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In recent years, a drop in the condition of the European sardine has been observed. Although several causes have been attributed to this issue, as overfishing and climate change, little is known about the link between ascaridoid nematode parasitisation and fish status. In this study, sardines were obtained from four fishing grounds along the Mediterranean (Alboran, Northern Spain, Northern Adriatic, and Aegean), and one location in the Atlantic Ocean (Southern Portugal). After analysing individual fish body condition (by direct tissue fat content measurements and condition indices), and reproductive status (by a detailed gonadal examination) throughout the entire annual cycle, ascaridoids were recognised by combining naked eye and UV-press method along flesh, viscera, and gonads. Afterwards, sequence analysis of the rDNA internal transcribed spacers region (ITS) and the mtDNA cox2 gene were used to identify and characterise the different species of ascaridoids from the fish host in the localities throughout the seasons. The main species found along different areas was Hysterothylacium aduncum , present in the Northern Adriatic (prevalence of 7.6%, mean intensity 1.700), the Atlantic (7.5%, 3.889), and the Northern Spain (3.9%, 1.600). Moreover, few individuals of Anisakis simplex (s.s.) and A. pegreffii were observed in the Atlantic (1.7% and 0.8%, respectively), and the latter species was also found in the Adriatic stock (0.8%). All ascaridoid specimens were found in viscera. Obtained results seem to indicate that in stocks with medium sizes, small variations in length are related to parasite intensity. This study highlights the importance of seasonal parasitological analyses at stock level and, especially, in capital breeders, as relationships between condition and reproduction parameters and parasitism are conditioned by seasonality., Competing Interests: The authors declare no conflict of interest/competing interest., (© 2022 The Authors.)

The Ross Sea, Eastern Antarctica, is considered a "pristine ecosystem" and a biodiversity "hotspot" scarcely impacted by humans. The sibling species Contracaecum osculatum sp. D and C. osculatum sp. E are anisakid parasites embedded in the natural Antarctic marine ecosystem. Aims of this study were to: identify the larvae of C. osculatum (s.l.) recovered in fish hosts during the XXVII Italian Expedition to Antarctica (2011-2012); perform a comparative analysis of the contemporary parasitic load and genetic variability estimates of C. osculatum sp. D and C. osculatum sp. E with respect to samples collected during the expedition of 1993-1994; to provide ecological data on these parasites. 200 fish specimens (Chionodraco hamatus, Trematomus bernacchii, Trematomus hansoni, Trematomus newnesi) were analysed for Contracaecum sp. larvae, identified at species level by allozyme diagnostic markers and sequences analysis of the mtDNA cox2 gene. Statistically significant differences were found between the occurrence of C. osculatum sp. D and C. osculatum sp. E in different fish species. C. osculatum sp. E was more prevalent in T. bernacchii; while, a higher percentage of C. osculatum sp. D occurred in Ch. hamatus and T. hansoni. The two species also showed differences in the host infection site: C. osculatum sp. D showed higher percentage of infection in the fish liver. High genetic variability values at both nuclear and mitochondrial level were found in the two species in both sampling periods. The parasitic infection levels by C. osculatum sp. D and sp. E and their estimates of genetic variability showed no statistically significant variation over a temporal scale (2012 versus 1994). This suggests that the low habitat disturbance of the Antarctic region permits the maintenance of stable ecosystem trophic webs, which contributes to the maintenance of a large populations of anisakid nematodes with high genetic variability.