A novel species of bothriocephalid cestode, <i>Clestobothrium brettiae</i> n. sp. in the European hake ( <i>Merluccius merluccius</i> ) revealed using an integrative taxonomic approach

<i>Clestobothrium</i> Lühe, 1899 is a genus of cestodes belonging to the order Bothriocephalidea, which infects marine fish from the Gadiformes order. Herein, a novel species of <i>Clestobothrium</i> is described from the intestine of the European hake <i>Merluccius merluccius</i> (Linnaeus, 1758) in the Ionian Sea (Eastern Mediterranean Sea), through an integrative taxonomic approach that combines morphological and molecular data. The new species, named <i>Clestobothrium brettiae</i> n. sp., can be distinguished from all congeners by its unique ovary shape, and a combination of characters including the arrangement and number of testes. It shares similar morphological characters, with <i>Clestobothrium crassiceps</i> (Rudolphi, 1819) Lühe, 1899, which overlaps in host and geographic distribution. However, morphological differences between <i>Clestobothrium brettiae</i> n. sp. and <i>C. crassiceps</i> also include the arrangement of gladiate spinitriches, as well as a larger scolex, proglottids, testes and cirrus sac, and different ovary morphology. The establishment of <i>Clestobothrium brettiae</i> n. sp. as a novel taxon is supported by detailed morphological description and biometric statistics, in addition to molecular characterisation (based on partial small subunit ribosomal ribonucleic acid [rRNA], partial large subunit rRNA, and internal transcribed spacer region 2), genetic distance, and phylogenetic analyses.