Genetic and morphological variation in Pennella sp. (Copepoda: Siphonostomatoida) collected from Pacific saury, Cololabis saira

In this paper, we describe three morphotypes of large parasitic copepods identified as Pennella Oken, 1815 (Siphonostomatoida, Pennellidae), which are found on the commercial fish Pacific saury Cololabis saira (Brevoort, 1856) (Beloniformes, Scomberesocidae). Of three morphotypes (Pennella sp. (PSP), parasite A (PA), and indeterminate individuals (IDI)), PSP had feather-like abdominal brushes and egg sacs which were characters typical of Pennella, PA lacked abdominal brushes and egg sacs, and IDI had few abdominal brushes but no egg sacs. Nucleotide sequence differences of mitochondrial cytochrome oxidase subunit I (COI) and nuclear ribosomal internal transcribed spacer 1 (ITS1) regions between types were small and comparable with differences within a type, indicating that the three types were conspecific. Mean body length of PSP was significantly larger than those of PA and IDI, suggesting types PA and IDI were immature morphs of PSP or individuals that had ceased development prior to maturation.