A fresh look at Clestobothrium crassiceps (Rudolphi 1819) Lühe, 1899 (Cestoda: Bothriocephalidea) two centuries later: first complete description and novel observations from its type-host in the NW Mediterranean Sea.

Clestobothrium crassiceps (Rudolphi, 1819) Lühe, 1899 is the type species of the genus Clestobothrium Lühe, 1899, originally described from Merluccius merluccius (Linnaeus, 1758) in the Mediterranean Sea. Despite its apparently wide host and geographic ranges, this species remains poorly understood due to the lack of detailed morphological, genetic and epidemiological data. In the present study, newly collected materials of C. crassiceps from M. merluccius off Barcelona, Spain (NW Mediterranean) were used to provide the first complete description from its type-host and locality. An integrative approach was applied, combining traditional morphological techniques with confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), molecular, histological and epidemiological data. Confocal microscopy revealed key diagnostic features of the female reproductive system (i.e. vagina, Mehli's gland, uterus tube), while SEM analysis showed, for the first time, different distribution patterns of microtriches along the scolex and strobila. Histological observations showed the attachment mode of the scolex to intestinal folds, causing mild epithelial alterations such as attenuation of the intestinal epithelium. Prevalence and intensity of infestation with C. crassiceps were higher in larger fish, suggesting a role for dietary shifts and potential paratenic hosts in transmission. Phylogenetic analysis based on newly generated 28S and cox1 sequences confirmed the monophyly of the genus Clestobothrium and highlighted an intraspecific variation comparable to the genetic divergence observed between congeners C. splendidum and C. cristinae. Based on these results, a morphological reexamination of paratypes of both species was conducted, proposing C. cristinae as a junior synonym of C. splendidum.