Thelohanellus species in common carp (Cyprinus carpio) and their life cycles from fish farms and Lake Balaton in Hungary

Eighteen Thelohanellus species have been reported from the common carp, Cyprinus carpio and its subspecies C. carpio haematopterus in the Far-Eastern countries such as Russia, China, Japan, Vietnam and Korea. In contrast, only T. nikolskii , T. hovorkai and actinospore of T. kitauei have been documented in Hungary to date. During a comprehensive parasitological survey on common carp, five Thelohanellus species and three actinospore types were identified from fish farms and Lake Balaton. Out of them, three new Thelohanellus species ( T. serosae n. sp., T. borkhanuddini n. sp., and T. paranikolskii n. sp.) were described by morphological, morphometric, and molecular analyses. The life cycles of T. borkhanuddini n. sp. and T. serosae n. sp. were inferred, with T. borkhanuddini n. sp. confirmed through both molecular data and experimental infection. Notably, the actinospore stage of T. borkhanuddini n. sp. was identified in this study, confirming that T. borkhanuddini n. sp. is associated with four distinct neoactinomyxum morphotypes. These differed primarily in the length and width of their caudal processes and were released from two oligochaete hosts: Branchiura sowerbyi and Isochaetides michaelseni . Additionally, two known species, T. hovorkai and T. nikolskii , were recorded. Thelohanellus hovorkai was found to be associated with three distinct aurantiactinomyxon morphotypes, two of which were identified in this study. These morphotypes, which varied notably in caudal process and polar capsule dimensions, were all collected from different B. sowerbyi individuals. Additionally, a novel species, T. paranikolskii n. sp., was described, resembling T. nikolskii in both morphology and 18S rDNA sequences. • Five Thelohanellus species from common carp are identified, including 3 novel species, from fish farms and Lake Balaton. • Three actinospore types are characterized, one of them is newly identified. • Three complete life cycles are elucidated: two newly described and one previously known. • The life cycle of T. borkhanuddini n. sp. is experimentally confirmed through infection trials and molecular analysis.