We tested the effect of silver nanoparticles (AgNPs) on the ability of the pond snail, Lymnaea stagnalis, to learn and form long-term memory (LTM) following operant conditioning of aerial respiration. We hypothesized that the AgNPs would act as a stressor and prevent learning and LTM formation. We tested snails exposed for either 72 h or only during training and testing for memory (i.e. 0.5 h) and found no difference between those treatments. We found that at a low concentration of AgNPs (5 μg/L) neither learning and nor memory formation were altered. When we increased the concentration of AgNPs (10 μg/L) we found that memory formation was enhanced. Finally, at a higher concentration (50 μg/L) memory formation was blocked. To determine if the disassociation of Ag+ from the AgNPs caused the effects on memory we performed similar experiments with AgNO3 and found similar concentration-dependent results. Finally, we found that snails perceive the AgNPs differently from Ag+ as there was context specific memory. That is, snails trained in AgNPs did not show memory when tested in Ag+ and vice-versa. We believe that changes in memory formation may be a more sensitive determination of AgNPs on aquatic organisms than the determination of a LC50.

Marine coastal ecosystems are affected by a vast array of human-induced disturbances and stresses, which are often capable of overwhelming the effects of natural changes. Despite the conceptual and practical difficulty in differentiating between disturbance and stress, which are often used interchangeably, the two terms bear different ecological meanings. Both are external agents, but the former causes mortality or physical damage (subtraction of biomass), whereas the latter causes physiological alteration (reduction in productivity). Sensitivity of marine organisms may thus have a dual connotation, being influenced in different ways by disturbance and by stress following major environmental change. Coralligenous assemblages, which shape unique biogenic formations in the Mediterranean Sea, are considered highly sensitive to change. In this paper, we propose a method to differentiate between disturbance and stress to assess the ecological status of the coralligenous assemblages. Disturbance sensitivity level (DSL) and stress sensitivity level (SSL) of the sessile organisms thriving in the coralligenous assemblages were combined into the integrated sensitivity level of coralligenous assemblages (ISLA) index. Changes in the coralligenous status were assessed in space, along a gradient of stress (human-induced pressures) at several sites of the western Mediterranean, and in time, from a long-term series (1961–2008) at Mesco Reef (Ligurian Sea) that encompasses a mass mortality event in the 1990s. The quality of the coralligenous assemblages was lower in highly urbanised sites than that in sites in both marine protected areas and areas with low levels of urbanisation; moreover, the quality of the assemblages at Mesco Reef decreased during the last 50years. Reduction in quality was mainly due to the increase in stress-tolerant and/or opportunist species (e.g. algal turfs, hydroids and encrusting sponges), the disappearance of the most sensitive macroalgae (e.g. Udoteaceae and erect Rhodophyta) and macro-invertebrates (e.g. Savalia savaglia, Alcyonium coralloides and Smittina cervicornis), and the appearance of invasive alien algal species. Although the specific indices of SSL or DSL well illustrated the changes in the spatial or temporal datasets, respectively, their integration in the ISLA index was more effective in measuring the change experienced by the coralligenous assemblages in both space and time.