Redescription of Sciadocephalus megalodiscus Diesing, 1850, An Unusual Neotropical Fish Tapeworm (Cestoda: Proteocephalidae).

The tapeworm Sciadocephalus megalodiscus Diesing, 1850 (Cestoda: Proteocephalidae), is redescribed based on newly collected specimens parasitizing tucunare peacock bass, Cichla monoculus Agassiz, 1831 (Perciformes: Cichlidae), in the Peruvian Amazon. Even though this cestode was redescribed 2 decades ago, that redescription did not report some of the unique features of this species. The most unusual characteristics of the species are (1) peculiar formation of the uterus, with fast formation of numerous, tightly packed diverticula protruding ventrally and dorsally, with simultaneous disintegration of the ovary and vitelline follicles in the first pregravid proglottids; (2) inverted umbrella-shaped scolex with a well-developed apical sucker; (3) a large-sized, follicular (grape cluster-like) ovary, which occupies most of the central (median) third of proglottids, with the ovarian isthmus situated almost equatorially; (4) regular alternation of genital pores; (5) a well-developed internal seminal vesicle; and (6) a small-sized strobila (shorter than 6 mm) consisting of few proglottids (15-20). Preliminary molecular data reveal S. megalodiscus to be most closely related to Cichlidocestus gillesi, the type species of the recently erected Cichlidocestus de Chambrier, Pinacho-Pinacho, Hernández-Orts, and Scholz, 2017. Species of both genera parasitize Neotropical cichlids and are unique among all proteocephalids in the shape and position of the ovary. They also share other morphological characteristics unusual among other proteocephalids, and thus these 2 genera can be considered good candidates to be placed in a new, higher-level taxon such as separate subfamily or even family when a new, more natural classification of the Proteocephalidae is proposed.