Although the irritant effects of quaternium-15 have been established, little is known about the toxicological consequences induced by this xenobiotic on aquatic invertebrates. The present article reports toxicological, histological and physiological effects of quaternium-15 following the exposure of Mytilus galloprovincialis for 18 days at three different concentrations (0.1, 1.0 and 2.0 mg/L). The results demonstrate that at higher concentrations histological damages to M. galloprovincialis gills occur, like melanosis, light exfoliations, increase of mucus production and infiltrative inflammation. In addition digestive gland cells of M. galloprovincialis, were not able to perform the regulation volume decrease (RVD) owing to osmotic stress following the exposure to the preservative. Overall, this first study on quaternium-15 highlights that it can jeopardize both the morphology and vital physiological processes in marine invertebrates, depending on the duration of exposure and the concentration of the preservative, indicating that further studies are necessary to increase our knowledge about the effects of this substance, commonly added to our products of daily use.

When it comes to predation, alarm signals enable individuals to assess risks and modulate their behavior accordingly. These signals, often chemical in aquatic environments, can be recognized across species boundaries and are typically released through injury-induced mechanisms in response to predation. While extensively documented in teleosts, particularly those possessing epidermal club cells, such mechanisms remain poorly understood in more basal vertebrates, such as lampreys, which possess unique epidermal structures called skein cells. The present study investigated the potential role of skein cells in the immune and alarm signaling systems of the sea lamprey (Petromyzon marinus), given their strategic location in the skin and distinctive ultrastructural characteristics, such as polarized nuclei and tonofilament-rich basal cytoplasm. Although originally misidentified as club cells, skein cells may be involved in mucus secretion and the release of compounds such as sialic acid and glycoconjugates, which provide defense against aquatic pathogens. This study employed histomorphological analysis, immunoperoxidase labeling, confocal microscopy, bioinformatics, and quantitative and statistical analysis to investigate the hypothesis that skein cells contribute to anti-predator defense via the release of alarm substances. These findings provide new insights into the evolutionary origins and functional diversity of chemical signaling in the early vertebrate.