This survey reports the spatial distribution of gastropods belonging to Caenogastropoda, Architaenioglossa, Littorinimorpha, Cycloneritida and Hygrophila orders, and malacostracans from Amphipoda and Mysida orders in the lower sector of the Danube River, Romania, using DNA barcoding based on the cytochrome C oxidase I (COI) gene sequence. Sampling was performed for eight locations of Danube Delta branches and Bechet area during three consecutive years (2019–2021). Molecular identification of sixteen gastropods and twelve crustacean individuals was confirmed to the species level, providing the first molecular identification of gastropods from the Lower Danube sector. Phylogenetic analysis showed that species of gastropods and crustaceans clustered in monophyletic groups. Among gastropods, Microcolpia daudebartii acicularis, Viviparus viviparus, Bithynia tentaculata, Physa fontinalis, Ampullaceana lagotis and Planorbarius corneus were identified in Chilia and Sulina branches; and the Bechet area was populated by Holandriana holandrii, Theodoxus transversalis and Gyraulus parvus. The amphipods and mysids were present along the three main Danube branches. The calculated density of these species revealed an abundant community of crustacean Chelicorophium robustum on Sulina branch, and Dikerogammarus haemobaphes and D. villosus in extended areas of the Danube Delta. The presence of these invertebrates along Danube River was reported in relation to the sediment type and water depth.

Biological invasions can have major impacts on freshwater and marine ecosystems. Therefore, it is vital that non-indigenous species are accurately identified and reported when potential or confirmed invasions occur. The present study reports the first occurrence of Marenzelleria neglecta (Annelida, Spionidae) and the spread of Laonome xeprovala (Annelida, Sabellidae) in the Danube Delta–Black Sea ecosystem. Spionidae is one of the most diverse families of annelid worms and is a dominant group in terms of the number of species that have been introduced to non-native areas, while the members of Sabellidae are among the most visible polychaetes commonly found in fouling communities and are colonizing new geographic areas. Based on 20 samples collected in 2021, we provide an overview of the distribution of the investigated species and possible arrival pathways for Marenzelleria neglecta. Specimens were identified based on morphological descriptions. Both species have invasive behaviour, colonizing large areas in relatively short time periods and reaching relatively high densities (M. neglecta—1400 ind.m⁻²; L. xeprovala—40 ind.m⁻²). Due to their distribution and high abundances, the biology and ecology of these species in the Danube River–Danube Delta–Black Sea system need to be investigated further in order to assess their impact on ecosystem structure and functioning.

The paper reports the record of Pseudochromadora gomoi sp.n. in circalittoral habitats (at depths of 47–59 m) of the Romanian Black Sea waters. The species is characterized by a unispiral amphidial fovea, the presence of lateral alae stretching from the posterior end of the cardia to the level of the cloacal opening, but also shows unique features found in other Desmodorida genera (i.e., the presence of a precloacal ventral ala in males similar to several Desmodora or Echinodesmodora species and eight subcephalic setae on the head capsule). The presence of a constrictor cuticle ring and orange elongated ducts in the anterior body part as in Pseudodesmodora lacrima, Leduc and Wharton, 2010, were also noted. The morphometric measurements of our species differ from most of the Pseudochromadora species reported in specialty literature. We extracted and counted all desmodorid individuals from 9 meiobenthic samples collected in June 2020, and performed a detailed morphological analysis of 12 individuals (5 females and 7 males) belonging to our species, also highlighting the dissimilarities with the related desmodorid species.

The Northwestern Black Sea shelf, strongly influenced by Danube discharge and coastal activities, provides an effective setting for separating lithogenic controls from localized anthropogenic inputs. We applied a multi-proxy geochemical–radiometric approach to Romanian shelf waters and surface sediments. A CTD–Rosette was used to quantify nutrients, chlorophyll-a, TOC, and TN. Dissolved metals and PAHs were measured in seawater, while surface sediments were analyzed for CaCO₃, TOC, trace metals, and γ-emitting radionuclides. Multivariate statistics (PCA/FA) were used to resolve the dominant environmental controls. Summer stratification was characterized by the bottom-layer maxima of PO₄³⁻, SiO₄⁴⁻, and NH₄⁺ and a pronounced subsurface chlorophyll-a maximum at 12–16 m. Surface-water Σ16PAH ranged from 134 to 347 ng L⁻¹ and was dominated by low-molecular-weight compounds, with episodic nearshore enrichment in high-molecular-weight species. In sediments, CaCO₃ ranged from 7.6 to 29.9% and TOC from 0.11 to 0.96%. Trace metals were generally low. Pb and Hg peaked at nearshore station S23, whereas mean Ni (38.88 ppm) slightly exceeded the 35 ppm guideline, consistent with natural Fe/Mn-oxide association. PCA/FA identified a terrigenous axis (Fe-Al-Ti-V-Ni-Cr), a carbonate axis (CaCO₃; Sr where available), and an anthropogenic factor (Pb, Hg, HMW-PAHs). γ-spectrometry provided a compatible radiometric baseline that supports the multi-proxy interpretation.