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 : 673-674

OF ARCHOLAEMUS

Archolaemus shares the following unique combination of characters that define the Sternopygidae : the presence of multiple rows of small needle-like teeth in the form of a villiform band on the dentary; the relatively large eyes; the enlargement of the antorbital and infraorbitals, with expanded bony arches over the laterosensory canal segments; the anterior naris located within the vertical limits of the gape; an anal-fin origin located along the isthmus; and the lack of a caudal fin and dorsal electroreceptive organ ( Hulen, Crampton & Albert, 2005).

Four proposed synapomorphies that delimit Archolaemus as monophyletic were identified in our analysis.

1. A free orbital rim. As reported by several authors (e.g. Nijssen & Isbrücker, 1972: 174; Mago-Leccia, 1994: 18; Meunier et al., 2011: 48), all species of the Gymnotiformes , except for Archolaemus and Sternopygus , have the orbit covered by skin, with this surface layer attached to tissues bordering the eye. Juveniles of Archolaemus share this apparently primitive condition of an orbit covered by skin and attached to the surrounding tissues, in some species up to approximately 135 mm TL; however, larger individuals of the genus all have the orbital rim free of the surrounding orbital margin. In the context of present concepts of relationships within the Gymnotiformes , the free orbital rim of Archolaemus and Sternopygus is considered a homoplastic derived character in these two genera.

2. Form of attachment of the teeth to the premaxilla. As in all other sternopygids, the species of Archolaemus bear teeth on the premaxilla (approximately 22–35 teeth, depending on the species). The villiform teeth in sternopygids are typically immobile and attached to the ventral surface of the premaxilla. Conversely, in Archolaemus only the anterobasal margins of the teeth of the first tooth row are attached to the dentigerous surface of the premaxilla. As a consequence, these teeth are variably mobile relative to the premaxilla with the range of movement ranging between a few and 90 degrees: an apparent apomorphy for the members of the genus.

3. Association of the posterior margin of the upper lip and the anterior margin of the premaxilla. The anterior margin of the premaxilla lies close to and contacts, or almost contacts, the posterior margin of the upper lip from a ventral view in all species of the Sternopygidae other than for Archolaemus . In contrast, all species of Archolaemus have a pronounced gap equal to approximately one-half the width of the eye between the anterior margin of the premaxilla and the posterior margin of the upper lip ( A. blax in Korringa, 1970: fig. 2A).

4. Morphology of the upper lip. The ventral surface of the upper lip is relatively flat and unelaborated from a ventral view in most members of the Sternopygidae , as well as across the Gymnotiformes . Conversely, the ventral surface of the upper lip is porous and sponge-like with raised papillae and fleshy anteroposteriorly elongate ridges of various extents in all of the species of Archolaemus . Elsewhere within the Sternopygidae this condition was encountered only in some populations of what is now considered to be Eigenmannia trilineata . Within the context of present hypotheses of relationships within the Sternopygidae (see the discussion under character 1), the occurrence of this form of upper lip in some specimens of E. trilineata and the species of Archolaemus would be homoplastic, but with the attribute a synapomorphy for the members of Archolaemus .

Two characters previously advanced as autapomorphies for what was then thought to be a monotypic Archolaemus proved not to apply across the expanse of the more species-rich genus in this study. Albert (2001: 71) proposed that a lateral ethmoid contacting four bones was an autapomorphy for A. blax . As such it potentially served as a synapomorphy for the six species of the genus we recognize herein. Our survey shows that the complex lateral ethmoid does not universally contact four bones across the genus. A posterior naris positioned closer to the tip of the snout than to the eye was hypothesized by Albert (2001: 71) to be an autapomorphy for A. blax . Comparisons reveal, however, that the posterior naris is sometimes located approximately in the middle of the snout among several of the previously undescribed species of Archolaemus recognized herein.

Relationships within Archolaemus . The data indicate that A. orientalis is the sister group of a clade composed by A. blax , A. ferreirai , A. janeae , A. luciae , and A. santosi . The hypothesis of the monophyly of a clade formed by these five species is supported by three synapomorphies.

1. Presence of a narrow stripe of dark pigmentation along the lateral line. The presence of a narrow stripe of dark pigmentation extending along the lateral line is absent in A. orientalis , and is unique to these five species (see species accounts) among the examined members of the Sternopygidae , other than for also occurring in some species of Eigenmannia such as E. trilineata . What appears as a dark line of pigmentation proximate to the lateral line in A. orientalis is in fact the line of contact between the epaxial and hypaxial musculature.

2. Presence of a band of dusky to dark pigmentation overlying the basal pterygiophores of the anal fin. The presence of a broad band of dusky to distinctly dark pigmentation formed of individual vertical bars paralleling the pterygiophores, with the bars sometimes conjoining in darker specimens to form a variably continuous broad stripe, is limited to the members of this clade (see species accounts).

3. Form of posterodorsal portion of the dentary. Whereas the posterodorsal portion of the dentary is ossified in other sternopygids, this region is occupied by cartilage in these five species. Arratia (1992) made the case that given that the dentary is a dermal bone, the cartilage in the area typically occupied by the posterodorsal portion of the dentary, the ‘cartilaginous coronoid process’ in her terminology, is at least in some catfishes more likely to be a dorsal extension of Meckel’s cartilage.

Within the clade of five species supported by characters 1–3, the evidence supports a hypothesis of two clades, each including two species ( A. blax plus A. janeae and A. ferreirai plus A. luciae ). Those two clades along with A. santosi form an unresolved trichotomy.

Archolaemus blax and A. janeae are hypothesized to be sister species based on their common possession of two derived characters.

4. Relative size of the posterior ceratohyal versus ventral hypohyal. In these species, the posterior ceratohyal is approximately 1.5 times the length of the ventral hypohyal versus the two bones being approximately the same size in their congeners and other sternopygids.

5. Number of tooth rows on the posterior portion of dentary. A single tooth row on the posterior portion of the dentary occurs in both of these species, whereas other congeners have two rows of teeth in this area. The remaining sternopygids with teeth present on the posterior portion of the dentary have two or more rows of teeth in that region.

The remaining pair of sister species, A. ferreirai and A. luciae , share one hypothesized derived character.

6. Relative size of the coronomeckelian bone versus Meckel’s cartilage. In these two species the elongate coronomeckelian bone extends along a considerable portion of the inner surface of the lower jaw, and corresponds to 50% or more of the length of Meckel’s cartilage. In other congeners and outgroups the more compact coronomeckelian bone is approximately 20% or less of the length of Meckel’s cartilage.