Molecular and morphological systematics of the Leptorhynchoides thecatus (Acanthocephala: Rhadinorhynchidae) complex of species

Recognition of species boundaries is fundamental to many biological disciplines, yet remains problematic. One difficulty is the existence of morphologically cryptic sibling species. Sibling species commonly may form in organisms, such as autogenic parasites, that are characterized by high speciation rates and conserved body plans. Leptorhynchoides thecatus is an acanthocephalan parasite of freshwater fishes that occurs throughout eastern North America. This species is autogenic and exhibits subtle variation in morphology, host use, habitat use, and developmental patterns, which led to the suspicion that L. thecatus comprised multiple species. Identification and characterization of possible species within the L. thecatus complex was undertaken using phylogenetic and morphometric analyses with specimens collected throughout its distribution range in the eastern U.S.A. and Canada. Phylogenetic analyses utilized sequences of the cytochrome oxidase I (cox1) gene and the internal transcribed spacer region (ITS). Morphometric analyses included multivariate and univariate statistical techniques using discrete and continuous characters. Additionally, three methods were developed for the classification of new specimens: classification functions based on a canonical discriminant analysis, a decision tree, and a dichotomous key. These methods were employed on new specimens to assess their utility. Results of the phylogenetic analyses indicated that L. thecatus comprises six species. Deep phylogenetic divergence among each of the proposed species and their exclusiveness indicate that each is an independent lineage with its own evolutionary tendencies and historical fate. A canonical discriminant analysis detected morphological divergence among the proposed species and correctly discriminated 92% of males and 90% of females. Two pairs of species are morphologically cryptic and one species is polytypic. The most powerful discriminating morphological characters were the length of the trunk, the number of longitudinal rows of hooks, and the length of the longest hook. The six proposed species also differ in their patterns of host use, habitat use, and development. The existence of multiple species explains some of the variation detected previously in the L. thecatus complex and gives a credible explanation for apparent discrepancies reported in the literature