Feeding strategy of Pleuragramma antarctica investigated by using ecological and ecomorphological approaches

Antarctic silverfish Pleuragramma antarctica is a keystone species of the coastal ecosystems of the Sothern Ocean. This species is the most abundant pelagic fish in the coastal waters of Antarctica, where it plays a pivotal role in the trophic web as the major link between lower and higher trophic levels. Due to the ecological importance of this species, over the last decades many studies on biology and ecology of P. antarctica were carried out. Despite previous studies have provided an overall picture of the trophic ecology of the Antarctic silverfish, important questions remain. Among them, the opportunistic versus selective feeding habit of the species wasn't still investigated, even if this information holds potential to highlight mechanisms and dynamics of the Antarctic silverfish relationship with the other components of its environment. The investigations on the capability of the species at using food resources can give information about its adaptability to ecological changes. Such a piece of information gains further value if contextualized in the frame of the environmental changes ongoing at a rapid pace in polar regions. The aim of my work is to improve present knowledge on the feeding ecology of the Antarctic silverfish highlighting its feeding habits to assess its possible ability to adapt to variations in the availability of prey caused by climate change or overfishing of krill. The research developed within my PhD program contributes to the PNRA (Italian National Program for Research in Antarctica) project RAISE (Integrate Research on Antarctic Silverfish Ecology in the Ross Sea). The study was divided in two parts, the first one was a purely ecological analysis based on the stomach contents of samples collected in different sectors of the Southern Ocean, the second one was an ecomorphological focus on traits involved in feeding activity of the species. In the ecological analysis, I detected evident differences in the diet of the specimens coming from three different zones (Ross Sea, Dumont d’Urville Sea, sea near Antarctic Peninsula). The diets were composed exclusively by planktonic organisms, which belong to different taxa depending on the area, mainly composed by copepods and euphausiids. Depending on the case, the range of prey items found was significantly broader or narrower. The observation of the condition of the different schools of fish and the investigation of selectivity versus generalism of the species let we deduce what item of the diet represented the preferential prey (through my results they appear to be the euphausiids) and if the species was able to adapt to situations of availability of prey different from the optimal ones. The ecomorphological study consisted in a depth study of the biomechanics of the buccal apparatus of P. antarctica. I measured some features of jaws, head and gill rakers and calculated significant index of jaws movements. The comparison of the values obtained with that of the same parameters found in other two species of the same family of nototheniids, Dissostichus mawsoni and Trematomus bernacchii, which live in different habitats and have different feeding habits, highlighted the adaptation to planktofagy and the feeding strategies of P. antarctica. It was relevant the confirmation of the capability of the species to feed on different planktonic items, having both structures to catch relatively large size zooplanktonic organisms and gill rakers conformation to particulate feeding. Both studies carried out lead us to the conclusion that P. antarctica is a plastic predator. Its preferential prey seem to be the euphausiids, towards which it performs a selective predation. In the case of paucity or absence of these organisms, it is able to vary its diet and feed on smaller zooplanktonic species, preyed by means of a less selective particulate feeding activity.