Ammoglanis obliquus sp. nov., a minute catfish species reaching a maximum adult size of 15.5 mm, is described from the Rio Preto da Eva drainage in the central Brazilian Amazon. It is distinguished from all of its congeners in possessing an exclusive combination of character states, including the presence and number of premaxillary and dentary teeth, number of interopercular and opercular odontodes, presence of cranial fontanel, number of dorsal-fin rays, number of anal-fin rays, number of caudal-fin rays, number of pelvic-fin rays, number of pectoral-fin rays, absence of pelvic splint, antorbital morphology, and absence of supraorbital and autopalatine morphology. It is considered to be a member of a clade also including A. pulex and A. amapaensis due to the unique oral, antorbital, and autopalatine morphology. Ammoglanis obliquus is regarded as more closely related to A. pulex than to any other congener, as both species exhibit a similar colour pattern, an absence of the metapterygoid, and the presence of two finger-like projections on the chin region.

Taxonomy, tropical rain forest, Sarcoglanidinae, systematics

The Amazon river basin exhibits the greatest diversity of fish species in the world, harbouring more than 2,400 species (Reis et al. 2016). Among this diversity, 956 species are representatives of the order Siluriformes (Dagosta and de Pinna 2019), popularly known as catfishes. Occupying a wide range of habitats throughout this basin (e.g. lakes, streams, waterfalls, rapids, and flooded forests; Aedriens et al. 2010), catfishes are known for their adaptability, but their presence in sandbanks highlights how specialized these fishes can be. Despite being widespread and structurally simple, sand bottom habitats shelter a unique fish fauna that bears some remarkable morphological features: cryptic colour (usually translucent or light-coloured body), small body size, eyes dorsally placed (sometimes reduced or absent), and a specialized head morphology (Zuanon et al. 2006). These characteristics can be found in several sand-dwelling (i.e. psammophilic) lineages of the family Trichomycteridae, such as the subfamily Sarcoglanidinae.

Members of Sarcoglanidinae are exclusively psammophilic, inhabiting loose patches of sand in both clearwater (Costa 1994; Zuanon and Sazima 2004) and blackwater streams (de Pinna and Winemiller 2000; Costa et al. 2019). They are miniaturized catfishes that do not exceed 25 mm of standard length (de Pinna 1989) and are generally translucent, which allows for camouflage with the surrounding environment (Myers and Weitzman 1966; de Pinna 1989; Costa 1994). Their association with sandy-bottom habitats is so strong that sarcoglanidines bury themselves in the sand when disturbed and when resting (Zuanon and Sazima 2004; Zuanon et al. 2006), to the extreme that Ammoglanis pulex de Pinna & Winemiller, 2000 burrows 20 cm deep into the substrate (de Pinna and Winemiller 2000).

Sarcoglanidinae was erected by Myers and Weitzman (1966) to allocate at that time two new distinct genera and species, Sarcoglanis simplex Myers & Weitzman, 1966 and Malacoglanis gelatinosus Myers & Weitzman, 1966. Few morphological characters (Myers and Weitzman 1966; de Pinna 1989; de Pinna and Starnes 1990; Costa 1994; de Pinna and Winemiller 2000) have been described to diagnose Sarcoglanidinae and its genera. However, a recent molecular analysis directed to establish relationships of interstitial trichomycterids from the Atlantic Forest indicated that Sarcoglanidinae, in the sense of previous studies, is a polyphyletic assemblage comprising unrelated lineages with convergent morphological adaptations to the psammophilic life-style (Costa et al. 2020). Currently, according to Costa et al. (2020), Sarcoglanidinae comprises five genera: Malacoglanis Myers & Weitzman, 1966, Sarcoglanis Myers & Weitzman, 1966, Stauroglanis de Pinna, 1989, Stenolicmus de Pinna & Starnes, 1990, and Ammoglanis Costa, 1994.

Ammoglanis was considered to be exclusively Amazonian, but recently, A. multidentatus Costa, Mattos & Santos, 2019 was described from northeastern Brazil, which expanded the geographical range of Ammoglanis 1000 km to the east (Costa et al. 2019). Three other species are currently recognized for the genus: Ammoglanis diaphanus Costa, 1994, from the Araguaia river basin; A. pulex from the Orinoco river basin; and Ammoglanis amapaensis Mattos, Costa & Gama, 2008 from the Jari river basin. The recent description of A. multidentatus illustrates that undocumented species may occur throughout South America (Costa et al. 2019). The current paper contributes to elucidating the species diversity of sarcoglanidines by describing a new species of Ammoglanis from the Amazon river basin, northern Brazil.

Collections were made with a permit provided by Instituto Chico Mendes de Conservação da Biodiversidade as approved by Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio; permit number 54636-3). Specimens were captured with small dip nets and euthanized in a buffered solution of ethyl-3-amino-benzoat-methanesulfonate (MS-222) at a concentration of 250 mg/l until completely ceasing opercular movements, according to animal welfare laws and guidelines (Leary et al. 2013; Close et al. 1996, 1997). Morphological characters were obtained from specimens fixed in formalin for a period of 10 days and then transferred to 70% ethanol. Specimens are deposited in the ichthyological collection of the Instituto de Biologia, Universidade Federal do Rio de Janeiro, Rio de Janeiro (UFRJ). Comparative material is listed by Costa (1994), Mattos et al. (2008), and Costa et al. (2019). Additional comparative material is listed in Supplementary file 1. Measurements and counts follow Costa (1992), with the addition of preanal length (measured from snout tip to origin of anal fin), first pectoral-fin ray length (measured from basis of first-pectoral fin to its tip), and opercular patch length (longitudinal length of the opercular patch of odontodes). Measurements are presented as percentages of standard length (SL), except for those related to head morphology, which are expressed as percentages of head length. Cleared and stained (CS) specimens for osteological examination were prepared according to Taylor and Van Dyke (1985). Osteological nomenclature follows Datovo and Bockmann (2010). Osteological illustrations were made using a stereomicroscope Zeiss Stemi SV 6 with a camera lucida. Fin-ray counts include all elements, and vertebral counts include all vertebrae except those part in the Weberian apparatus; the compound caudal centrum was counted as a single element. Roman numerals represent unbranched rays and Arabic numerals represent branched rays in each fin-ray count. In the description, the frequency of each fin-ray count is given in parentheses after each count.

Ammoglanis obliquus is allocated to the genus Ammoglanis by the presence of a long and slender quadrate, its greatest length about 75% of the length of the hyomandibula excluding the anterior process and its greatest depth about 30% of the quadrate total length (Fig. 3). This character state was first proposed by Costa (1994) to diagnose the genus and was confirmed to occur in all other species of Ammmoglanis, as stated by de Pinna and Winemiller (2000), Mattos et al. (2008), and Costa et al. (2019). Ammoglanis obliquus is probably more closely related to A. pulex and A. amapaensis than to its other congeners. Ammoglanis pulex and A. amapaensis have been considered closely related taxa due to their unique oral, antorbital, and autopalatine morphology (Mattos et al. 2008; Costa et al. 2019a). As seen in A. pulex and A. amapaensis, A. obliquus also exhibits a subventral mouth, a scythe-shaped antorbital with an anterior facet (Fig. 5; de Pinna and Winemiller 2000: fig. 6; Mattos et al. 2008: fig. 5), and a compact autopalatine with the lateral margin slightly concave and the medial margin slightly convex, bearing a small posterolateral process (Fig. 5; de Pinna and Winemiller 2000: fig. 6; Mattos et al. 2008: fig. 5). According to Costa et al. (2019a), the autopalatine morphology recorded for A. pulex and A. amapaensis is apomorphic and indicates a close relationship between these two species. Therefore, because A. obliquus has the same autopalatine shape, we consider this character state synapomorphic for A. obliquus, A. pulex, and A. amapaensis. Ammoglanis obliquus and A. pulex share some unique character states indicating that they are sister taxa: absence of the metapterygoid, a condition unique among sarcoglanidines (Fig. 3; de Pinna and Winemiller: fig. 5), and a similar colour pattern, with eight poorly-defined bands of chromatophores visible in dorsal view and dark chromatophores in the interior of the body forming a banded pattern (Figs 1A, B, 2).

Figure 4. 

Ammoglanis obliquus, UFRJ 12478, 13.2 mm SL. A. Ventral view of the left premaxilla; B. Medial view of left lower jaw.

Figure 5. 

Ammoglanis obliquus, UFRJ 12478, 13.2mm SL; dorsal view of mesethmoidal region and associated structures.

Figure 6. 

Geographic distribution of species of Ammoglanis. Red circle – A. pulex; black circle – A. obliquus; orange circle – A. amapaensis; yellow circle – A. diaphanous; blue circle – A. multidentatus.

Figure 7. 

Habitat of Ammoglanis obliquus.

Thanks are due to J.L.O. Mattos for laboratorial assistance and to P. Vilardo for helping to edit images. A. Katz gently helped photographing fixed specimens. E. Henschel was funded by National Geographic Society (Early Carreer Grant Number EC-316R-18) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Ministério de Ciência e Tecnologia). This study is part of E. Henschel’s PhD thesis. This study was also supported by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Finance Code 001) through Programa de Pós-Graduação em Genética/UFRJ.