A new species in the rubbernose catfish genus Chaetostoma is described from the Aguaytia, Pisqui and Palcazu Rivers, which drain the Pampa de Sacramento Region in the Ucayali River drainage of central Peru. The new species is distinguished from congeners, except C. anomalum, C. branickii, C. dorsale, C. leucomelas, C. microps, C. nudirostre, C. palmeri and C. thomsoni by having distinct, white, variably-shaped spots or vermiculations ½–2× nostril diameter on dark grey to black background on the head (vs. spots absent or black on light-coloured background). The new species is distinguished from C. anomalum, C. branickii, C. dorsale, C. microps, C. nudirostre and C. thomsoni by having highly variable, distinct white spots, vermiculations or bands ½–10× nostril diameter on the body, from C. leucomelas by having dorsal and caudal fin indistinctly and variably-patterned with zero to four bands (vs. dorsal and caudal fin consistently having five or more uniform bands) and from C. palmeri by having two predorsal plates (vs. three), supraoccipital excrescence present (vs. absent) and pelvic-fin insertion slightly posterior to dorsal-fin insertion (vs. pelvic-fin insertion at middle of dorsal-fin base). Species delimitation analyses of the COI and Cytb genes further support the recognition of this new species.

Amazon, Andes, Chaetostoma clade, freshwater, molecular, Neotropical, taxonomy

With over 1060 currently valid species, 19% of which (200) were described in the last decade, the suckermouth armoured catfish family Loricariidae is the fifth largest vertebrate family and one of the fastest growing (Fricke et al. 2024). Of the four vertebrate families that are more species-rich (Characidae, Cichlidae, Cyprinidae and Gobiidae), none is growing as quickly or is as geographically restricted as Loricariidae, which naturally occurs only in fresh or estuarine waters from southern Costa Rica to northern Argentina. Within Loricariidae, the rubbernose catfish genus Chaetostoma Tschudi 1846 is the third most species-rich genus with 49 valid species (Meza-Vargas et al. 2022), being superseded only by Hypostomus (> 150 valid species) and Ancistrus (76 valid species) (de Queiroz et al. 2020; Neuhaus et al. 2023). Chaetostoma is also the most geographically restricted of these species-rich genera, with 46 species being restricted to flanks of the Andes and Caribbean Coastal mountains from Panama to southern Peru and three species (C. jegui, C. orientale and C. vasquezi) occurring in rivers draining the Guiana or Brazilian shields (Meza-Vargas et al. 2022).

All but one species of Chaetostoma (C. platyrhynchus) are externally distinguished from most other loricariid genera by having the anterior and anterolateral snout margins free of plates, with this region instead being covered by a broad band of naked (i.e. unplated) skin, lacking tentacles. Five other loricariid genera also have unplated snout margins (Ancistrus Kner, 1854, Corumbataia (Britski, 1997), Paulasquama Armbruster & Taphorn, 2011, Soromonichthys Lujan & Armbruster, 2011 and Transancistrus Lujan, Meza-Vargas & Barriga-Salazar, 2015), but only Ancistrus and Transancistrus are sympatric with Chaetostoma. Chaetostoma can be easily distinguished from Ancistrus by lacking snout tentacles (vs. tentacles present), having at least five longitudinal series of plates at the shallowest part of the caudal peduncle (vs. three) and having eight or more branched dorsal-fin rays (vs. seven). The only Chaetostoma with a fully-plated snout (C. platyrhynchus) is restricted to the upper Caqueta and Napo drainages of Colombia and Ecuador, respectively; it shares all other traits above with congeners.

Chaetostoma has never undergone a comprehensive species-level taxonomic revision, though Lujan et al. (2015b) reviewed species from the centre of the genus’s geographic range in and around Ecuador and provided a multi-locus molecular phylogenetic hypothesis spanning many species and the entire geographic range of the genus. This analysis was recently enlarged and updated by Meza-Vargas et al. (2022). Both studies used combinations of up to 18 external morphological traits including colouration, meristics and morphometrics to distinguish amongst 13 species from rivers draining east of the Andes (cis-Andean), making taxonomy of the cis-Andean distribution of Chaetostoma the most well-studied (Salcedo 2006a, 2006b; Salcedo and Ortega 2015; Ballen et al. 2016). Within this distribution, the Huallaga River in central Peru is the epicentre of species diversity, with seven species currently recognised: C. changae, C. daidalmatos, C. lexa, C. marmorescens, C. stroumpoulos, C. taczanowskii, and C. trimaculineum. In the much larger Ucayali River drainage, which borders the Huallaga to the east and south, only two species are currently known: C. lineopunctatum and C. loborhynchos. In this paper, we describe a new, third species for the Ucayali River drainage. Seidel (2011) first reported this species as undescribed in the ornamental aquarium fish literature, simultaneously highlighting the species’ considerable colour pattern variation by assigning it three different L-numbers (L455, L456, L457). L-numbers are codes used by the ornamental aquarium fish industry to differentiate amongst and market distinct, often scientifically undescribed colour morphs (Stawikowski 1988).

In scientific literature, the species was first recognised as new by Lujan et al. (2015a), who referred to it as ‘Chaetostoma n.sp. Ucayali’ and included it in a multi-locus molecular phylogenetic analysis. This analysis found the new species to be part of a polytomy containing C. breve, C. dermorhynchus, C. dorsale, C. formosae, C. orientale (then Chaetostoma n.sp. Xingu), C. trimaculineum and an undescribed species from the Orinoco River Basin. In a re-analysis and expansion of the Lujan et al. (2015c) dataset, Meza-Vargas et al. (2022) found the new species to be sister to Chaetostoma breve Regan, 1904, a species whose nomenclature we revisit in light of these phylogenetic results and a large series of more recently collected topotype material for Chaetostoma branickii Steindachner, 1881.

Fourteen Chaetostoma species are currently recognised from Peruvian drainages, most of which (12) inhabit the Marañon River Basin, including the Huallaga, Pastaza and Napo Rivers, with only three now described from the Ucayali River Basin: C. lineopunctatum from the Pachitea River, C. loborhynchos from the Tulumayo Rivers and C. sacramento. Until now, the colour pattern described for most species of Chaetostoma in Peru comprises black spots on the body and head (C. daidalmatos, C. lineopunctatum, C. stroumpoulos, C. trimaculineum) or irregular black spots on the body alone (C. loborhynchos, C. spondylus). There are also six species described as having uniform or marbled colouration (C. branickii, C. changae, C. marmorescens, C. lexa and C. taczanowskii). White patterns on a darker base colour are observed in only two species in Peru, C. dermorhynchus and C. microps, from which C. sacramento differs by having distinct and highly variable small to large white spots on the body, head and fins (vs. white spots on only the dorsal fin in C. dermorhynchus) and by having eight dorsal-fin branched rays (vs. nine in C. microps). Our recognition of C. sacramento is further supported by our molecular results, which consistently identify it as a distinct MOTU using the mitochondrial genes COI and CytB. Additionally, our findings demonstrate that C. dermorhynchus and C. microps each represent a separate MOTU and are phylogenetically distant species to C. sacramento, as shown in Lujan et al (2015b).

One of the fundamental criticisms of DNA-based approaches to species delimitation is that such analyses are incapable of distinguishing between genetic structure due to population-level processes and that due to species boundaries (Sukumaran and Knowles 2017). We advocate for a conservative approach in which MOTUs are treated as candidate species to be recognised taxonomically only if they are also morphologically diagnosable. Thus, the combined molecular and morphological evidence supports our recognition of Chaetostoma sacramento as a distinct species. Alternatively, the molecular species delimitation analyses consistently subdivided the Chaetostoma branickii clade into six MOTUs that we do not recognise as distinct species because of the overall morphologically similarity of these subunits and the strong monophyly of the parent clade. Future research may yet yield morphological characteristics capable of distinguishing these populations, thereby supporting their recognition as distinct species. Such a potentiality would be unlikely to resurrect Chaetostoma breve, however, given the very close genetic relationship between individuals from the Santiago (breve) and Marañon (branickii) river type locality populations, consistent with our decision to recognise Chaetostoma branickii as the senior synonym for this species.

Although Chaetostoma sacramento has been known in the aquarium fish trade for almost 30 years, it was originally assigned three L-numbers in 2011 (L455, L456 and L457; Seidel (2011)) and is known in the aquarium trade most commonly as L455. Other common aquarium trade names for the species include ‘Tiger Chaetostoma’, ‘Tingo Red Chaetostoma’ or ‘Ornate Chaetostoma’. Specimens on which the present description is based were collected in the San Carlos River, part of the Palcazu River Basin in Yanachaga Chemillen National Park (PNYC) in 2007 and in the Aguaytia River in 2008. According to aquarium fish exporters, this species also occurs in the Negro River, a small tributary of the upper Pisqui River near the Boqueron del Padre Abad and in the Chiu River, a tributary of San Alejandro River (Seidel 2011).

We thank Dario Faustino, Junior Chuctaya, Claudia Nolasco, Omar Quispe, Jerry Arana and Miguel Paniagua for field assistance, Raphael Covain at MHNG for sharing specimens and data and Julian Dignall at PlanetCatfish.com for sharing photos, aquarium literature and his thoughts on L455. This work was financed by CONCYTEC through the PROCIENCIA programme within the framework of the contest “Projects for the incorporation of postdoctoral researchers in Peruvian institutions” according to contract [074-2021] and [PE501084299-2023-PROCIENCIA-BM] within the framework of the call “Interinstitutional Alliances for Doctorate Programs”.