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Four ovigerous females of the parasitic copepod Pseudosarcotretes omorii gen. et sp. nov. (Siphonostomatoida: Pennellidae) were found on North Pacific lightfish Maurolicus japonicus Ishikawa, 1915, collected from Suruga Bay, eastern Japan. Pseudosarcotretes gen. nov. shares some features with pennellids parasitic on mesopelagic fish such as Cardiodectes Wilson, 1917, Protosarcotretes Ohtsuka, Lindsay, and Izawa, 2018, and Sarcotretes Jungersen, 1911. The morphological differences between the new genus and other related genera are having a pair of lateral cephalothoracic holdfast, the presence of distinct caudal rami, and the absence of leg 4. The hatching stage of the new genus is the copepodid. A molecular phylogenetic analysis suggested that the reduction in the number of developmental stages in pennellid life cycles had occurred at least in two cases. According to the molecular data, the new genus consists of a clade accommodating Cardiodectes, Lernaeenicus Lesueur, 1824 and Pennella Oken, 1815. All these genera have a distinct cephalothoracic holdfast embedded into the host's muscle tissues, which seems to be advantageous for expanding the range of the hosts to small mesopelagic fish.
Yumura et al. (Wed,) studied this question.
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