Characterization of a diversity of tetraphyllidean and rhinebothriidean cestode larval types, with comments on host associations and life-cycles

Abstract

Life-cycles of marine tapeworms of the orders Tetraphyllidea and Rhinebothriidea are poorly known primarily because their larvae typically lack species level, taxonomically distinguishing adult characteristics and using morphology they can be identified to genus, family or order only. This large-scale study conducted in the northern Gulf of Mexico includes adult cestodes (25 species) collected from sharks and rays (Elasmobranchii, eight species) and larval cestodes (27 species) collected from teleosts (Neopterygii, 46 species), bivalves and gastropods (Mollusca, 24 species), and shrimps (Crustacea, five species), comprising a phylogenetically (75 species in three phyla, 14 orders and 46 families) and ecologically (e.g., benthic, epibenthic, pelagic, euryhaline, stenohaline) diverse array of hosts of larval cestode. Molecular biology and morphology informed larval identification and facilitated the circumscription of suites of morphological features representing distinct larval types (i.e., collective groups). A total of 198 specimens comprising adult and larval tetraphyllideans and rhinebothriideans assigned to 12 genera were characterized for the partial (D1–D3) lsrDNA gene and analyzed separately and in combination with data derived from species belonging to an additional 21 genera available from GenBank. Eight larval types were identified and matched to one or several genera of Tetraphyllidea or Rhinebothriidea; morphological variation within these larval types was also documented. In combination with published reports of unique larval morphologies, 15 larval types were established and a key to their larvae presented. Overall, teleosts figured prominently in the life-cycles of tetraphyllideans and rhinebothriideans. Intermediate host specificity at the level of cestode genus was euryxenous, but limited host records suggest that host specificity at the level of cestode species may be more strict. To our knowledge, this is the first published study that approaches the elucidation of marine tapeworm life-cycles by incorporating morphological, molecular biological and phylogenetic methods using specimens collected on a regional scale and from wild-caught hosts from four metazoan phyla.