Poulin and Morand (2000, 2004) have used several approaches to further examine the potential diversity of parasitic helminths. They point out that many of the problems that beset estimates of free-living biodiversity also confound estimates of parasite diversity. In particular, the rate of discovery of new parasite species has grown linearly or exponentially in some well-studied helminth taxa. In contrast, sampling of parasite diversity from the most diverse parts of the world is thin at best. For example, Cribb et al. (2002) estimated that in groupers (Epinephelinae)—one of the largest and most common groups of marine fish—parasitic trematodes have been recorded from only 62 of the 159 species, and from only 9 of 15 genera. The absences reflect a paucity of sampling; most species were examined at only one location. Moreover, not only are most host species unstudied, but no tropical species of grouper has been exhaustively sampled for trematodes. This creates a significant problem for estimating global species richness of parasites based on extrapolations from known patterns of host specificity.
While acknowledging these problems, Poulin and Morand (2000, 2004) extrapolated estimates of specificity from studies of parasites in the relatively well-surveyed vertebrates. Their summary table suggests that there are at least 50% more parasitic helminth species (≈75,000) than there are vertebrate hosts (45,000) (Table 4.1). [The number of parasite species could actually be much higher, especially because fish species are hugely undersampled (Cribb et al., 2002; Hoberg and Klassen, 2002), as are the reptiles, amphibians, and indeed all vertebrate groups in the tropics (Brooks and Hoberg, 2000).]
Modern molecular methods have revealed a further bias that suggests that we have underestimated parasite species richness. These methods have revealed significant numbers of “cryptic species” of parasite that look morphologically similar but are sufficiently genetically distinct so as to represent different species [e.g., see Hung et al. (1999); Jousson et al. (2000); Haukisalmi et al. (2004); Perrot-Minnot (2004)]. The number of cryptic parasite species previously classified as a single morphologically recognized species can sometimes be disconcertingly high [for example, Miura et al. (2005) distinguished eight genetic species for a single morphospecies]. The issue of cryptic species will significantly distort estimates of global parasite species richness based on extrapolations from host specificity and mean numbers of parasites observed per host species. One of the basic elements of Poulin and Morand’s extrapolation is the number of hosts used by a parasite (Table 4.1). As parasites use more hosts, estimates of global diversity go down. However, many studies have found that cryptic species parasitize only a subset of the species originally recognized as hosting a parasite morphospecies [e.g., see Reversat et al. (1989) and Jousson et al. (2000)]. Thus, considerations of cryptic species might well