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Recently, large numbers of Polyclad flatworms (Stylochus mediterraneus) have been observed in Istrian County shellfish farms connected to intensive ascidian fouling. To assess a possible threat and determine whether the flatworm feeds only on weakened dead mussels or can also infest healthy mussels, we have conducted a field research and laboratory mussel predation experiments. Performed field observations (August 2022–May 2023) indicated increased mussel mortality and, at the same time, increased abundance of flatworms from 70 % time filtrating with the valve open > 50 %, with 1–5 resting periods), the behavioural changes of exposed mussels compared to control mussels, the timing of flatworm attack, the response of mussels during predation, and the timing of mussel death. We conclude that the flatworm S. mediterraneus is most likely a very opportunistic species that prefers weaker mussels but can also feed on healthy mussels. This investigation brings new knowledge and assumptions on the behavioural patterns of investigated flatworms. Further studies should consider that ''experienced'' flatworms attack and kill the mussels, while the rest simply eat a free meal using the strategy of the predator pack.

The intensive colonial tunicate Clavelina oblonga invasion was observed in the summer of 2020 at shellfish farms located on the western and eastern coasts of the Istrian peninsula (NE, Adriatic, Croatia). Aquaculture facilities create an ideal environment for ascidians, therefore, as a possible environmentally friendly eradication measure, laboratory experimental exposure of C. oblonga to different reduced seawater salinities of 37, 30, 20 and 11 was examined in November 2020. During the total observation time (14 days exposure + 14 days of recovery at 37) at all seawater salinities, there was no sexual reproduction of Clavelina specimens and larvae settlement in general. Salinities of 37 and 30 didn’t provoke any significant effect in colonial corpus weight change and morphology. Salinities of 20 and 11 caused disorder in specimens feed uptake and feeding itself which led to change of colour, tissue necrosis and finally body fragmentation. Similar separate laboratory exposure of mussel Mytilus galloprovincialis specimens did not provoke significant differences in meat yield between control 37 salinity (14.0 %) and reduced salinities of 30 (14.2 %), 20 (14.6 %) and 11 (15.3 %) measured at the end of the recovery experiment. Based on our experimental results of C. oblonga salinity susceptibility and field observations (2020–2021), we can assume that field translocation of overgrown mussels and infrastructures to locations of seawater

Silver nanoparticles represent a threat to biota and have been shown to cause harm through a number of mechanisms, using a wide range of bioassay endpoints. While nanoparticle concentration has been primarily considered, comparison of studies that have used differently sized nanoparticles indicate that nanoparticle diameter may be an important factor that impacts negative outcomes. In considering this, the aim of the present study was to determine if different sizes of silver nanoparticles (AgNPs; 10, 20, 40, 60 and 100 nm) give rise to similar effects during embryogenesis of Mediterranean sea urchins Arbacia lixula and Paracentrotus lividus, or if nanoparticle size is a parameter that can modulate embryotoxicity and spermiotoxicity in these species. Fertilised embryos were exposed to a range of AgNP concentrations (1–1000 µg L−1) and after 48 h larvae were scored. Embryos exposed to 1 and 10 µg L−1 AgNPs (for all tested sizes) showed no negative effect in both sea urchins. The smaller AgNPs (size 10 and 20 nm) caused a decrease in the percentage of normally developed A. lixula larvae at concentrations ≥50 µg L−1 (EC50: 49 and 75 μg L−1, respectively) and at ≥100 µg L−1 (EC50: 67 and 91 μg L−1, respectively) for P. lividus. AgNPs of 40 nm diameter was less harmful in both species ((EC50: 322 and 486 μg L−1, for P. lividus and A. lixula, respectively)). The largest AgNPs (60 and 100 nm) showed a dose-dependent response, with little effect at lower concentrations, while more than 50% of larvae were developmentally delayed at the highest tested concentrations of 500 and 1000 µg L−1 (EC50(100 nm); 662 and 529 μg L−1, for P. lividus and A. lixula, respectively. While AgNPs showed no effect on the fertilisation success of treated sperm, an increase in offspring developmental defects and arrested development was observed in A. lixula larvae for 10 nm AgNPs at concentrations ≥50 μg L−1, and for 20 and 40 nm AgNPs at concentrations >100 μg L−1. Overall, toxicity was mostly ascribed to more rapid oxidative dissolution of smaller nanoparticles, although, in cases, Ag+ ion concentrations alone could not explain high toxicity, indicating a nanoparticle-size effect.

The widespread occurrence of plastic has become a significant problem in the natural environment and may give rise to a range of deleterious impacts in biota, particularly where plastic particles overlap in size with that of the particles that are naturally ingested by filter-feeders. In this context, the effects of two particle sizes (10 μm and 50 μm) of polymethylmethacrylate (PMMA) on ecologically and commercially significant mussel Mytilus galloprovincialis have been investigated. Mussel health status was evaluated by measuring the condition index and stress-on-stress test during and after 72 h exposure to PMMA microparticles in the 0.1–10 mg L−1 concentration range. The decreased condition and fitness indices in the exposed mussels pointed to significant physiological effects at the entire organism level. The 10 μm and 50 μm PMMA particles were noted to rapidly increase the total haemocytes count in haemolymph, and significantly reduce cell viability at higher concentrations of both particle sizes. The results also indicated a significant increase in levels of vacuolised haemocytes as a result of PMMA exposure. While both microparticles were detected in the haemolymph, only the 10 μm PMMA was observed in the gill tissue and digestive gland by histological cryosections, indicating their rapid uptake, transport, and accumulation in tissue. Lack of accumulation of 50 μm microparticles in tissue may be related to a combination of chemical identity and size considerations, enabling more efficient depuration of microparticles in pseudofaeces. The PMMA particles did not induce significant changes in activity of a range of enzymes involved in neurotransmission and responses to oxidative stress.

As nanoparticles have been found to cause a range of harmful impacts in biota, understanding processes and transformations which may stabilize and increase their persistence time in the environment are of great importance. As nanoparticles carried in riverine or wastewaters will eventually reach estuaries, understanding their behavior and transport potential in this transition zone from fresh to marine waters is essential, particularly as estuaries are sensitive ecological zones, oftentimes encompassing ornithologically important areas. In this direction, we report on the influence of combined gradients of riverine and marine natural organic matter (NOM) on the temporal stability of biocorona-encapsulated silver nanoparticles in terms of ion release kinetics. In parallel, salinity, pH and oxygen saturation were simultaneously varied to create a model to mimic the complex estuarine environment. While humic acid (HA) and alginate (Alg) disrupted the stabilizing ability of the nanoparticle protein corona to a greater and lesser degree, respectively, they slowed the rate of ion release in freshwater at pH 6.6 and in saltwater at pH 8, respectively, while oxygen saturation was also found to be an important factor. Thus, as the type of NOM changes with pH along a salinity gradient in an estuary, conditions required to increase the persistence time of nanoparticles are serendipitously met, with greater colloidal stability achieved in cases where there is more rapid replacement of HA with Alg. Despite the strong gradients in ionic strength, pH and oxygen saturation, the protein corona was not sufficiently disrupted at the nanoparticle surface to be substituted by NOM indicating the greater adsorption energy of the protein’s hydrophobic domains. Ultimately, it is the specific NOM profile of individual estuaries that may provide the best indicator for predicting the stability and persistence of silver nanoparticles as they transition from fresh to salt water environments.