The story began on October 1, 2024, when a fisherman from Sanlúcar de Barrameda retrieved a cylindrical baited trap net, 4.5 meters long with a 50-centimeter diameter and 6-centimeter mesh, that had been deployed for 24 hours at a depth of about one meter. The trap, baited with a chicken carcass, was targeting the invasive blue crab Callinectes sapidus, and the haul was substantial: 613 blue crabs with an estimated total biomass of roughly 150 kilograms. Among them was one animal the fisherman had never encountered in his traps before. Instead of preserving the specimen, he recorded it on video, a decision that proved decisive because the footage contained enough detail for scientists to make a confident identification.

Back at the laboratory, researchers extracted frames from the video at a rate of 10 frames per second using FFmpeg software, yielding 364 usable images. Morphometric measurements were then estimated with ImageJ2 image analysis software, using a standardized reference object in the frames, the fisher’s fingernail, to calibrate scale. The specimen’s total length was estimated at approximately 7 centimeters, with a carapace length of 28 millimeters and a rostrum length of 21.5 millimeters. The identification relied on diagnostic morphological traits and taxonomic keys, including the rostral formula, the relative proportions of the segments of the second pereiopods, and chela ornamentation, particularly the presence of tubercles and setae. The animal displayed a mostly transparent body with small dark-brown dots, a nearly straight rostrum bearing 12 to 13 dorsal teeth spaced at regular intervals, and a finely tuberculated carapace.

The most telling features were the second pair of pereiopods, which were well developed and elongated, dark brown with beige spots, and bore claws covered with numerous setae. These elongated limbs, which in large adults of Macrobrachium can exceed the body length, are structurally identical to each other, a trait that distinguishes this specimen from other species in the genus and from any known native palaemonid shrimp in the region. The Iberian Peninsula hosts 18 species of the family Palaemonidae, 11 of which are present in the Gulf of Cadiz and the Guadalquivir estuary, including several Palaemon species and the non-native P. macrodactylus, itself introduced around 1997 to 1999. None of these native species display the elongated, setose second pereiopods characteristic of Macrobrachium, which is why researchers conservatively referred to the Guadalquivir animal as M. cf. nipponense, pending future integrative taxonomy combining morphological and molecular approaches if additional individuals are collected.

The oriental river prawn is native to East and Southeast Asia, with a natural distribution spanning eastern China, Japan, Korea, Vietnam, Myanmar, and Taiwan. Since the 1960s, however, it has expanded far beyond this range through a combination of aquaculture transfers, ballast water discharge, and the ornamental pet trade. The literature review conducted for this study, using Web of Knowledge searches cross-verified with GBIF database records and citizen science platforms, compiled 191 records from 21 countries, drawn from 33 publications. In the former Soviet Union, the species was introduced via aquaculture transfers from the Yangtze River basin as early as the 1950s, initially into lakes of the Arnasay system and fish farms in Uzbekistan, from where it spread through the movement of fish stocking material into Kazakhstan, Turkmenistan, and power station reservoirs across Eastern Europe. By the 1980s it was recorded in natural freshwater systems in Belarus, Moldova, and Ukraine, and more recently in Romania, Bulgaria, Germany, and Hungary.

The species’ success in temperate regions is linked to remarkable environmental tolerance. It reproduces above 20 degrees Celsius but can survive winter temperatures as low as 2 to 4 degrees Celsius, enabling persistence across European climates. It typically grows to 5 to 8 centimeters, prefers clean, slow-flowing waters with a pH of 6.5 to 7.5 and temperatures between 20 and 28 degrees Celsius, and can tolerate salinities up to 10. These thresholds align closely with conditions in the Guadalquivir estuary, where temperatures range from 12.5 to 27 degrees Celsius and salinity follows a horizontal gradient, from oligohaline and mesohaline waters in the inner stretch to values of 6 to 22 along the estuary during the dry season. In the last decade the prawn has rapidly expanded in the Danube and Dniester River basins, where it is now reproducing and well established, and molecular analyses suggest multiple introduction events, with haplotypes tracing back to Chinese populations.

How the prawn reached the Guadalquivir remains uncertain, but the researchers hypothesize that accidental transport in ballast water is the most plausible vector, potentially facilitated by increased maritime traffic from the Danube River Basin since Russia’s invasion of Ukraine in February 2022. The Port of Seville, reachable by commercial shipping extending 80 kilometers inland along one of Europe’s major navigable rivers, experienced an increase in goods received from the Danube Basin in 2023, intensifying connections with regions where M. nipponense is established and, in some cases, considered invasive. Data from the port’s annual reports between 2015 and 2023 show that the total volume of goods unloaded from countries where the prawn has been reported varied over time, although these figures represent total national imports rather than specific shipments from confirmed regions. Notably, Bulgaria and Germany recorded their first detections only after this reporting period. Introductions via aquaculture or the pet trade cannot be ruled out either, given that the species is one of the world’s major farmed aquatic animals.

The ecological stakes are considerable. In the Danube and Dniester basins, the prawn’s expansion has raised concerns about its interactions with native Palaemon species and its role as a vector for pathogens affecting wild and farmed crustaceans. The species feeds on a wide range of items, including phytoplankton, detritus, mollusks, and benthic invertebrates such as chironomid larvae, and can act as a trophic link in aquatic food webs. Its growing presence in commercial catches in invaded systems suggests rising population sizes and potential economic impacts. In the Guadalquivir estuary, already recognized as a bioinvasion hotspot driven by habitat modification and climate change, an established Macrobrachium population could compete with native decapods for resources, disrupt benthic community structures, and potentially transmit pathogens, pressures that would compound the effects of the euryhaline blue crab and other non-native species already reshaping the estuary’s ecology.

Only a single individual has been detected so far, and the researchers emphasize that early detection and continuous monitoring are essential to determine whether the species is merely a transient arrival or the leading edge of an establishment event. They recommend a combined strategy that includes systematic field surveys, environmental DNA monitoring, and collaboration with local fishers through citizen science initiatives. Based on the experience of countries where Macrobrachium populations have become established or invasive within the last decade, they advise implementing an early detection system that records each sighting or capture, including the coordinates of the detection site, the number of individuals observed, and their sex, for example the presence of ovigerous females, which would signal local reproduction. The Guadalquivir estuary is already covered by a long-term ecological research program, and the researchers argue that layering citizen science on top of systematic monitoring can multiply the spatial and temporal coverage of biodiversity surveillance.

The episode is a striking demonstration of how much a motivated non-expert can contribute to invasion biology. Marine and coastal ecosystems are vast and difficult to monitor systematically, so many non-native species remain undetected for years, sometimes reaching high population densities before being officially recorded, by which point the window for control measures has narrowed or closed. Citizen science initiatives engaging recreational divers, fishers, and local communities have proven effective at tracking the spread of non-native marine fishes, crustaceans, and mollusks in European waters, and they raise public awareness about biological invasions along the way. In this case, a fisherman’s surprise at an unfamiliar animal in his trap, and his decision to film it rather than discard it, gave scientists the evidence needed to document a genus-level first for the Iberian Peninsula and to begin planning a response before the oriental river prawn can dig in along the Atlantic coast of southern Europe.

Subject of Research: A new non-native species detected through citizen science: the presence of a Macrobrachium species ( Decapoda , Palaemonidae ) in the Guadalquivir River estuary (SW Spain), with an overview of its global spread

Article Title: A new non-native species detected through citizen science: the presence of a Macrobrachium species ( Decapoda , Palaemonidae ) in the Guadalquivir River estuary (SW Spain), with an overview of its global spread

Article References: de Carvalho-Souza, G. F., Cuesta, J. A., & González-Ortegón, E. (2025). A new non-native species detected through citizen science: the presence of a Macrobrachium species (Decapoda, Palaemonidae) in the Guadalquivir River estuary (SW Spain), with an overview of its global spread. Estuarine Management and Technologies, 2, 71-86. https://doi.org/10.3897/emt.2.165887

Image Credits: AI Generated

DOI: 10.3897/emt.2.165887

Keywords: non-native, species, detected, citizen, science, presence, Macrobrachium, Decapoda, Palaemonidae, Guadalquivir, River, estuary