Numerous rivers, interrupted by large waterfalls and extensive rapids, drain the geologically ancient Guiana Shield Highlands. We describe a new armoured catfish genus and two new species endemic to the upper Ireng and Kuribrong rivers, respective tributaries of the Amazon and Essequibo basins in western Guiana. Corymbophanes ameliae sp. nov. is distinguished by having vermiculations on the abdomen, bands on the caudal fin, the anal fin i,5 and narrow caudal peduncle. Yaluwak primus gen. & sp. nov. is distinguished by having evertible cheek odontodes, a plated snout, a tall caudal peduncle and absence of adipose fin and iris operculum. We present a new molecular phylogenetic analysis inclusive of these and several related genera that suggests that the Corymbophanes clade (Araichthys, Corymbophanes, Cryptancistrus, Guianancistrus, Hopliancistrus and Yaluwak) originated in the Guiana Shield with secondary dispersal to the Brazilian Shield. Within the Guiana Shield, relationships among Corymbophanes and Yaluwak are consistent with geodispersal between drainages via headwater capture, although an uplift-mediated relictual distribution cannot be ruled out. ND2 haplotype structure among C. ameliae populations suggests that ichthyofaunal diversity on the Guiana Shield escarpment is shaped not only by inter-, but also intrafluvial barriers to gene flow.
KEYWORDS: Amaila Falls – Amazonian Craton – Guianas – Ireng River – Kuribrong River – Rio Maú – Potaro River – sexual dimorphism.
INTRODUCTION
Eigenmann (1909) described the genus and species Corymbophanes andersoni based on a single specimen (FMNH 52675) collected in the upper Potaro River basin above Kaieteur Falls in what was then British Guiana (now Guiana; Fig. 1). Eigenmann’s collections above Kaieteur were the culmination of a landmark 1908 ichthyofaunal survey of Guiana; an expedition whose many discoveries were more fully detailed in a subsequent monograph (Eigenmann, 1912). Eigenmann (1909) described C. andersoni as being primarily distinguished from other loricariids by lacking an adipose fin and instead having ‘a low median ridge extending from the tip of the dorsal to the caudal’ and ‘no externally visible occipital crest’ (Eigenmann, 1912: 5). In 1912, he provided the first illustrations of C. andersoni, but otherwise did not expand on his original description, adding only that C. andersoni ‘differs from Plecostomus [now Hypostomus] in trifling characters only’ (Eigenmann, 1912: 103). Like Hypostomus, Corymbophanes is distinguished from most other members of subfamily Hypostominae by lacking evertible cheek odontodes. Because of this similarity to Hypostomus – a genus known to be widespread in South America – Eigenmann hypothesized that Corymbophanes may only be ‘a local modification of a comparatively recent immigrant to the [Guiana Shield] plateau’ (ibid.).
Ninety years after Eigenmann’s expedition to the upper Potaro River, one of us (JWA) participated
in an expedition to resample many of the same sites as Eigenmann, with results of this temporal
comparison being summarized by Hardman et al. (2002). In addition to collecting five new specimens of
C. andersoni from its type locality at Chenapou Falls (known to Eigenmann in 1908 as Aruataima Falls)
in the Potaro River main channel, members of this expedition collected five specimens of a new congener
from the nearby Oung Creek (Fig. 1: OC). This new species, Corymbophanes kaiei Armbruster & Sabaj
2000, differs from C. andersoni by having a dark brown abdomen with white vermiculations (vs. mostly white), white vermiculations on the sides (vs. spots) and by having three to four plates beneath the adpressed pectoral-fin spine (vs. five; Armbruster et al., 2000).
Interpretation of the phylogenetic and biogeographical history of Corymbophanes changed
with Armbruster’s (2004) morphology-based phylogenetic analysis of the Hypostominae. Armbruster
found Corymbophanes to be monophyletic, diagnosed by 11 non-unique synapomorphies, and sister to
all other members of the Hypostominae, including Hypostomus. Corymbophanes was, therefore, placed
in its own tribe Corymbophanini. This interpretation of the phylogenetic placement of Corymbophanes held until the first comprehensive molecular phylogenetic analysis of Hypostominae (Lujan et al., 2015), which again dramatically changed our understanding of relationships within this subfamily. Both Lujan et al. (2015) and Fisch-Muller et al. (2018), using different sets of markers, found Corymbophanes to be nested within the Hypostominae tribe Ancistrini and to be well supported as sister to either the Guiana Shield endemic species Cryptancistrus similis (Fisch-Muller et al., 2018) or the Brazilian Shield endemic genus Hopliancistrus [according to Lujan et al. (2015) who did not examine Cr. similis]. Together, the clade of either Corymbophanes + Hopliancistrus (Lujan et al., 2015) or Hopliancistrus + (Corymbophanes + Cryptancistrus) (Fisch-Muller et al., 2018) was sister to the genus Guianancistrus, which is distributed across large lowland rivers of the eastern Guiana Shield in north-eastern Brazil, French Guiana and Suriname (Cardoso & Montoya-Burgos, 2009; Covain & Fisch- Muller, 2012). Problematically, Corymbophanes lacks the evertible cheek odontode mechanism of most other members of Ancistrini, but that mechanism has likely been reduced or lost at least four times within Hypostominae (Armbruster, 2004; Lujan et al., 2015). These taxa formed a largely Guiana Shieldrestricted clade within which the evertible cheek odontode mechanism is highly variable, being absent in Corymbophanes, modified into a three-hooked, anteriorly highly evertible pinching mechanism in Hopliancistrus and present but unspecialized in Cryptancistrus and Guianancistrus.
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