Nodular Lesions in the Pyloric Caeca of <i>Sardinella aurita</i> (Teleostei) Associated With the Cestode <i>Anthobothrium</i> sp. 1 Larvae (Platyhelminthes: Eucestoda) Off Tunisia, Mediterranean Sea

Examination of round sardinella Sardinella aurita Valenciennes, 1847 (Clupeidae) from the Gulf of Tunis (Tunisia) revealed the first local and Mediterranean record of Anthobothrium sp. 1 sensu Jensen & Bullard, 2010 (Tetraphyllidea incertae sedis, Anthobothrium). Nodular lesions containing multiple larvae were detected at the proximal end of the pyloric caeca, and free plerocercoids were observed in the intestinal lumen and stomach. The newly generated LSU rDNA sequences shared 100% nucleotide identity with adult Anthobothrium sp. 1B from the blacktip shark Carcharhinus limbatus Valenciennes, 1839 and larvae from the largehead hairtail Trichiurus lepturus L., both collected from the Gulf of Mexico (Florida and Mississippi, USA). Furthermore, they exhibited a minor 2.1% sequence divergence from related lineages within the same complex found in the Atlantic sharpnose shark Rhizoprionodon terranovae Richardson, 1836 and the hardhead sea catfish Ariopsis felis (Linnaeus, 1766) from the same Gulf of Mexico geolocations. COI sequences showed low variability, with p-distances ranging from 0.18% to 0.92% (corresponding to 1-5 nucleotide differences across 544 base pairs), consistent with the presence of a single species. Scanning electron microscopy of the cestode larva revealed aristate gladiate spinitriches and capilliform filitriches within the acetabula, and capilliform filitriches on the scolex peduncle. Histopathological analysis of nodular lesions showed clusters of multiple larvae, predominantly located within the caecal lumen. Host tissues exhibited localised mucosal alterations, characterised by elements of mucosal atrophy, villus compression and epithelial degeneration. These changes were primarily the result of mechanical pressure exerted by the larvae within the caecal lumen. A focal accumulation of mucous cells (presumptive goblet cells) was observed in affected areas. While primary structural alterations were confined to the mucosa, inflammatory infiltration within the submucosa appeared minimal. Despite this typical localised tissue response, S. aurita appears to be a regular second intermediate host, whereas other clupeids may also act as second intermediate hosts and clupeid-feeding teleosts as paratenic hosts. This finding extends the distribution of Anthobothrium sp. 1 from the western Atlantic to the western Mediterranean Sea, where the definitive host remains unknown.