This research underscores the effectiveness of eDNA metabarcoding in shedding light on the biodiversity studies of river affected by anthropogenic pollution such as Batanghari River ecosystem in Indonesia. We also emphasized its potential in enhancing monitoring and conservation efforts for ecologically and economically significant species, including those crucial for Batanghari river fisheries. eDNA metabarcoding serves as a valuable complement to traditional methods such as visual census and morphology-based identification, enabling the collection of data on freshwater biodiversity that was previously inaccessible through conventional means (Piggott et al. 2021; Minamoto et al. 2012; Porter and Hajibabaei 2018; Beng and Corlett 2020). Despite its relatively limited application in the sampling sites of the Batanghari River, a hotspot for freshwater biodiversity spanning much of its course, this study represents one of the few endeavors in Indonesia to utilize eDNA metabarcoding, yielding promising outcomes.
In general, 80% of the reads were successfully classified to the species level, whereas 20% remained unclassified. The presence of numerous unknown taxa suggests that many of the species identified in this study are not documented in global databases. However, considering some previous studies discovery of new taxa in the region, the limited research on biodiversity in Indonesian rivers, such as the Batanghari River, and the vast unexplored areas, it is possible that some of these unidentified ASVs may represent previously undiscovered freshwater species. This potential discovery could greatly improve our understanding of freshwater biodiversity. It highlights the significant portion of Indonesia's diverse biodiversity that remains unexplored and emphasizes the insufficient representation of taxa from the Batanghari River and surrounding areas in global COI databases. Moreover, the substantial abundance of unidentified taxa in this study emphasizes the need to address the gap in global metazoan databases to ensure accurate taxonomic classification. Nevertheless, this study has provided essential foundational sequence data that can be compared with species currently documented in these databases, proving valuable for future biodiversity and conservation research efforts.
The Batanghari River, Sumatra's longest river, is believed to harbor a vast array of multicellular species, of which only a small fraction has been identified so far (Marnis et al. 2024). These underscore the significant gap in our understanding of the river's biodiversity and emphasize the need for further research. It is imperative to focus research efforts on areas with extensive biodiversity that have been insufficiently explored, such as the upstream to downstream stretches of the river. Additionally, attention should be directed towards habitats facing significant anthropogenic pressures, such as illegal gold mining and expanding palm oil plantations along the downstream Batanghari River (Febrianti et al. 2023; Marnis et al. 2024). Despite ongoing efforts, biodiversity records in the region suffer from various shortcomings, including ineffective methods for building comprehensive inventory catalogues. For instance, biodiversity monitoring primarily relies on visual surveys conducted by observers with varying levels of expertise, which might not capture the full extent of biodiversity. In this context, our study has made significant strides by introducing a new method for biodiversity inventory and has underscored the vast disparity between our current knowledge and the unknown aspects of the river's biodiversity. Although still relatively new in Indonesia and arguably not yet developed to its full potential for Indonesian river, the application of eDNA metabarcoding can speed up auditing process for species identification by providing a general picture regarding the magnitude of biodiversity. Furthermore, while some studies applied eDNA in the freshwater areas have successfully identified plankton residing in the Cliwung river Indonesia (Aprilia et al., 2023), fish species and vertebrates in The Maninjau and Singkarak lakes (Roesma et al., 2021a, 2021b; Roesma et al., 2023), this approach has not been widely utilized to explore the biodiversity inhabiting concealed other freshwater environments like in Batanghari rivers. Our study outcomes have reinforced the significance of freshwater biodiversity, particularly within the Batanghari River in Indonesia, highlighting the necessity for additional taxonomic investigations aimed at offering comprehensive taxonomic data accompanied by DNA information. Furthermore, the results further underscore the crucial role of eDNA in uncovering concealed Batanghari river biodiversity, which encompasses the discovery of novel organisms (Penaluna et al. 2023; Jerde 2021) and the addition of updated information to public databases (Kulaš et al. 2022; Parrondo et al. 2018; Belle, Stoeckle, and Geist 2019; Coble et al. 2019).
The exploration of taxonomic composition in freshwater biodiversity hotspots like Indonesia poses significant challenges, especially when considering species that are under-researched, have small populations, or are rare and uncommon (Madduppa et al. 2021; Madduppa et al. 2022). The challenges in exploring the taxonomic composition of freshwater biodiversity hotspots, such as Indonesia, arise from various factors. Firstly, many species in freshwater ecosystems have not received sufficient attention due to limited resources, funding, and scientific focus (Darwall et al. 2018; Harper et al. 2021). This lack of investigation hinders our understanding of their taxonomic makeup and ecological functions. Secondly, some species exhibit cryptic diversity, where morphologically similar species have genetic distinctions (Grupstra et al. 2024; Cheng et al. 2024). Identifying and characterizing these species requires advanced molecular techniques, which may not be readily accessible. Thirdly, habitat loss, degradation, and fragmentation pose significant threats to freshwater ecosystems, leading to declines in species populations and overall biodiversity (Mayer and Pšenička 2024; Reid et al. 2019). This makes it challenging to study and conserve less prevalent species that are vulnerable to environmental changes. Additionally, conducting sampling activities in freshwater environments presents logistical challenges, including remote locations, rugged terrain, and fluctuating water conditions. These factors may limit access to certain habitats and species for research purposes. To address these challenges, collaborative efforts among researchers, conservationists, policymakers, and local communities are essential. Investing in biodiversity research, utilizing advanced modern molecular techniques such as eDNA, implementing effective conservation measures, and advocating for habitat restoration and protection are crucial steps toward enhancing our understanding and safeguarding the taxonomic composition of freshwater biodiversity hotspots like Indonesia.
The diversity analyses conducted in our study underscored the disparities in species richness and biotic community composition between the upstream and downstream locations of the Batanghari River. Our results were consistent with the widely accepted idea that areas with a history of environmental pollution, like downstream river areas, often have reduced levels of biodiversity. The results indicated that eDNA could expedite the identification of previously unnoticed diversity. The relatively comprehensive data from ecological surveillance and visual census of Batanghari river showed that this area suffers from a variety of anthropogenic stresses, including large numbers of introduced species induced by fish farming activities (Kaban et al. 2019; Kaban et al. 2020). In addition, numerous previous studies in these areas are poorly documented in terms of accessions deposited in public databases, including the BOLD Systems database (www.boldsystems.org) and the NCBI sequence database (https://www.ncbi.nlm.nih.gov/).
The utilization of eDNA metabarcoding involves applying both standard and taxon-specific COI mini barcodes, typically with lengths ranging up to approximately 200 bp but usually below 400 bp, has been developed for examining degraded DNA materials found in aquatic environments. Through this analysis, we successfully identified various taxa spanning three major phyla (Chordata, Arthropoda, and Mollusca) using short mitochondrial DNA COI sequences. However, we observed an unequal distribution of these phyla across the sampled areas. Regarding the sampling methodology, the relatively limited number of taxa detected in the water samples could be attributed to factors such as the minimal amount of DNA material released into the environment by present organisms, rapid degradation processes post-DNA shedding, or local water current movements consistently transporting DNA within the water column.
An analysis conducted across all sites in the Batanghari River globally revealed a higher proportion of reads originating from the phylum Chordata compared to other phyla, particularly mammals and fish. We identified a total of 587 chordate species, with 303 fish species surpassing the previously reported 297 fish species documented or presumed to inhabit this river between 1994 and 2003 (Hui and Kottelat 2009). Furthermore, this number exceeds the species count identified in 2022 using a combination of conventional methods and DNA barcoding (Marnis et al. 2024). Given that our study marked the first utilization of an eDNA approach in these regions, the findings imply that eDNA has the capability to unveil concealed and cryptic diversity that may remain unnoticed by traditional monitoring and DNA barcoding techniques.
The dominant fish order across all locations in the Batanghari River is Cyprinidae. This finding closely resemble those of previous studies conducted using both DNA barcoding (Marnis et al. 2024) and conventional technology (Hui and Kottelat 2009). However, our research utilizing the eDNA approach revealed a higher number of families, genera, and species, including unidentified species accounting for 3.39%. This presents an opportunity for the discovery of new fish species within the Batanghari River. Furtherome, in our study, we detected Scleropages formosus, Chitala borneensis, Chitala lopis, Notopterus notopterus, and Anguilla bicolor. These fish species have been exploited for an extended period, resulting in their classification within different at-risk categories on the IUCN Red List due to their global population status (IUCN 2023). Another significant observation was the substantial presence of cichliformes and Characiformes reads, suggesting the dominance of introduced fish species in the Batanghari River.
Regarding Arthropods, we observed a substantial portion of unidentified taxa at Batanghari Hulu, Teluk Kayu Putih, which includes freshwater shrimp, freshwater crab, and freshwater lobster. Batanghari River is one of the River in Sumatra with the potential for freshwater shrimp diversity and high freshwater crabs (Purnamasari 2013; Susilo 2013). The highest count of reads for the Order Decapoda was also found in the upper stretches of the Batanghari River, specifically at Batanghari Hulu (Teluk Kayu Putih), with Batanghari Tebo, also situated in the upper stretches of the river, showing similar patterns. The scarcity of shrimp and other Decapoda presence in the downstream Batanghari river might be attributed to the elevated levels of antopogenic pollution in the downstream river (dos Anjos Guimarães et al. 2023; Nguyen and Fisher 2014; Hukom et al. 2020). Nonetheless, numerous species of shrimp inhabit the lower stretches of the Batanghari River, including: Macrobrachium rosenbergii, also referred to as the giant river prawn or giant freshwater prawn, holds considerable commercial importance as a palaemonid freshwater prawn species. It is widely distributed across tropical and subtropical regions of the Indo-Pacific, ranging from India to Southeast Asia and Northern Australia. While the larvae of this species develop exclusively in brackish water, juveniles and adults primarily inhabit areas with lower salinity levels and freshwater. Adult M. rosenbergii migrate to estuaries for breeding purposes (Pillai et al. 2022). This observation may account for the higher prevalence of this species in the lower stretches of the river compared to the upper reaches of the Batang Hari River. Apart from Macrobrachium rosenbergii, Macrobrachium lar, which exhibit nearly identical characteristics to M. rosenbergii, were more prevalent in the lower sections of the Batang Hari River compared to the upper regions. Moreover, Macrobrachium malayanum, the most prevalent species in the rivers of the South Malaya forest, is distributed from Thailand to Borneo, including Sumatra (Johnson 1963). This species inhabits habitats with varying water currents, ranging from strong to slow. Macrobrachium lanchesteri thrives in pond and swamp environments and exhibits exceptional resilience in extreme conditions compared to other shrimp species. Its presence poses a potential threat to various native shrimp species (Wowor et al. 2009). Other genus of Macrobrachium is Macrobrachium pilimanus prefers habitats characterized by strong currents and rocky substrates. It is one of the freshwater shrimp species capable of surviving in areas with heavy currents, spanning both lowland and highland regions (Wowor 2010). This type of shrimp is commonly found in the upper reaches of the Batang Hari River.
The Atyidae family comprises Caridina sumatrensis, Caridina excavatoides, Caridina propinqua, Caridina gracilipes, and Caridina sp. These species are exclusively found in rivers within oil palm plantations and containing aquatic vegetation. This is supported by research indicating that the Atyidae family is typically associated with environments featuring aquatic, rocky, and subterranean vegetation. Caridina sumatrensis is distributed in Sumatra, the Malay Peninsula, and the Philippines, inhabiting rivers and streams (Johnson 1963). C. excavatoides is found in the Malay Peninsula and Sumatra. Caridina propinqua' distribution spans Malay Peninsula, Philippines, Sri Lanka, India, China, and Japan, (Cai, Ng, and Choy 2007). Caridina gracilipes is distributed across Sulawesi, Borneo, Taiwan, Philippines, Malay Peninsula, and mainland China, with its habitat primarily in the lower reaches of rivers and streams influenced by seawater (Yule and Sen 2004). Moreover, in this research, all Caridina genera are classified as Least Concern according to the IUCN 2023 assessment.
The current study identified seven freshwater crab species, a greater number than those identified in previous studies using conventional techniques (Nursyahra and Purnamasari 2018), including Parathelphusa pantherina, Parathelphusa maculate, Parathelphusa maindroni, Parathelphusa tridentata, Parathelphusa batamensis, Parathelphusa sp, and Geosesarma sp. The crabs' habitat has faced significant risks due to anthropogenic activities, including domestic and urban wastewater, and they have endured prolonged periods of exploitation. Consequently, these habitats are classified into various at-risk categories on the IUCN Red List. For example, Parathelphusa maindroni has been categorized as vulnerable since 2008 (Esser 2008). Interestingly, we found sequence reads of Parathelphusa batamensis species listed as endangered since 2008 (Esser 2008) and Parathelphusa pantherine is also listed as an endangered species since 2018 (Schubart 2018).
The presence of Cherax quadricarinatus in the Batanghari River resulted from human-mediated translocation, resulting in its designation as an introduced species (Austin 2010). C. quadricarinatus originates from permanent freshwater streams, billabongs, and lakes located along the northern coast of the Northern Territory, northeastern Queensland, and Papua New Guinea. In various rivers and freshwater habitats spanning multiple countries, including Indonesia (Patoka et al. 2016), this species formed indigenous populations, leading to its classification as an invasive species.
The application of eDNA metabarcoding has demonstrated its efficacy as a tool for identifying species across a range of Batanghari river invertebrate groups, including Mollusca. In this study, we identified 24 orders, 40 families, 102 genera, and 153 species belonging to the Phylum Mollusca. Unidentified Mollusca taxa were present across all sites, with the bivalve group exhibiting a notably higher percentage of unidentified species compared to Gastropods. In the Batanghari River, the Thiaridae family demonstrated the highest prevalence among all sites, surpassing other families within the gastropod class. The abundant presence of Gastropods belonging to the Thiaridae family is linked to the abundance of their food sources, encompassing detritus, moss, and diverse algae. Moreover, factors such as their adaptability to tranquil, slow-moving, and swiftly flowing waters contribute to their proliferation. Furthermore, members of the Thiaridae family are capable of thriving in water bodies characterized by high turbidity and elevated levels of Total Suspended Solids (Djajasasmita 1985, 1999). The family Ampullariidae and Viviparidae from Gastropods class were general find in the Batanghari river. The Viviparidae family thrives in a variety of aquatic environments such as swamps, rivers swift water currents, and typically resides in communal groups within their habitat (Viza 2018). Furthermore, the Ampullariidae family possesses the broadest distribution and demonstrates the capacity to endure dry environmental conditions for a comparatively extended period. In our study, Pomacea canaliculata was discovered, a species previously identified as invasive in small rivers flowing into the Batanghari River in several prior investigations (Susilowati 2017; Isnaningsih and Marwoto 2011). This species exhibits a remarkable capacity to adjust to varying environmental conditions, enabling it to inhabit diverse habitats. This adaptability is believed to contribute to its status as an invasive species.
This study revealed limited diversity within the bivalve order, with only species classified in the order Unionida and family Unionidae being detected, while the other orders remained unidentified. This might be attributed to the fact that many of the bivalve species identified in this study are not recorded in databases. Additionally, the limited variety of bivalves might also be associated with various activities carried out by communities surrounding the Batanghari River, such as the discharge of liquid waste from households and industries alongside illegal gold mining operations, potentially contributing to river contamination. These activities could lead to water pollution, resulting in a deterioration of water quality (Desrizal, Carlo, and Syah 2019; Kaban et al. 2020; Yanova et al. 2022).
Monitoring extensive areas of the Batanghari River presents a significant challenge. Utilizing eDNA protocols to effectively detect specific important species could be an efficient approach to support the achievement of river protection objectives. Environmental DNA (eDNA) metabarcoding surveys have shown their utility across various ecosystems and have the potential to provide an efficient and cost-effective complement to traditional ecosystem surveys. This could benefit river conservation managers and fisheries officers responsible for monitoring their designated areas.