Sandonella sandoni (Lynsdale, 1960), an Enigmatic and Morphologically Unique Cestode Parasitic in the Osteoglossiform Fish Heterotis niloticus in Africa

Sandonella sandoni (Lynsdale, 1960) is the type and only species of the Sandonellinae, a cestode subfamily of unclear phylogenetic position. It is redescribed here on the basis of a re-examination of its syntypes, voucher specimens from museum collections, and freshly collected material from the intestine of Heterotis niloticus (Osteoglossiformes: Arapaimidae) from Benin, Nigeria, Senegal, and the Sudan. The species possesses several unique morphological characters, such as (1) a vitellarium formed by 2 compact, but deeply lobulated, postovarian masses near the posterior margin of proglottids; (2) a scolex with a highly modified apical structure formed by 4 muscular retractile lappets; (3) a well-developed circular musculature, which is external to the inner longitudinal muscles; (4) a dilated, vesicle-like proximal part of the external sperm duct; (5) the unique morphology of the uterus and its development, which represents an intermediate form between the 2 basic types recognized in the Proteocephalidea; (6) the growth of eggs during their development within the uterus; and (7) the complex proglottization with intermingled smaller and larger (wider) proglottids. The morphology of S. sandoni, including the form and distribution of microtriches, was studied by scanning electron microscopy for the first time, and the lectotype and paralectotypes of S. sandoni are designated. Sequences of the 28S rRNA gene of 4 specimens (2 from the Sudan and 2 from Senegal) were identical, which confirms conspecificity of geographically distant samples. Sandonella sandoni sequences have also shown that it actually belongs among the Proteocephalidea, being a sister taxon of a relatively derived clade of Palaearctic proteocephalideans, containing Glanitaenia osculata and Paraproteocephalus parasiluri from catfish and Palaearctic species of the Proteocephalus aggregate.