......................... B. beebei (upper,

central, lower Amazon, lower Madeira, lower Tocantins)

7A. Pectoral-fin rays 16-19 ....... B. arrayae (upper Madeira)

7B.Pectoral-fin rays 13-15 ........ B. pinnicaudatus (central

and lower Amazon, lower, upper Madeira, lower

Tocantins, Mearim)

8A. Presence of scales in entire middorsal region .............. 9

8B.Absence of scales in middorsal region of anterior third of

body................................ B. benjamini (upper Amazon)

9A. Bilateral columns of electrocytes at anal-fin terminus

3-4................................................................................ 10

9B. Bilateral columns of electrocytes at anal-fin terminus

6......... B. bennetti (upper, central, lower Amazon, upper

Madeira, lower Tocantins)

10A. Presence of scattered conspicuous black or charcoal

flecks on flank.............................................................. 11

10B. Absence of scattered conspicuous black or charcoal

flecks on flank.............................................................. 13

11A. Presence of horizontal dark band or heavy concentration

of dark flecks along entire anal-fin base ..................... 12

11B. Absence of horizontal dark band or heavy concentration

of dark flecks along entire anal-fin base........ B. sullivani

(upper, central, and lower Amazon, upper Madeira, upper

Tapajós, lower Tocantins)

12A. Mouth width 24.8-38.0% HL........................ B. cunia

(upper Madeira)

12B. Mouth width 15.7-22.6% HL ................. B. hendersoni

(central Amazon)

13A. Absence of dark suborbital stripe............................. 14

13B. Presence of dark suborbital stripe ................ B. walteri

(upper, central, and lower Amazon, upper Madeira, Tapajós,

upper Xingu, Araguaia, upper and lower Tocantins).

14A. Anal-fin rays 163-208............................................... 15

14B. Anal-fin rays 226-293............................ B. brevirostris

(upper, central, and lower Amazon, upper Madeira,

upper Xingu)

15A. Head depth at occiput 69.9-87.9% HL..................... 16

15B. Head depth at occiput 61.0-67.9% HL........... B. batesi

(central Amazon)

16A.Mouth width 15.9-20.7% HL, absence of conspicuous

patches of shiny yellow guanine on operculum and

anterior to pectoral-fin base in live individuals........... 17 16B. Mouth width 21.2-37.8% HL, presence of conspicuous patches of shiny yellow guanine on operculum and anterior to pectoral-fin base in live individuals.......... B. flavipomus (upper and central Amazon)

17A. Pectoral-fin rays 15-16 (mode 16) ................ B. alberti (upper Madeira)

17B. Pectoral-fin rays 12-15 (rarely 15), (mode 13)............. ........................ B. hamiltoni (upper and central Amazon)

Key to the species of Brachyhypopomus occurring in the Paraná, Paraguay and Uruguay drainages, the Patos- Mirim lagoon system and nearby Atlantic drainages of Rio Grande do Sul, and coastal drainages of Brazil from states of São Paulo to Rio de Janeiro

1A. Absence of electric accessory organ over opercular region............................................................................. 2 1B. Presence of electric accessory organ over opercular region............................................................ B. bombilla (lower Paraná, upper, central, and lower Paraguay, Uruguay, Patos-Mirim)

2B. Absence of pale prominent pale uninterrupted middorsal stripe from occipital region to base of caudal filament.... 3 2A. Presence of prominent pale uninterrupted middorsal stripe from occipital region to base of caudal filament ... ...................... B. gauderio (lower Paraná, upper, central, and lower Paraguay, Uruguay, Patos-Mirim, Tramandaí) 3A. Absence of dark suborbital stripe................................. 4 3B. Presence of dark suborbital stripe .................. B. walteri (lower Paraná, upper Paraguay)

4A. Anal-fin rays 155-223................................................... 5 4B. Anal-fin rays 226-293 ....... B. brevirostris (upper Paraguay) 5A. Dorsal rami of the recurrent branch of the anterior lateral line nerve visible ........................................................... 6 5B.Dorsal rami of the recurrent branch of the anterior lateral line nerve not visible........... B. draco (lower Paraná, central, and lower Paraguay, Uruguay, Patos-Mirim, Tramandaí) 6A. Snout to pectoral-fin base 10.7-12.3% LEA .................. .................. B. janeiroensis (São João, Paraíba and small intervening coastal drainages)

6B.Snout to pectoral-fin base 12.3-15.7% LEA..... B. jureiae (Ribeira de Iguape, Una do Prelado)

Brachyhypopomus alberti , new species

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( Fig. 4 ; Tables 2-6)

Brachyhypopomus sp. “alb”. – Crampton, 2011: 176, table 10.2, species list; 179, figs. 10.2-10.3 (phylogeny, geographical and ecological distributions, gymnotiform biology).

Brachyhypopomus sp. “alberti ”. – Crampton et al., 2016: 1-66, table 1, 3-4, figs. 1-7, 11, 18-20 (phylogeny, biogeography and ecology of Brachyhypopomus ).

Holotype. CBF 10284 , male, 97 mm TL, 79 mm LEA, Bolivia, Beni, mun. Riberalta, stream nr. San José, nr. Riberalta , affl. río Beni, affl. rio Madeira , Amazonas dr., 10°55′32″S, 066°00′36″W, 28 Jun 2007, W. Crampton, J. Albert & M. Arraya.

Paratypes. 7 specimens, localities from mun. Riberalta, río Beni dr., affl. rio Madeira , Amazonas dr., collected by W. Crampton, J. Albert & M. Arraya. Bolivia. Beni. ANSP

197573, 1, female, 91 mm, 25 Jun 2007, CBF 10279 , 1 , female, 70 mm, 25 Jun 2007 , CBF 10280 , 1 , immature, 64 mm, 25 Jun 2007, stream nr. lago San José, 10°54′47″S, 065°59′49″W . CBF 10281 , 1 , female, 72 mm, 28 Jun 2007 , CBF 10282 , 1 , immature, 48 mm, 28 Jun 2007 , CBF 10283 , 1 , immature, 58 mm, 28 Jun 2007, stream nr. San José, 10°55′32″S, 066°00′36″W . UMSS 07042 , 1 , male ( CS), 84 mm, 6 Jul 2007, stream nr. Bocerón, on Riberalta-Guayaramerín rd. , 11°02′51″S, 065°50′06″W .

Non-types. 9 specimens, localities from rio Madeira dr., Amazonas dr. Bolivia. Beni (localities from mun. Riberalta , río Beni dr.). UF 177345 , 1 , immature, 68 mm, Riberalta-Guayaramerín rd. , nr. km 43, 11°00′30″S, 065°39′49″W . UMSS 7041 , 1 , male, 96 mm, stream nr. Bocerón, on Riberalta-Guayaramerín rd. , 11°02′51″S, 065°50′06″W . UMSS 7043 , 1 , immature, 70 mm, stream nr. Hormiga, on Riberalta-Guayaramerín rd. , 11°01′34″S, 065°52′58″W . UMSS 7044 , 1 , male, 111 mm , UMSS 7045 , 1 , female, 100 mm, Riberalta- Guayaramerín rd. , nr. km 43, 11°01′58″S, 065°44′53″W . Brazil. Rondônia. UFRO-I 6482 , 4 , 53-57 mm, rio Guaporé upstream community Pedras Negras, affl. rio Mamoré , 12°52′03″S 62°52′38″W .

Diagnosis. Brachyhypopomus alberti is diagnosed from congeners by the following combination of characters: depigmented stripe along middorsal region of body absent, vs. prominent pale uninterrupted middorsal stripe from occipital region to base of caudal filament in B. arrayae , B. beebei , B. belindae , B. gauderio , B. pinnicaudatus , and B. verdii ; precaudal vertebrae 20-22 vs. 15-19 in B. batesi , B. benjamini , B. bennetti , B. bombilla , B. bullocki , B. cunia , B. diazi , B. hendersoni , B. menezesi , B. provenzanoi , B. regani , and B. sullivani ; anal-fin rays 182-202 vs. 226-293 in B. brevirostris ; continuous or discontinuous dark vertical or diagonally oriented bands or saddles present on body surface dorsal to lateral line, often extending across lateral line into ventral lateral surface, vs. absence of oblique bands or saddles on body surface dorsal to lateral line in B. draco , B. flavipomus B. jureiae , and B. palenque ; bilateral columns of electrocytes at the anal-fin terminus 3 vs. 4-5 in B. janeiroensis and B. occidentalis (except some populations in Colombia and Venezuela, see redescription of B. occidentalis ); dark suborbital stripe absent, vs. present in B. walteri . Brachyhypopomus alberti can be distinguished from most but not all specimens of B. hamiltoni by a higher number of pectoral-fin rays – 15-16 (mode 16) vs. 12-15 (mode 13) (only 2 of 18 measured specimens of B. hamiltoni exhibited an overlapping number of pectoral-fin rays with B. alberti ). Brachyhypopomus alberti can be further distinguished from B. hamiltoni by the absence of the first of five branchiostegal rays vs. presence in B. hamiltoni Mago-Leccia (1994: 175, fig. 77).

Description. Head and body shape, and pigmentation illustrated in Fig. 4 . Meristic and morphometric data for examined specimens presented in Tables 2-6. Body shallow to moderate in depth. Head short to moderate in length and shallow to moderate in depth. Dorsal profile of head slightly convex from occiput to snout, ventral profile of head straight to slightly convex between operculum and snout, snout rounded and bulbous. Eye moderate in size. Upper jaw with slight to moderate sigmoidal angle between premaxillary and maxillary portions in lateral view. No accessory electric organ over operculum. Gill filaments on first gill arch 30-37 (median 36, n = 4). Precaudal vertebrae 20-22 (mode 22) including 1-2 (mode 2) transitional vertebrae. Pectoral fin narrow to moderate in width, pectoral-fin rays 16 in all specimens examined. Anal-fin origin slightly (<0.25 HL distance) anterior to, or near, tip of pectoral fin. Anal-fin rays 172-191 (median 184). Dorsal rami of recurrent branch of anterior lateral line nerve not visible. Middorsal region of body scaled. Rows of scales above lateral line 4-6 (mode 5). Lateral line discontinuous in all examined specimens; area in midbody with no pored lateral-line scales for ca. 1-2 head lengths. Sparse groove-like depigmented epidermal canals found mainly in posterior half of body – as parallel lines either side of lateral line, on flank midway from lateral line to dorsal midline, and as a pair of long irregular lines either side of dorsal surface in dorsal portion of flank. Three bilateral horizontal columns of electrocytes at anal-fin terminus and at mid-point between anal-fin terminus and tip of caudal filament in immature, mature female, and mature male specimens. Caudal filament short to moderate in length.

Coloration. ( Fig. 4 ). Immature specimens and males with pale straw to tan background. Several diffuse and poorly defined vertical darker vertical bands comprising higher chromatophore densities located on lateral surface of body, traversing lateral line without disrupting ventrally. Dorsal region approximately uniformly pigmented with a speckling of brown chromatophores, without uninterrupted pale stripe along dorsal midline from occipital region to base of caudal filament. Irregular dark markings near upper portion of lateral surface but not on dorsal midline in some specimens. Very poorly defined light chocolate darker bands also located over anal-fin pterygiophores, sometimes forming inverted Y-shaped markings. Series of diffuse horizontal dash-like dark markings often present along lateral line in posterior third of body, anterior to anal-fin terminus. Head with evenly scattered dark chromatophores, darker dorsally. Eye without prominent suborbital patch, or stripe, of chromatophores/subcutaneous pigmentation. Pectoral and anal-fin membranes hyaline. Pectoral and anal-fin rays hyaline with light scattering of brown chromatophores. Anal-fin ray pigmentation darker in posterior half of fin. Mature female specimens distinctly darker, with much higher density of brown chromatophores over entire body, head and dorsal regions, background color of lateral surface of body light brown with indistinct oblique darker bands of dark brown. Color in live individuals similar to preserved specimens, with opercular region usually rosy due to underlying gills.

Size. Small adult size, largest specimen examined 111 mm TL, 89 mm LEA (n = 17). Largest male specimen examined 111 mm TL, 89 mm LEA (n = 4). Largest female specimen examined 100 mm TL, 77 mm LEA (n = 4).

Sexual dimorphism. Brachyhypopomus alberti exhibits an unusual sexual difference in pigmentation, with sexually mature females exhibiting a much darker overall coloration (a character shared with B. arrayae ) ( Figs. 4 a-b). No other secondary sexually dimorphic characters known.

Geographic distribution. Bolivia and Brazil ( Fig. 5 ). Known from the upper rio Madeira (Amazonas dr.), above its series of falls beginning at the Cachoeira de Santo Antônio, near Porto Velho, Rondônia, Brazil; in the lower río Beni near the town of Riberalta, Bolivia; and from the rio Guaporé of Brazil.

walteri . In the region of the type locality it exhibits an allotopic distribution with the following whitewater floodplain occurring species: B. arrayae , B. bombilla , and B. pinnicaudatus . Brachyhypopomus alberti co-occurs in geographical sympatry with its sister species B. arrayae , but the species exhibit a noteworthy difference in ecological distribution: B. alberti occurs in low-conductivity (ca. 5-15 μScm-1) terra firme forest and savanna streams, while B. arrayae mainly occurs in higher-conductivity whitewater floodplain systems (ca. 150 μScm-1 at the time of sampling). Nonetheless, B. alberti was sampled together with B. arrayae (and also B. pinnicaudatus ) in the lower reaches of terra firme streams, at the ecotone with the río Beni floodplain.

Etymology. The specific name is a patronym (noun in the genitive case) in honor of James S. Albert, American ( USA) ichthyologist, collector of part of the type series, for his enormous contributions to the systematic biology of gymnotiform fishes.

Local names. Bolivia: cuchillo; Brazil: sarapó. Brachyhypopomus arrayae , new species

Ecological notes. The type series was collected from small low-conductivity clearwater and blackwater terra firme streams in tropical forest and savanna near Riberalta , Bolivia. It was encountered mostly commonly in marginal root mats, and in emergent or submerged aquatic vegetation. The following water parameters were recorded at the sample sites: conductivity 5-15 µScm-1, dissolved oxygen 3.0-5.0 mgl-1, temperature 22-24°C, and pH 5.2-5.5. Adults in breeding condition were found during the dry season in June-July 2007. However, the duration and timing of breeding are otherwise unknown. Stomach contents of specimens from the type locality comprise aquatic insect larvae and other small aquatic invertebrates ( WGRC unpublished data) .

Co-occurring congeners: Brachyhypopomus alberti is known to co-occur in geographical sympatry and ecological syntopy with the following terra firme stream-occurring species: B. brevirostris , B. sullivani , and B.

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Figures

Fig. 4. Brachyhypopomus alberti. a. CBF 10284 (WC24.280607), holotype, male, 97 mm TL (head in lateral view, and body in lateral and dorsal views); b. ANSP 197573 (WC03.250607), paratype, female, 91 mm TL (body in lateral view); Bolivia, río Beni, Amazonas dr. Note the sexual dimorphism in body pigmentation, in which females are darker. Specimens fixed in formalin and preserved in EtOH. Scale bars = 5mm.

Fig. 5. Collection records for Brachyhypopomus alberti (circles), B. batesi (squares), and B. benjamini (triangles). Holotype locations are marked with open symbols. Elevation data refers to altitude above mean sea level (see Fig. 2 for legend).