Mediterranean-wide transfer of the polyopisthocotylean Sparicotyle chrysophrii between wild sparids and farmed gilthead seabream (Sparus aurata) inferred by ddRAD loci

The polyopisthocotylean Sparicotyle chrysophrii is considered one of the most harmful pathogens in Mediterranean farmed gilthead seabream (Sparus aurata). For the first time recorded in aquaculture in the mid-eighties, it has rapidly spread through facilities in the whole Mediterranean basin, causing mortalities due the anemia, the mechanical damage of the host's gill tissues and the consequent secondary bacterial infections. Even though one of the recognized risk factors for spreading of sparicotylosis is the transfer of the parasite from wild fish, there is a need to infer the extent of this event over a wider geographic area, as well as using more informative multilocus genotyping, such as typing based on the discovery of SNPs. Therefore, we sampled a total of 330 S. chryspohrii specimens from Italian, Croatian, and Greek wild and farmed gilthead seabream, as well as from Spanish farmed fish. After a double digest restriction-site associated DNA sequencing (ddRADseq), a total 173 samples were used to infer the genetic structure of the parasite and draw conclusions on its migratory patterns in the Mediterranean. Our results confirm a panmictic distribution of the polyopisthocotylean in the Mediterranean basin, with no clear genetic structure. However, three discrete genetic pools or clusters, probably driven by geographic constraints, have been observed within the parasite populations of origins. We speculate that the dispersion of polyopisthocotylean eggs via the circulation of dominant currents and fronts, as well as anthropogenic influence, facilitates the gene flow within the genetic clusters. In addition, gene flow between polyopisthocotyleans parasitizing wild and farmed gilthead seabream is evident, but other wild sparids (i.e., the auxiliary and annular seabream, blotched pickerel, bogue, and common pandora) likely play a role in the event. This indicates that S. chrysophrii should be considered a family-specific rather than a host-specific parasite.