Tissue-level biomarkers and histopathological alterations in mussels (Mytilus trossulus) from the Baltic Sea exposed to Oil WAF and WAF-D in extreme brackish conditions
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The brackish Baltic Sea has been identified as a fragile ecosystem potentially sensitive area to Oil Spills. Chemical dispersants are an effective method to mitigate coastal impacts of oil spills; however, these dispersants could pose potential unknown toxic effects onto brackish ecosystems. Particularly, Baltic blue mussels (Mytilus trossulus) are a particular variety of marine mussels acclimatized to low salinities of the Baltic. Early winter mussels were collected scuba diving from Tvarminne area of Baltic Sea, Finland in November 2016, taken to laboratory facilities (SYKE) and acclimatized to two different salinity regimes: native 5.6 psu and artificially increased salinity 15 psu (Outer Baltic) at 5ºC. Experimental mussels where exposed to consensus-produced Water Accommodated Fractions (WAF) and dispersed WAF (Finasol OSR 51) mixtures (WAF-D) and sampled at 0, 1, 7 and 21days. To understand biological response of mussels, several biomarkers were investigated.

Tissue level biomarkers, including cell type composition (volume density of basophilic cells, VvBAS) in digestive gland epithelium, structural changes of digestive alveoli (mean luminal radius to mean epithelial thickness, MLR/MET), mean epithelial thickness to mean diverticular radius (MET/MDR), lumen to epithelial thickness (LTE), connective to diverticula ratio (CTD), gonadal development and other histopathological alterations of the digestive gland, gonad and gill were analyzed. VVBAS significantly increased after 1 d in mussels exposed to WAF and WAF-D at 15 psu, and decreased afterwards. Digestive alveoli structural parameters were not significantly affected after exposure to WAF and WAF-D. MLR/MET significantly changed with exposure time at 15 psu whereas MET/MDR showed no response. The LTE RATIO behaved in a similar manner to MLR/MET. Higher CTD values obtained in mussels maintained at 5.6 psu, possibly indicating poorer morphofuctional condition of digestive gland in mussels acclimated to the lower salinity. Gametogenic development progressed in both salinities in the experiment. Pathological responses e.g. atrophy, necrosis, vacuolization, haemocytic infiltration, brown cell aggregate, granulocytomas, decrease in adipogranular cell etc. were observed in digestive alveoli, gill and mantle. Despite large inter-individual variation, alterations were more evident and of greater intensity in those mussels exposed to WAF and WAF+D evidencing early stages of affection at 21d. Likewise, parasites e.g. trematode, Rickettsia /Clamydia-like Organisms (R/CLO), intracellular ciliates, Ancistrum sp., sporocysts, Mytilicola intestinalis etc were commonly observed; being prevalence low. It can be concluded that mussels acclimatized to increased salinity i.e. 15 psu are likely dwelling in more physiologically favorable conditions compared to those subject to the somehow challenging conditions of native salinity in the Gulf of Finland to respond in the event of a winter oil spill. The study herein is the first approach to understand histological response of Baltic mussels in native brackish salinities and cold temperatures, thus, further research is envisaged comprising cold-water specific biological responses in the particular environmental conditions of the Baltic.