Host specificity of adult versus larval cestodes of the elasmobranch tapeworm order Trypanorhyncha.

Host specificity between the adult and final larval stages (plerocercus, plerocercoid, or merocercoid) of a diversity of trypanorhynch species was compared using the host specificity index (HS s). Index values were generated for a total of 63 species representing all five trypanorhynch superfamilies and 11 families. Host specificity of both adults and final larvae was found to be widely variable among species, ranging from very high (oioxenous) to very low (euryxenous) for both stages. However, in general, host specificity was highest for the adult stage in the definitive host (mean HS s=3.86) and lowest for the final larval stage in the second intermediate host (mean HS s=6.29). This difference was found to be significant using the Wilcoxon signed-rank test. Limited data available for procercoids in the first intermediate host suggest that this stage exhibits a degree of specificity intermediate between that of the former two stages (mean HS s=4.23). No taxonomic trend was seen. Species with a plerocercoid final larval stage (mean HS s=8.62) were significantly less host-specific than those with plerocerci or merocercoids (mean HS s=5.56). This result may reflect the use of paratenic hosts by species possessing the relatively more resilient plerocercoid as their final larval stage. These results provide an example of how HS s can be used to compare levels of host specificity, in this instance, among stages of polyxenous life cycles. They also emphasise the importance of articulating the life cycle stage under consideration when general statements are made about host specificity.