First Molecular Characteristics and Morphology Updates of <em>Priapocephalus eschrichtii</em> (Cestoda: Tetrabothriidae) from a Gray Whale (<em>Eschrichtius robustus</em>)

Parasites of marine mammals are of significant interest in the fields of evolutionary biology, veterinary medicine, human health, conservation, and ecology. Therefore, every opportunity to study them should be seized. This study aimed to report on specimens of cestodes found in the small intestine of a gray whale (Eschrichtius robustus) in Chukotka, Russia, in 2024. The specimens were examined using light and scanning electron microscopy, histology, and molecular analysis. Based on their morphology and host specificity, the cestodes were identified as Priapocephalus eschrichtii. Analyses of the 18S and 28S ribosomal RNA genes confirmed a close relationship between P. eschrichtii and species of Tetrabothrius, which infect seabirds and whales. Sequences of the 18S, 28S, ITS2 rDNA, and gene CoxI mtDNA are reported here for the first time. More than 50 specimens of P. eschrichtii were detected in 1 meter of intestine of the whale with a maximum length of 50 cm. Chronic lymphoplasmacytic inflammation of the intestinal mucosa was observed in the infected whale. The eggs of the cestodes were clustered within an irregular membrane, which may contribute to multiple infestations of the host and lead to diseases that affect the whale’s health and survival. This study should be continued.