Diagnostic strategies for Sparicotyle chrysophrii detection based on size-variability and site-location

Routine monitoring of monogenean loads is a crucial step for infection management in aquaculture facilities. This analysis is usually performed from partial gill analyses and thus, knowledge of parasite distribution and size can be helpful to optimise diagnosis, estimate infection levels and improve management procedures. Gill location preferences and intraspecific size variations relative to parasite age, host factors and parasite abundance are herein determined for the monogenean Sparicotyle chrysophrii, one of the most challenging pathogens in gilthead sea bream aquaculture. Present findings on parasite location show that the life-history stages of S. chrysophrii have distinct location preferences. Analysis of intraspecific size variations of adult stages of S. chrysophrii revealed that the size of most morphological features of S. chrysophrii increased significantly with parasite age and host size. Positive and significant relationships between the size of the posteriormost clamp and host size pointed out the adaptation of the parasite to the gilthead sea bream size from the initial settlement. Host size also seems to restrict the development of parasites after maturity as the oldest specimens were exclusively found in mid-large fish (>18 cm). By contrast, no clear patterns were observed for the relationship between parasite size and location or abundance. Post-maturity size variations of S. chrysophrii should be considered for parasite identification and the clamp pair number should be fixed for accurate taxonomic diagnosis. Based on the site location of S. chrysophrii, the dorsal sections or the outermost holobranches would be suitable for adult detection while the analysis of the central and dorsal sections of the outermost holobranches may be a better option for finding most of the adult, juvenile or post-larval stages.