Pseudophyllidean Cestodes from Alaskan Pinnipeds

Some thirty-three species of pseudophyllidean cestodes have been described from seals and sea-lions but the descriptions are so incomplete and imperfect that it is virtually impossible to recognize any of them with certainty. Redescriptions of species, on the basis of incorrectly determined specimens, have undoubtedly added to the confusion. In the absence of information concerning life cycles, consideration must be given to the possibility that representatives of a single species may complete their development in different host species and furthermore that, as a result of development in different hosts, individuals of the same species may manifest differences in size and shape, in rate of growth and sexual maturity, and in extent of development of various tissues and organs. Consequently, specific determination of specimens presents a particularly difficult problem. The pseudophyllidean cestodes of seals and sea-lions have been assigned to nine different genera, but the generic concepts are so indefinite and their limits are so poorly defined that there is no agreement concerning either the number of valid species or the generic groups to which they should be allocated. The uncertainty and confusion regarding the morphology and taxonomy of these cestodes were discussed by many earlier authors and more recently by Mueller (1937) and Wardle, McLeod and Stewart (1947). Most of the species have been referred, at one time or another, to the genus Diphyllobothrium Cobbold, 1858 as emended by Liihe (1910). The genus Diphyllobothrium, as defined by Liihe, is a heterogeneous assemblage and taxonomic revision of these tapeworms was proposed by Mueller (1937) and by Wardle, McLeod and Stewart (1947). The latter authors stated: "It is a cumbersome group of about 70 species-many of them of dubious validity-and comprises forms from toothed-whales, seals, sea-lions, carnivorous land mammals and fisheating birds. Several species have been recorded from humans and one even from a snake. It has always been an unsatisfactory genus to define and analyze, and particularly difficult to evaluate have been the forms from seals and sea-lions that have been recorded by numerous writers." Wardle et al erected a new genus, Cordicephalus, to contain the species found in seals-and sea-lions and recognized four species: Cordicephalus phocarus (Fabricius, 1780) ; C. tectus (Linstow, 1892); C. arctocephalinus (Johnston, 1937) ;and C. quadratus (Linstow, 1892). All other species from seals and sea-lions were regarded as identical with one or other of the four accepted species. The remaining species of Liihe's genus were distributed among six other genera: Diphyllobothrium Cobbold, 1858; Diplogonoporus Loennberg, 1892; Dibothriocephalus Liihe, 1899; Glandicephalus Fuhrmann, 1921 ; Adenocephalus Nybelin, 1931; and Spirometra Mueller, 1937.