Melanomacrophage centers and mucus cells of shorthorn sculpin (Myoxocephalus scorpius) as biomarkers of contaminants

Marine pollution has been a global issue as most marine ecosystems are affected by anthropogenic pollutants. Some common pollutants such as persistent organic pollutants (POPs) and heavy metals have become a widespread concern to environment and public health due to their toxicity, persistence and bioaccumulation. Environmental monitoring programs need to detect potential harmful effects of these pollutants before they affect high levels of biological organization such as population or ecosystem. This can be done using biomarkers in appropriate indicator species. Shorthorn sculpin (Myoxocephalus scorpius) has been used in this project as an indicator species for environmental studies in Greenland because the fish is abundant, easy to sample, benthic, sedentary and long-lived. Although sculpins have been used extensively in environmental monitoring programs in Greenland, most studies focused on increased concentrations of trace elements in various organs and some histopathology in fishes caught from polluted areas. Most significant histological alterations, in particular, melanomacrophage centers (MMCs) or mucus cells (MCs) were evaluated using qualitative or semi-quantitative methods. The overall aims of this thesis were to study MMCs and MCs in shorthorn sculpins and evaluate their biomarker potential for pollutant exposure. These were achieved by pursuing the following specific aims: a) Investigate MMCs in various organs of shorthorn sculpins and assess the biomarker potential of the splenic and renal MMC populations, and b) Investigate the MCs in the gills and skin of shorthorn sculpins using different quantification methods and examined their biomarker potential for pollutant exposure. This study characterised MMCs in all main organs including the gills, liver, spleen, kidney and pancreas of shorthorn sculpins. The variations in morphology, density, size, area, distribution, pigments and response to pathogens of the MMC populations in these organs were described and can be used as baselines for future monitoring studies. The biomarker potential of splenic and renal MMCs, the two most numerous and largest MMC populations in shorthorn sculpins, was evaluated. Splenic MMCs may have a biomarker potential for POPs chronic exposure. This was, however, not the case for renal MMCs. Effects of body size on the splenic and renal MMCs responses were confirmed and this factor needs to be considered if MMCs are used as biomarkers. This study developed a novel method to reconstruct 3D structure of splenic MMCs from histological samples. This method can be applied to reconstruct any structures or organs of interest from histological samples. The 3D structure of splenic MMCs was reported for the first time and showed spherical shape and limited variation in sizes. These characteristics were critical to develop a proper quantification for MMCs. An investigation into 3D splenic MMCs revealed that 97% of examined splenic MMCs were closely associated with blood vessels in the spleen of shorthorn sculpins. This study also indicated the presence of pheomelanin in MMCs of a fish species for the first time. Responses of MCs in the gills and skin of shorthorn sculpin were investigated using 16 mucosal indices generated by both mucosal mapping and histological methods. Three mucosal indices including size of skin MCs, density of lamellar MCs (mucosal mapping) and number of MCs per interlamellar unit (ILU, traditional methods) had biomarker potential for heavy metal exposure. Size of skin MCs (mucosal mapping) were significantly smallest in the least polluted station whereas the gill filament MCs were largest and most dense in the fishes from the most contaminated site. Density of the gill lamellar MCs followed a toxicity gradient and was highest at the most polluted site and lowest at the least polluted site. A strong positive correlation between hepatic Pb level and size of filament MCs, suggested the role of filament MCs in reducing the somatic Pb in sculpins. The biomarker potential of MC responses was also uncovered using traditional quantification as number of MCs per ILU was significantly different in fishes from sites with different levels of pollutants. This study investigated MC responses using two different quantification methods. Overall, both traditional methods and mucosal mapping were able to detect the biomarker potential of MC responses to pollutants and their links to parasitic infection. MCs quantified by traditional methods were related to body size of fishes whereas MCs quantified by mucosal mapping were linked to levels of Pb in the liver of fishes. Both methods detected the differences between the skin and gill MCs and only mucosal mapping distinguished the differences between filaments and lamellar MCs. Results of this study were valuable for environmental monitoring programs in the Northern Hemisphere where shorthorn sculpins are naturally distributed.