Does the number of genital organs matter? Case of the seal tapeworm Diphyllobothrium (syn. Diplogonoporus) tetrapterum (Cestoda: Diphyllobothriidea)

Abstract

The seal tapeworm Diphyllobothrium tetrapterum (von Siebold, 1848) Baer, 1932 (syn. Diplogonoporus tetrapterus) is exceptional among cestodes because it possesses two types of the strobila, one with a multiple set of genitalia per proglottid and another with a single set of reproductive organs per proglottid. In this study, Diph. tetrapterum is redescribed on the basis of extensive, well-fixed material from the northern fur seal (Callorhinus ursinus (Linnaeus, 1758)) from Alaska, USA. A critical morphological and molecular study of comprehensive material from several hosts throughout the Northern Hemisphere is provided. As a result, Diplogonoporus mutabilis Belopolskaia, 1960 and Diplogonoporus violettae Yurakhno, 1986 become junior synonyms of Diph. tetrapterum. Our study provides evidence of intraspecific and even individual variability of Diph. tetrapterum in the number of genital complexes, thus making this generic feature questionable for circumscription of the diphyllobothriid genera. The seal tapeworm has been found exclusively in the Northern Hemisphere and exhibits a wide (euryxenous) specificity at the level of the definitive host, having been found in a number of seals, the sea otter (Enhydra lutris (Linnaeus, 1758)), and exceptionally, in other terrestrial mammals. Plerocercoids of Diph. tetrapterum are reported from the second (fish) intermediate host for the first time, in this case the pink salmon (Oncorhynchus gorbuscha (Walbaum, 1792)) from Alaska.