An investigation of a parasite species that is broadly host- and habitat-specific and exhibits alternative transmission strategies was undertaken to examine intraspecific variability and if it can be attributed to cryptic speciation or environmentally induced plasticity. Specimens of an acanthocephalan parasite, Leptorhynchoides thecatus, collected throughout North America were analysed phylogenetically using sequences of the cytochrome oxidase I gene and the internal transcribed spacer region. Variation in host use, habitat use, and transmission were examined in a phylogenetic context to determine if they were more likely phylogenetically based or due to environmental influences. Results indicated that most of the variation detected can be explained by the presence of cryptic species. The majority of these species have narrow host and microhabitat specificities although one species, which also may comprise a complex of species, exhibits broad host and habitat specificity. Alternate transmission pathways only occurred in two of the cryptic species and correlate with host use patterns. Taxa that mature in piscivorous piscine hosts use a paratenic fish host to bridge the trophic gap between their amphipod intermediate host and piscivorous definitive host. One potential example of environmentally induced variation was identified in three populations of these parasites, which differ on their abilities to infect different host species.

Although the relationship between intestinal parasitism, the ingestion and use of energy, and host survival is expected, little work has been done to outline the effect of such organisms upon their host's nutritional requirements in an ecological context. This study is the first to demonstrate that an intestinal helminth previously reported to be of little or no histopathological consequence, Plagiorhynchus cylindraceus, has a significant detrimental impact upon the flow of food energy through a definitive host, the European starling, Sturnus vulgaris. Within both male and female adult European starlings reductions in standard metabolic rates occurred as the result of initial infection, indicating that the host's basal metabolism/thermal regulatory abilities were altered. Moreover, initially infected male starlings, but not females, had an increased consumption and excretion of energy and maintained lower average daily body weights versus controls when temperature stressed. These results appear to be due to either a parasite-mediated alteration in host activity and/or to the disruption of host-digestive abilities. Additionally, these data indicate that, overall, male and female S. vulgaris respond differently to infection and that intestinal helminths normally thought to be of little or no pathological consequence to the host are factors that should be addressed in future studies regarding animal energetics, ecology, and behaviour.

The complete sequence of the mitochondrial genome of Leptorhynchoides thecatus (Acanthocephala) was determined, and a phylogenetic analysis was carried out to determine its placement within Metazoa. The genome is circular, 13,888 bp, and contains at least 36 of the 37 genes typically found in animal mitochondrial genomes. The genes for the large and small ribosomal RNA subunits are shorter than those of most metazoans, and the structures of most of the tRNA genes are atypical. There are two significant noncoding regions (377 and 294 bp), which are the best candidates for a control region; however, these regions do not appear similar to any of the control regions of other animals studied to date. The amino acid and nucleotide sequences of the protein coding genes of L. thecatus and 25 other metazoan taxa were used in both maximum likelihood and maximum parsimony phylogenetic analyses. Results indicate that among taxa with available mitochondrial genome sequences, Platyhelminthes is the closest relative to L. thecatus, which together are the sister taxon of Nematoda; however, long branches and/or base composition bias could be responsible for this result. The monophyly of Ecdysozoa, molting organisms, was not supported by any of the analyses. This study represents the first mitochondrial genome of an acanthocephalan to be sequenced and will allow further studies of systematics, population genetics, and genome evolution.

Because of the likelihood that Corynosoma magdaleni Montreuil, 1958. has been confused with C. strumosum (Rudolphi, 1802) in reports of parasites from seals and to clarify its distribution in the Baltic Sea, acanthocephalans from 26 young gray seals from the southwestern Finnish archipelago (western Baltic Sea) were examined. All harbored C. semerme (Forssell. 1904). In addition to C. semerme, 12 had both C. strumosum and C. magdaleni, 3 had only C. strumosum, and 9 had only C. magdaleni. Most anatomical structures of C. strumosum are similar to, but larger than, those of C. magdaleni. The most conspicuous differences are the longer and more robust trunk of C. strumosum, the routinely longer proboscis (mean, 653 vs. 476 microm) and larger proboscis hook (mean, 69 vs. 58 microm) of C. strumosum, and the greater extent of ventral trunk spines (mean, 61 vs. 47%) in C. magdaleni. In addition, C. strumosum consistently possesses 18 longitudinal rows of proboscis hooks, whereas C. magdaleni has 17-23, with 20 being the usual number by far. In seals aged 3.6 mo, on average, C. strumosum was more prevalent and abundant than was C. magdaleni, whereas in seals of age 2.0-3.0 mo the reverse was true, with C. strumosum being nearly absent. These differences might reveal the age-dependent food habits of the very young seals. No site segregation was found between C. strumosum and C. magdaleni in the small intestine, but they clearly segregated from C. semerme, which occurred mainly in the cecum, large intestine, and rectum. All species matured equally well in gray seals, with 62, 67, and 53% of the C. magdaleni, C. strumosum, and C. semerme, respectively, comprising gravid worms (possessing eggs with fully formed acanthors).

Because of the likelihood that Corynosoma magdaleni Montreuil, 1958. has been confused with C. strumosum (Rudolphi, 1802) in reports of parasites from seals and to clarify its distribution in the Baltic Sea, acanthocephalans from 26 young gray seals from the southwestern Finnish archipelago (western Baltic Sea) were examined. All harbored C. semerme (Forssell. 1904). In addition to C. semerme, 12 had both C. strumosum and C. magdaleni, 3 had only C. strumosum, and 9 had only C. magdaleni. Most anatomical structures of C. strumosum are similar to, but larger than, those of C. magdaleni. The most conspicuous differences are the longer and more robust trunk of C. strumosum, the routinely longer proboscis (mean, 653 vs. 476 microm) and larger proboscis hook (mean, 69 vs. 58 microm) of C. strumosum, and the greater extent of ventral trunk spines (mean, 61 vs. 47%) in C. magdaleni. In addition, C. strumosum consistently possesses 18 longitudinal rows of proboscis hooks, whereas C. magdaleni has 17-23, with 20 being the usual number by far. In seals aged 3.6 mo, on average, C. strumosum was more prevalent and abundant than was C. magdaleni, whereas in seals of age 2.0-3.0 mo the reverse was true, with C. strumosum being nearly absent. These differences might reveal the age-dependent food habits of the very young seals. No site segregation was found between C. strumosum and C. magdaleni in the small intestine, but they clearly segregated from C. semerme, which occurred mainly in the cecum, large intestine, and rectum. All species matured equally well in gray seals, with 62, 67, and 53% of the C. magdaleni, C. strumosum, and C. semerme, respectively, comprising gravid worms (possessing eggs with fully formed acanthors).