New records of Endangered Slender Walking Catfish, Clarias pseudonieuhofii Sudarto, Teugels & Pouyaud, 2004 (Siluriformes, Clariidae) from Central Mahakam River, East Kalimantan, Indonesia

the journal of biodiversity data NOTES ON GEOGRAPHIC DISTRIBUTION 1171 Academic editor: Pradeep Kumkar Received: 4 August 2025 Accepted: 13 November 2025 Published: 26 November 2025 Copyright © The authors. This is an open‑access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0) Abstract. Clariidae, commonly referred as air-breathing or walking catfishes, are a family of catfish dis‑ tributed in Africa, Asia Minor, the Indian subcontinent, and Southeast Asia. Some species are highly valu‑ able as food fish. As part of ongoing ichthyofaunal surveys in the Central Mahakam River, East Kalimantan, Indonesia, we present our findings from Clarias specimens collected from Central Mahakam River. We document C. pseudonieuhofii Sudarto, Teugels & Pouyaud, 2004, in the Central Mahakam River, Kutai Kar‑ tanegara Regency, East Kalimantan, which is located approximately 1200 km from the type locality in West Kalimantan. Key words. Air-breathing fish, distribution, fish fauna, Southeast Asia, morphology Gustiano R, Haryono H, Harefa T, Jusmaldi J, Nurjirana N, Rusdianto R (2025) New records of Endangered Slender Walking Catfish, Clarias pseudonieuhofii Sudarto, Teugels & Pouyaud, 2004 (Siluriformes, Clarii‑ dae) from Central Mahakam River, East Kalimantan, Indonesia. Check List 21 (6): 1171–1178. https://doi. org/10.15560/21.6.1171 Introduction The fish family Clariidae Bonaparte, 1846, often known as air-breathing or walking catfishes, is a group widely found in Africa, Asia Minor, the Indian subcontinent, and Southeast Asia. Some species are of high economic importance as food fish. Clarias gariepinus (Burchell, 1822), is currently Indonesia’s second most-produced freshwater aquaculture commodity, trailing only tilapia. Farming of Clarias Scopoli, 1777 was established because these species can be easily farmed in areas with limited space and scarce water supplies (Gustiano et al. 2021). Many catfish species probably have significantly greater aquaculture potential than C. gariepinus (Teugels 1996). The family Clariidae can be recognized by the distinctive structure of its respiratory system, specifically a suprabranchial organ formed by the expansion of the second and fourth epibranchials (Teugels and Adriaens 2003). Currently, the family contains 14 genera. Of these, 12 are found in Africa and three occur in India to Southeast Asia. The genus Clarias is one of the genera in Asia and is known as the world’s most widespread genera of fish. Species of Clarias are distinguished by an elongated body with long dorsal and anal fins, no adipose fin, a mouth with bands of filiform teeth on the premaxillary and vomerine, small eyes, a bony top and sides of the head or only covered by very thin skin, four pairs of barbels, six soft rays in the pelvic fins and only two hard rays in the pectoral fins, and two swim bladders. Following a systematic review of Southeast Asian Clarias (Sudarto and Pouyaud 2005), 10 species are recognized from Indonesia (Gustiano et al. 2020). They are C. nieuhofii Cuvier & Valenciennes, 1840; C. pseudonieuhofii Sudarto, Teugels & Pouyaud, 2004; C. intermedius Teugels, Sudarto & Pouyaud, 2001; C. meladerma Bleeker,1846; C. leiacanthus Bleeker, 1851; C. olivaceus Fowler, 1904; C. batrachus Linnaeus, 21 (6) · https://doi.org/10.15560/21.6.1171 21 (6): 1171–1178. https://doi.org/10.15560/21.6.1171 New records of Endangered Slender Walking Catfish, Clarias pseudonieuhofii Sudarto, Teugels & Pouyaud, 2004 (Siluriformes, Clariidae) from Central Mahakam River, East Kalimantan, Indonesia Rudhy Gustiano1, Haryono Haryono1, Tonisman Harefa1, Jusmaldi Jusmaldi2, Nurjirana Nurjirana3, Rusdianto Rusdianto1 1 Laboratorium Ichthyology, Museum Zoologicum Bogoriense, Research Center for Biosystematics and Evolution, National Research and Innovation Agency, Jl. Raya Jakarta Bogor Km 46, Cibinong 16911, Jawa Barat, Indonesia 2 Department of Biology, Mulawarman University, Jl. Barong Tongkok No. 4, Kampus Gunung Kelua, Samarinda 75123, Kalimantan Timur, Indonesia 3 Hasanuddin University, Jl. Perintis Kemerdekaan Km 10, Tamalanrea, Makassar 90245, Sulawesi Selatan, Indonesia Corresponding author: Rudhy Gustiano (rudhy[email protected]) Check List 21 (6) · https://doi.org/10.15560/21.6.1171 Gustiano et al. · New record Clarias pseudonieuhofii from East Kalimantan 1172 1758; C. microstomus Ng, 2001; C. pseudoleiacanthus Sudarto, Teugels, & Pouyaud, 2003; and C. kapuasensis Sudarto, Teugels, & Pouyaud, 2003. C. nieuhofii and C. pseudonieuhofii are the Clarias species with cylindrical and extremely elongated bodies in Southeast Asia (Sudarto et al. 2004). One additional species not included in the above publication is Clarias nigricans described by Ng (2003). Until now, detailed information on the population status of C. nieuhofii has been lacking. Consequently, it is currently assessed as Least Concern in the IUCN Red List (Ng 2019). Weber and de Beaufort (1913) were the first to report C. nieuhofii in East Kalimantan, followed by Christensen (1992). For nearly three decades thereafter, surveys of the Mahakam River in East Kalimantan failed to find this species (Haryono 2006; Nasution et al. 2008; Kasim et al. 2015; Jusmaldi et al. 2019; Auliansyah et al. 2021; Batubara et al. 2025). Mahakam River, the second-largest river in Kalimantan, extends approximately 920 km and drains an area of 77,700 km² (Christensen 1992). As part of an ongoing catfish diversity survey of the Mahakam River, we report Clarias specimens collected from the Central Mahakam River. We found that C. nieuhofii maintains a substantial population in the Central Mahakam River. Confirming its occurrence after more than three decades without records, except those of Suyatna et al. (2021). Clarias nieuhofii is typically found in blackwater habitats such as swamps, streams, and rivers associated with peat-swamp forests. However, its primary habitat has undergone severe degradation due to human activities, with major consequences for population persistence. This species is an important food fish in peat swamps and contributes significantly to local food security and livelihoods. Nevertheless, unregulated harvesting, without restrictions on season or size, poses a serious threat to future population stability. The results of this study also showed the presence of C. pseudonieuhofii, Figure 1. Three habitat localities: A. Melintang Lake, B. Semayang Lake, C. Muara Wis; Traditional equipments used to collect fish samples: D. pangilar; E. bubu; F. anco. Check List 21 (6) · https://doi.org/10.15560/21.6.1171 Gustiano et al. · New record Clarias pseudonieuhofii from East Kalimantan 1173 which is an endemic species from West Kalimantan, namely a sympatric species, as a new record. Clarias pseudonieuhofiihas been assessed forthe IUCN Red List of Threatened Species as Endangered (Ng 2019). In this study, we offer the first records of C. pseudonieuhofii in the Central Mahakam River, East Kalimantan, which has not been previously recorded there. We determined its identity based on morphological traits. Methods Ichthyofaunal surveys were conducted during the dry season (August 2024) and in the rainy season (February 2025). Fifteen specimens of Clarias catfish were collected from three different localities: Semayang Lake, Melintang Lake, and Kota Bangun District (Figure 1). Fish samples were collected using “bubu”, a fish trap made from bamboo or wire; “pangilar”, a fence embedded in the water in the form of a cage; “banjur”, a net pulled together to trap fish; and “anco”, a manual fish-lift net (Figure 1). The fishing gear was installed in the evening, and the harvest took place the next day. Prior to fixation, fresh coloration of all specimens was documented with a digital camera. All samples were preserved in 10% formalin then transferred to 70% ethanol for long-term storage. Fish were measured point-to-point using digital dial calipers following the protocol of Teugels (1986). Subsequently, fish specimens were deposited in the laboratory of Ecology and Animal Systematics, Faculty of Mathematics and Natural Sciences, Mulawarman University (EAS-MSC-UNMUL), and the Marine Science and Fisheries Faculty, University of Hasanuddin, Makassar (MSFUH). Species identification followed the taxonomic keys of Kottelat et al. (1993) and Sudarto and Pouyaud (2005), Sudarto et al. (2004), Ng (2003), and Ng et al. (2011). Results Family Clariidae Bonaparte, 1846 Genus Clarias Scopoli, 1777 Clarias pseudonieuhofii Sudarto, Teugels & Pouyaud, 2004 Figure 2, Table 1 New records. INDONESIA — East Kalimantan • Kenohan District, Central Mahakam River, Semayang Lake; 00°11’15”S, 116°27’19.8”E; alt. ± 2 m a.s.l.; 8.II.2025; R. Gustiano, Haryono, Rusdianto leg.; 1♂, 364 mm SL, MSFUH 1965 • Muara Wis District, Central Mahakam River, Melintang Lake; 00°16’47.28”S, 116°26’26.484”E; ± 2 m a.s.l.; 7.II.2025; R. Gustiano, Haryono, Rusdianto leg.; 2♀,1♂, 143–192 mm SL, EAS-MSC-UNMUL 035 • Kota Bangun District, Fish market; 00°07’31.08”S, 116°26’57.12”E; ± 8 m a.s.l.; 7.IV.2025; T. Harefa leg. 1♀,1♂, 161–180 mm SL, MSFUH 1966. Figure 2. Clarias pseudonieuhofii Sudarto, Teugels, & Pouyaud, 2004. A. MSFUH 1966, 161 mm SL, collected in Melintang Lake, Muara Wis District, Kutai Kartanegara Regency, East Kalimantan. B. MZB 10966, 311 mm SL, holotype . Check List 21 (6) · https://doi.org/10.15560/21.6.1171 Gustiano et al. · New record Clarias pseudonieuhofii from East Kalimantan 1174 Identification. Morphometric measurements indicate that the six specimens we collected from Central Makakam confirm that they are C. pseudonieuhofii (Table 1). Clarias pseudonieuhofii differs from all other Southeast Asian Clarias species except the slender walking catfish group (C. nieuhofii, C. nigricans, C. gracilentus Ng, Hong & Tu (2011)) by its anguilliform body and relatively short (HL, 16.5–21.1% SL) and narrow (HW, 12.7–14.3% SL) head. Clarias pseudonieuhofii differs from C. nieuhofii by its shorter anal fin (56.8–58.9 vs. 59.7–66.8% SL), where posterior of anal fin reaches the beginning of the caudal fin in C. pseudonieuhofii, while exceeds the caudal fin and even merges with it in C. nieuhofii. Our specimens of C. pseudonieuhofii differ from C. nigricans by its longer head length (18.4–21.1 vs. 17.4–18.3% SL), shorter the length between occipital process and dorsal fin (6.1–8.0 vs. 8.1–9.8% SL), head width (12.7–14.6 vs 11.7–12.3% SL), and shorter snout length (21.7–24.5 vs. 29.2–34.9% HL). Our specimens of C. pseudonieuhofii differ from C. gracilentus by its longer head width (13.3–14.6 vs. 11.9–12.9% SL) and shorter snout length (21.0–23.8 vs. 23.0–31.5% HL). In living specimens of C. pseudonieuhofii, the skin is dark brown to black, with darker pigmentation on the dorsal body surface, and the fins are paired. The ventral surface is light brown. Overall, this species closely resembles C. nieuhofii, hence the species epithet. Previously, C. pseudonieuhofii was only known from the upper basin of the Kapuas River, in West Kalimantan, near Sentarum Lake (Sudarto et al. 2004). Our new records document this species in the Central Mahakam River, Kutai Kertanegara Regency, East Kalimantan (Figure 3). Discussion Prior to the identification of Clarius pseudonieuhofii (Sudarto et al. 2004), C. nigricans, C. gracilentus, all Clarias species in Southeast Asia with an anguilliform or slender body and a short, small head were known as C. nieuhofii (Kottelat et al. 1993; Teugels et al. 1998). Based on recent specimens from the Central Mahakam River, we concur that C. pseudonieuhofii is distinguished from C. nieuhofii by the length of its anal fin. Further investigation of samples from the Central Mahakam River reveals that C. nieuhofii and C. pseudonieuhofii are sympatric in this river. They share the same environment, with a balanced abundance ratio. Consequently, our study provides the first confirmed record of C. pseudonieuhofii in the Mahakam River system. Until now, only C. gariepinus has been cultivated, with fingerling obtained from the outside the area. Farmers justify this Table 1. Morphometric measurements of newly collected specimens of Clarias pseudonieuhofii from central Mahakam River in East Kalimatan, Indonesia. MSFUH 1965 MSFUH 1966 MSFUH 1966 EAS-MSC-UNMUL 035 EAS-MSC-UNMUL 035 EAS-MSC-UNMUL 035 Standard length (mm) 161 180 143 187 192 364 As % standard length Maximal body depth 11.7 12.5 11.9 13.1 12.3 12.9 Caudal peduncle depth 5.3 5.0 4.6 4.4 4.1 4.8 Head length 19.0 19.0 18.4 20.3 19.1 19.0 Predorsal length 26.8 26.0 27.3 28.6 26.3 25.7 Preanal length 39.1 39.9 41.0 41.9 42.8 43.5 Prepelvic length 36.6 34.1 35.4 36.6 35.9 36.2 Prepectoral length 15.8 15.0 16.0 16.1 14.2 14.8 Dorsal fin length 74.6 68.6 72.0 73.2 71.3 70.4 Length between occipital process and dorsal fin 7.2 6.9 7.8 8.0 6.2 7.1 Pectoral spine length 5.9 6.3 6.8 5.8 6.3 5.2 Pectoral fin length 8.4 9.8 9.4 10.3 10.0 7.9 Pelvic length 5.3 6.1 5.2 5.2 5.6 5.2 Anal fin length 58.1 56.8 58.9 58.5 58.3 58.4 Head width 13.8 13.6 13.3 14.6 13.5 14.1 As % head length Snout length 22.4 22.3 21.0 21.7 24.5 23.8 Interorbital width 41.4 42.0 38.1 42.2 41.1 41.4 Eye diameter 5.6 5.9 5.8 6.8 5.1 6.6 Occipital process length 10.1 14.5 11.2 13.1 13.3 9.7 Occipital process width 35.0 39.1 37.8 37.1 35.8 33.9 Frontal fontanelle length 11.5 15.1 12.9 14.6 15.3 16.1 Frontal fontanelle width 5.0 5.5 5.2 5.6 5.5 5.4 Premaxillary toothplate width 27.3 26.3 27.5 24.6 26.4 25.6 Vomerine toothplate width 19.4 21.8 20.0 22.9 20.5 18.4 Check List 21 (6) · https://doi.org/10.15560/21.6.1171 Gustiano et al. · New record Clarias pseudonieuhofii from East Kalimantan 1175 because C. garipinus is technically easier to maintain in a cage, adapts well to the feed provided, and is more cost effective and efficient for trade because to its greater size. The Mahakam River has been reported to have a high amount of endemism, ranging from 46% (Popta 1906) to 23% (Molengraaf and Weber 1921; Taki 1978; de Beaufort 1951). The ichthyofauna of the Mahakam River is particularly notable for its high diversity of indigenous species. Its high degree of endemism also suggests that the basin has either been isolated from other systems for a long period or has a unique ecology that supports speciation (Gustiano 2003). This biogeographic pattern is closely linked to the evolutionary history of the Mahakam River, which was not hydrologically connected to the major river systems of the Sunda Shelf region during the Pleistocene epoch (Molengraaf and Weber 1921; de Beaufort 1951; Rainboth 1996). According to McConnell (1987), the Oriental region, which includes the Indian subcontinent, Southeast Asia’s mainland, and the Indonesian Archipelago islands (Sumatra, Java, and Kalimantan), was periodically connected to the mainland during intervals of lowered sea levels in the Pleistocene glaciations. Kottelat (1995) reported that 23 of the 82 species (28%) collected from the Mahakam Basin were previously undescribed by science. Our findings support his hypothesis, that a significant number of Figure 3. Geographic distribution of Clarias pseudonieuhofii: ● = new records; ▲ = previously known occurrence (type locality). Check List 21 (6) · https://doi.org/10.15560/21.6.1171 Gustiano et al. · New record Clarias pseudonieuhofii from East Kalimantan 1176 species in the Mahakam River remain scientifically unclassified (Kottelat 1995; Gustiano and Pouyaud 2005). The occurrence of C. pseudonieuhofii provides concrete evidence supporting this proposition. The type locality of this species is characterized by peat-swamp forest, which is like the habitat of specimens collected from East Kalimantan, specifically from Lake Semayang and Melintang. Lake Semayang, about 1,200 km from the type locality, spans approximately 13,000 ha on the left bank of the Mahakam River and is adjacent Lake Melintang, which occupies approximately 11,000 ha on the right bank of Mahakam. Lake Semayang is one of the largest wetlands in Kalimantan. It supports a highly diverse aquatic biota and provides abundant water supplies for a range of community activities. The lake’s ecosystem constitutes a rich reservoir of genetic resources and represents an ecologically significant region with considerable economic potential for multiple sectors. Therefore, this area needs to be properly managed regarding fishing and habitat protection from damaging activities, especially for threatened fish such as C. pseudonieuhofii. According to de Beaufort (1951), species occurring in the rivers of eastern Sumatra and Kalimantan but absent from Mahakam River, East Kalimantan are likely recent Pleistocene migrants, while currently inhabiting the Mahakam River may represent relics of earlier colonization events. Studies on the fish families such as Cyprinidae (Bănărescu 1992), Bagridae (Dodson et al. 1995), and Pangasiidae (Gustiano 2003) support the hypothesis of an ancient river that once connected the major rivers of Java, Sumatra, and Kalimantan, excluding the Mahakam River in East Kalimantan. Acknowledgements We thank Alvin Pranata and Eddy Irawan of Conservation Area Management Office in the Waters of the Upstream Kutai Kartanegara Mahakam Region for guiding us in the field and providing facilities; Brooklyn Pramindra and Muhammad Hilmi of Biology Department, Faculty of Science, Mulawarman University for fieldwork and specimen collection. We thank the editors, Rafael Benzi Braga, Pradeep Kumkar, Robert Forsyth, and the reviewers, Ng Heok Hee, Tedjo Sukmono, Nian-Hong Jang-Liaw, and Veryl Hasan, for their contributions and suggestions that improved and enhanced the manuscript. Additional information Conflict of interest The authors declare that no competing interests exist. Ethical statement No ethical statement is reported. Funding This work was supported by RIIM Funding of National Research and Innovation Agency, Indonesia for Siluridae Fish Family Biodiversity Expedition in the Mahakam Watershed Area, Kalimantan (Official note # B-197/III.5/FR.06.01/1/2024). Postdoctoral fellowship in the Research Center for Biosystematics and Evolution (PRBE), National Research and Innovation Agency, Indonesia (BRIN) (NOMOR 149/II/ HK/2024). Author contributions Conceptualization: RG. Data curation: RG, HH, JJ, TH. Funding acquisition: HH, RR, TH. Methodology: RG, NN, TH. Validation: RG. Visualization: RG, HH, JJ, TH. Writing – original draft: RG. Writing – review and editing: RG, TH, NN, JJ. Author ORCID iDs Rudhy Gustiano https://orcid.org/0000-0001-7205-0101 Haryono Haryono https://orcid.org/0000-0002-5143-6790 Tonisman Harefa https://orcid.org/0000-0003-2308-8669 Jusmaldi Jusmaldi https://orcid.org/0000-0002-1002-3084 Nurjirana Nurjirana https://orcid.org/0000-0003-4801-1481 Rusdianto Rusdianto https://orcid.org/0000-0002-9776-2946 Data availability All data that support the findings of this study are available in the main text Check List 21 (6) · https://doi.org/10.15560/21.6.1171 Gustiano et al. · New record Clarias pseudonieuhofii from East Kalimantan 1177 References Auliansyah, Aprianti Y, Sulindrina A (2021) The fisheries economy in the Mahakam River:Benefits and factors that affect it. 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