Two new Stephanostomum-like cercariae (Digenea: Acanthocolpidae) from Nassarius dorsatus and N. olivaceus (Gastropoda: Nassariidae) in Central Queensland, Australia

Abstract

Two new Stephanostomum-like cercariae, Cercaria capricornia VII and Cercaria capricornia VIII (Digenea: Acanthocolpidae), are described from the nassariid gastropods Nassarius dorsatus and Nassarius olivaceus collected from the intertidal zone in the Capricornia region of Central Queensland, Australia. Morphological analysis of these new cercariae was augmented with DNA sequence data from the large subunit (LSU) ribosomal DNA region to aid in identification. Bayesian inference analysis of the LSU rDNA revealed that these putative acanthocolpid cercariae nested well within a clade containing species of Stephanostomum, which along with morphological data, suggests they are species of Stephanostomum. Comparative analysis of LSU rDNA sequences also indicates that these two cercariae are not S. adlardi, S. bicoronatum, S. tantabiddii or S. cf. uku, all species known from Australian fishes. The secondary structure of the internal transcribed spacer 2 (ITS2) rDNA region was inferred for these two cercariae using minimum free energy modelling algorithms. Both cercarial types displayed a four helix ITS2 secondary structure model and differed from each other by two compensatory base changes (CBCs) and nine hemi-CBCs.