Our molecular analyses confirm the occurrence of two genetically distinct invasive Sinanodonta lineages in Iraq, namely Sinanodonta woodiana and S. pacifica. The newly generated Iraqi COI sequences clustered within the S. woodianasensu stricto clade and were identical to haplotype SWA01, the dominant invasive haplotype widely distributed across introduced populations in Europe and Central Asia (Douda et al. 2025). In contrast, the previously published Iraqi sequences associated with the Euphrates River near Najaf (LC651629, LC651630, LC656037, LC658986) grouped within the S. pacifica clade and matched haplotype SWB02, previously reported from Peninsular Malaysia. These findings demonstrate that morphologically similar Sinanodonta populations in Iraq represent two genetically distinct invasive lineages and provide the first molecular evidence for cryptic co-invasion within the Tigris–Euphrates Basin.

The Iraqi S. pacifica sequences are particularly important because the original records were morphologically identified as S. woodiana (Abdul Razak and Zwair 2021). However, phylogenetic analyses by Douda et al. (2025), confirmed by our results, clearly place these sequences within S. pacifica. This inconsistency highlights the constraint of shell morphology for recognizing invasive Sinanodonta lineages. Previous studies have shown that shell shape in S. woodiana is highly plastic and affected by external factors, which may mask species limits and hinder identification based on morphological traits (Guarneri et al. 2014). More broadly, recent molecular studies demonstrated that invasive populations formerly treated as a single widespread species comprise several cryptic lineages, including S. woodianasensu stricto, S. pacifica, and S. lauta (Bolotov et al. 2016; Douda et al. 2025). This research points to the significance of genetic identification in tracing the origins of invasions and getting an exact picture of species distribution. Our results further indicate that the two invasive lineages currently occupy different sectors of the Tigris–Euphrates river system. Sinanodonta woodiana has so far been confirmed from the southeastern lowlands of the Al-Gharraf River and associated marsh systems, whereas S. pacifica has been recorded farther northwest in the middle Euphrates River system. The closest confirmed records of the two lineages remain separated by approximately 200 km in straight-line distance or roughly 360 km following river-course connectivity. Despite the lack of documented local sympatry, comprehensive surveys might uncover zones of contact or overlapping ranges for the two invasive lineages in this basin. The Iraqi records are biogeographically notable because co-invasion by multiple Sinanodonta lineages within the same country or river basin remains relatively uncommon. Outside Iraq, simultaneous occurrence of S. woodiana and S. pacifica has only rarely been documented, including in the Irrawaddy Basin of Myanmar (Vikhrev et al. 2017; Bolotov et al. 2022). Comparable cases involving other invasive Sinanodonta lineages have also been reported from the Volga Basin and Kazakhstan (S. woodiana and S. lauta) and from Malaysian Borneo (S. pacifica and S. lauta) (Kondakov et al. 2020a; Zieritz et al. 2024; Douda et al. 2025). The convergence of unique invasive Sinanodonta lineages in the Tigris–Euphrates Basin suggests that multiple, distinct introduction events occurred through fish trade and aquaculture practices.

Although both Sinanodonta lineages recorded in Iraq were most likely introduced unintentionally during their parasitic glochidial stage via fish hosts, the available molecular evidence suggests that their invasion pathways differ. Earlier genetic investigations showed that invasive populations of Sinanodonta are organized into multiple primary lineages linked to various geographical sources and aquaculture systems (Bolotov et al. 2016; Douda et al. 2025). The widespread temperate invasive lineage of S. woodianasensu stricto occurring throughout Europe and Central Asia is characterised by a dominant COI haplotype originating from the Yangtze River basin in China (Konečný et al. 2018; Kondakov et al. 2018; Urbańska et al. 2021). The Iraqi S. woodiana sequences we identified are part of this identical invasive haplotype, which reinforces the theory that populations in Iraq emerged via the same broad introduction route responsible for invasions across Europe and Central Asia. The most likely vector for introduction of the temperate S. woodiana lineage into Iraq is aquaculture-mediated translocation of East Asian cyprinids. In Europe and temperate Asia, S. woodiana spread repeatedly with silver carp (Hypophthalmichthys molitrix) and grass carp (Ctenopharyngodon idella) imported from China for aquaculture and stocking programmes (Konečný et al. 2018; Urbańska et al. 2021; Douda et al. 2025). In Iraq, Asian carp (Cyprinus carpio) aquaculture is widespread across the Tigris–Euphrates Basin and includes hatcheries, fish farms, and floating-cage operations (Al-Khafaji and Al-Amary 2016; Khudair and Fahd 2024). Therefore, the existence of the same invasive COI haplotype in Iraq strongly suggests that it was introduced through aquaculture routes related to carp. By contrast, the Iraqi S. pacifica sequences matched haplotype SWB02 previously reported from Peninsular Malaysia, indicating a potentially different invasion history associated with tropical aquaculture networks. Tropical invasive Sinanodonta lineages have frequently been linked to fish hatcheries and warm-water aquaculture systems in Southeast Asia and Central America (Bauer et al. 2021; Douda et al. 2025). Since the late 1990s, tilapia aquaculture has grown significantly in Iraq. Oreochromis species and red belly tilapia (Coptodon zillii) are now commonly cultivated and naturalized in a number of freshwater systems in Iraq (Jawad et al. 2018; Mohamed and Al-Wan 2020, 2023; Jawad and Abdulsamad 2021). Continuous propagation and movement of cultured fish stocks may therefore have facilitated introduction of S. pacifica into Iraqi inland waters.

The ornamental fish trade may represent an additional secondary introduction pathway. In Germany, ornamental-shop S. woodiana populations were shown to be genetically similar to wild invasive populations, demonstrating that the aquarium trade can contribute to regional spread of invasive Sinanodonta (Benedict et al. 2024). Since the early 2000s, importation and sale of tropical ornamental fishes have increased considerably in several Iraqi cities, with many traded species originating from Southeast Asia (Jawad et al. 2021). This introduction pathway cannot currently be ruled out, even though there is currently no concrete evidence connecting ornamental fish imports to Sinanodonta introductions in Iraq. Therefore, it is possible that the Tigris–Euphrates Basin serves as a secondary contact zone where various invasive Sinanodonta lineages brought in via various aquaculture routes come together within a single fluvial network. Similar cases of cryptic co-invasion involving multiple Sinanodonta lineages have been documented elsewhere, including the Yenisei River in Siberia, the Volga Basin, Kazakhstan, and the Irrawaddy Basin of Myanmar (Bespalaya et al. 2017; Kondakov et al. 2020a; Bolotov et al. 2022; Douda et al. 2025). Such systems are biogeographically important because they reflect multiple independent introduction events rather than local diversification.

Variations in climatic affinities may also have an impact on future expansion and establishment success. The temperate S. woodiana lineage has demonstrated a remarkable ability to establish populations in colder environments far beyond its native climatic range, whereas S. pacifica appears more strongly associated with tropical and subtropical regions (Urbańska et al. 2019; Yermoshyna and Pavliuchenko 2021; Douda et al. 2025). These ecological distinctions imply that the two invasive lineages might react differently to the Middle East’s ongoing hydrological change and climate change.

Although the proposed invasion pathways remain partly hypothetical, the available molecular and biogeographic evidence strongly supports the interpretation that Iraqi populations of S. woodiana and S. pacifica originated through separate introduction histories linked to different fish-translocation networks. Additional studies using higher-resolution genetic markers combined with detailed analysis of aquaculture trade pathways will be required to reconstruct the chronology and directionality of these invasion events more precisely.

Although ecological impacts are much better documented for Sinanodonta woodiana than for S. pacifica, the occurrence of two invasive Sinanodonta lineages within the Tigris–Euphrates Basin raises important ecological concerns. Invasive Sinanodonta species share several life-history characteristics associated with invasion success, including rapid growth, high fecundity, broad environmental tolerances, extended reproductive periods, and use of a wide range of fish hosts during the parasitic glochidial stage (Konečný et al. 2018; Urbańska et al. 2019, 2021; Douda et al. 2025). Experimental and field studies from Europe further demonstrated that S. woodiana can establish dense populations with high reproductive output and successful recruitment even under suboptimal climatic conditions (Dobler et al. 2022; Urbańska et al. 2021). Similar reproductive patterns have also been reported for S. pacifica in tropical Southeast Asia, suggesting strong dispersal and establishment potential for both invasive lineages (Sahidin et al. 2024).

Once established, invasive Sinanodonta populations may negatively affect native unionids through several mechanisms. Previous studies showed that S. woodiana glochidia successfully develop on a broad range of fish hosts and may even exhibit higher metamorphosis success than native unionids (Douda et al. 2011; Huber and Geist 2019). Experimental evidence additionally demonstrated that prior infestation of host fish by S. woodiana glochidia can significantly reduce metamorphosis success of native mussel larvae through host-mediated immune responses and competition for suitable hosts (Donrovich et al. 2017; Dobler et al. 2022). Other studies reported negative physiological effects on infested fish, including reduced body condition, altered blood chemistry, impaired swimming performance, and increased mortality under some conditions (Douda et al. 2017; Anil et al. 2025). In addition, invasive Sinanodonta may alter benthic habitats and ecosystem processes through intensive filter feeding, biodeposition, and competition for substrate and food resources (Douda and Čadková 2018; Poznańska-Kakareko et al. 2021).

In Iraq, establishment of S. woodiana coincides with the probable extirpation of the endemic freshwater mussel Anodonta vescoiana from large parts of its historical range (Naser et al. 2025b). Although direct causation cannot yet be demonstrated, the spread of invasive Sinanodonta together with hydrological alteration, salinisation, habitat fragmentation, and pollution may collectively have contributed to the decline of native unionid assemblages in the southern marshes and rivers of Iraq.

Potential future sympatry between S. woodiana and S. pacifica may further increase ecological risks. Co-invasion by multiple Sinanodonta lineages has previously been documented in several Eurasian river systems, including the Yenisei River, Kazakhstan, and the Irrawaddy Basin (Bespalaya et al. 2017; Kondakov et al. 2020b; Bolotov et al. 2022; Douda et al. 2025). Where multiple invasive lineages occur together, additive or synergistic ecological impacts may arise through increased propagule pressure, broader exploitation of host-fish assemblages, intensified competition for habitat and food resources, and cumulative effects on ecosystem functioning. There have also been reports of hybridization and complex lineage interactions within Sinanodonta, indicating further ecological and evolutionary complexity where invasive lineages come into secondary contact. (Kondakov et al. 2018; Douda et al. 2025).

Despite increasing knowledge of invasive Sinanodonta elsewhere, virtually no information is available regarding comparative ecology, host use, filtration impacts, or competitive interactions between S. woodiana and S. pacifica under semi-arid riverine conditions such as those characterising the Tigris–Euphrates Basin. To better understand their effects on native freshwater biodiversity in Iraq and the surrounding areas, targeted ecological and experimental studies addressing population dynamics, host associations, ecological interactions, and potential hybridization between the two lineages are therefore desperately needed.