Archolaemus, KORRINGA, 1970

Vari, Richard P., Santana, Carlos David De & Wosiacki, Wolmar B., 2012, South American electric knifefishes of the genus Archolaemus (Ostariophysi, Gymnotiformes): undetected diversity in a clade of rheophiles, Zoological Journal of the Linnean Society 165 (3), pp. 670-699 : 676-679

ARCHOLAEMUS KORRINGA, 1970

Archolaemus Korringa, 1970: 267 (Type species Archolaemus blax Korringa, 1970 ; by original designation).

Diagnosis: Archolaemus is diagnosed by the synapomorphies described and discussed above, and distinguished from all remaining genera of the Sternopygidae by a pronounced gap between the anterior margin of the premaxilla and the posterior margin of the upper lip, with this separation equal to approximately one-half the width of the eye. It is further separated from Distocyclus , Eigenmannia , Japigny , and Rhabdolichops by the possession of a free orbital rim in adults (a feature uniquely shared with Sternopygus within the Gymnotiformes ). Archolaemus is distinguished from Sternopygus by various features, with those of particular note being the possession of a postcleithrum (versus the absence of that ossification), the presence of a foramen in the scapula (versus the absence of that aperture), and the presence of some branched anal-fin rays (versus all anal-fin rays unbranched). One of the attributes that Korringa (1970) used to delimit Archolaemus from Sternopygus was the relative orbital diameter. The subsequent description of S. astrabes by Mago-Leccia (1994), with a relatively large eye (13.8–19.5% HL; Hulen et al., 2005), resulted in an overlap of the range of orbital diameters in the species of Sternopygus , with values for this feature sitting among the range of values found in species of Archolaemus ( Tables 1 and 2).

Dentition: Jaw dentition of all species of Archolaemus consists of small, needle-like teeth embedded in the fleshy jaws. Consequently, details of the number of teeth and their exact distribution in each jaw are difficult to ascertain other than in cleared and counterstained specimens. Korringa (1970: fig. 2) provides a good illustration of the general dentition pattern (see, however, our comment concerning the dentary dentition in the next paragraph), with the dentary dentition consisting of bands of small teeth extending for varying distances posteriorly along the dorsal margin of the bone. Premaxillary dentition consists of two rounded to anteroposteriorly elongate patches that correspond to the form of the overlying premaxillae, and are separated from each other medially.

Korringa (1970: 267) reported that larger specimens of A. blax have ‘a number of dentary teeth outside the mouth and project forward in larger specimens.’ Schwassmann & Carvalho (1985: 232) were unable to confirm this condition: an observation that is in line with our results. Some larger specimens of Archolaemus can have the dentary dentition somewhat more prominent anteriorly, but not projecting forwards.

Distribution: Species of Archolaemus are known from the Rio Branco, Rio Madeira, Rio Tapajós, and Rio Xingu catchments within the Amazon basin, and outside that system in the Rio Tocantins, the easterly flowing Rio Araguari in the state of Amapá, Brazil, and the Rio São Francisco system of eastern Brazil (see species accounts).

Secondary sexual dimorphism: Secondary sexual dimorphism is manifest in several features among the species of Archolaemus . These include the distinctly darker overall pigmentation of the head and body in mature males, relative to the condition in females and juveniles, in A. ferreirai , the pronounced elongation of the snout in mature males of A. luciae , versus the condition in females and immatures, and the presence of teeth on the upper lip of large males of A. ferreirai , compared with the absence of such dentition in mature females. Limited samples of many species and the absence of mature males for others make it impossible to determine the generality of these features across the genus. This limitation notwithstanding, these modifications represent the first reported instances of secondary sexual dimorphism in the Sternopygidae .

N, number of specimens; H, holotype; range includes holotype of species.

N, number of specimens; H, holotype; range includes holotype of species.

Remarks: In their description of Japigny, Meunier et al. (2011) differentiated that genus from Archolaemus on the basis of pigmentation patterns and the total number of anal-fin rays. Their concept of Archolaemus was based on samples from the Rio Xingu, which presumably represent what we consider to be A. janeae , the only member of the genus known from that river system. Archolaemus orientalis described in this paper, however, has a range of total anal-fin rays (164–186) overlapping, albeit barely, with the values in Japigny (132–164), and lacks the pattern of longitudinal dark and light pigmentation common to the other species of Archolaemus . As such, the characters originally proposed to distinguish the two genera are no longer applicable. Archolaemus and Japigny differ in the association of the eye with the surrounding regions (free versus attached) and pigmentation pattern (dark pigmentation either absent or, when present, in the form of a dark stripe along the lateral line and a broad dusky to dark band over the basal pterygiophores of the anal fin versus wide alternatively dark and light vertical bars along the body).

In their analysis of Archolaemus blax, Schwassmann & Carvalho (1985) detailed the apparent pronounced variation in head form between populations within what they considered to be that species. In part, this variation was a consequence of ontogenetic modifications in snout and overall head morphology, as reflected in differences in the head profile from a lateral view. Supplementing these intraspecific ontogenetic alterations are alternative degrees of snout development across the species in Archolaemus , four of which ( A. blax , A. ferreirai , A. janeae , and A. luciae ) were encompassed within A. blax as delimited by Schwassmann & Carvalho (1985). Of note is the particularly elongate and broader snout of A. luciae , more so in mature males ( Fig. 7 ). This factor contributes significantly to the range of snout profiles within what those authors considered to be A. blax .

Conservation status: The rapids and waterfalls patchily distributed across the gradients of the Brazilian and Guiana shields ( Lima & Ribeiro, 2011; Lujan & Armbruster, 2011) are home to a number of specialized communities of plants and animals, including rheophilic fishes. Such high-energy settings significantly hinder the sampling of these communities, and as such our knowledge of the ichthyofaunas in these settings is often deficient, even within the context of the still inadequate overall understanding of the Neotropical freshwater fish fauna ( Vari & Malabarba, 1998). At the same time the high-energy conditions that complicate the thorough sampling of these habitats provide the attributes that are ideal for hydroelectric facilities. The development of hydroelectric generating capacity is accelerating across the Neotropics, with the consequent submergence under reservoirs of multiple habits preferentially occupied by rheophiles. Moreover, proposed impoundments (i.e. the Belo Monte project on the lower Rio Xingu; Fearnside, 2006) threaten many as yet undeveloped high-energy settings. It is likely that various rheophilic species have been extirpated from their type localities by hydroelectric developments. Within the Gymnotiformes , this eventuality probably applies both in Archolaemus (e.g. A. santosi from the Rio Jamari at Usina Hidroelétrica Samuel in Rondônia) and Sternarchorhynchus (e.g. Sternarchorhynchus britskii in the upper Rio Paraná; Camposda-Paz, 2005: 399). The same is likely to befall many of the locations inhabited by the species of Archolaemus and other rheophiles.

Figures

Figure 7. A, Archolaemus luciae sp. nov., holotype, female, 267 mm total length, INPA 20960; Brazil, Pará, Rio Trombetas, Cachoeira Porteira, below Furo nas Pedras (approximately 01°05′S, 057°02′W); B, Archolaemus luciae sp. nov., male, 497 mm total length, INPA 6767; Brazil, Pará, Rio Jamanxim, Rio Tapajós basin, Ilha Terra Preta (05°27′04″S, 055°52′40″W).