Sea louse species infecting Sparus aurata farmed in Croatia: a new threat for Mediterranean aquaculture?

Caligids of the genus Caligus (Copepoda, Caligidae) are marine ectoparasites of considerable concern for finfish aquaculture due to their capacity to compromise fish health and productivity. These copepods, characterized by a dorsoventrally flattened body and a life cycle with both free-living and parasitic stages, feed on mucus, skin and blood, causing lesions, stress, and immunosuppression that may predispose fish to secondary infections. In the Mediterranean mariculture context, where European seabass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata) dominate production, Caligus minimus has been consistently reported as a relevant ectoparasite of seabass, with potential economic impact under intensive farming conditions. Conversely, caligid infestations in seabream appear sporadic and are often poorly characterized, highlighting a gap in knowledge on host specificity and epidemiology. Research was conducted at a fish farm located in eastern Adriatic Sea, farming S. aurata and D. labrax. In this farm, mortalities in gilthead seabream are usually associated with the gill fluke Sparicotyle chrysophrii. In May 2025, five cages of one-year-old gilthead seabream (50–140 g) experienced an unusual increase in daily mortality above the levels normally caused by S. chrysophrii infections. Affected fish exhibited reduced feeding activity and abnormal swimming behavior. This prompted a targeted parasitological investigation. A random sample of 10 fish was collected from one affected cage. Parasitological examination included macroscopic inspection of the skin and the first two gill arches on the right side of the fish. Skin samples with attached parasites were fixed in 10% neutral buffered formalin for histology. Sections were stained with Hematoxylin and Eosin. Copepods collected from the skin and gills were examined under a stereomicroscope for morphological identification. Morphometric observations were made under a light microscope and by Scanning Electron Microscopy. A subset of specimens was subjected to molecular analysis for species identification. In all the fish, numerous mobile copepods were observed on the skin. Morphological analysis identified the parasites at genus level as belonging to a Caligus species. Female specimens (n=13) and male specimens (n=7) were measured for the total length (excluding caudal rami) the cephalothoracic shield (length and width, excluding extensive marginal membrane), the genital complex (length and width) and the abdomen (length and width). No match in morphometry was found within the species of genus Caligus already described in the Mediterranean Sea. Histopathology of the skin reveals, around the attachment site of the parasite, erosion of the epidermis and, in the hypodermis, the presence of inflammatory infiltrate with lymphocytes and macrophages. In some sections presence of bacterial aggregates together with necrosis and mild hemorrhages were also observed. The images obtained with the Scanning Electron Microscope allowed to observe further taxonomic characters useful for future description of the specimens under study. Morphological features and molecular studies, are currently underway to assess the possible presence of an undescribed parasite in Mediterranean aquaculture.