Estimates of the total number of species that inhabit the Earth have increased significantly since Linnaeus's initial catalog of 20,000 species. The best recent estimates suggest that there are ≈6 million species. More emphasis has been placed on counts of free-living species than on parasitic species. We rectify this by quantifying the numbers and proportion of parasitic species. We estimate that there are between 75,000 and 300,000 helminth species parasitizing the vertebrates. We have no credible way of estimating how many parasitic protozoa, fungi, bacteria, and viruses exist. We estimate that between 3% and 5% of parasitic helminths are threatened with extinction in the next 50 to 100 years. Because patterns of parasite diversity do not clearly map onto patterns of host diversity, we can make very little prediction about geographical patterns of threat to parasites. If the threats reflect those experienced by avian hosts, then we expect climate change to be a major threat to the relatively small proportion of parasite diversity that lives in the polar and temperate regions, whereas habitat destruction will be the major threat to tropical parasite diversity. Recent studies of food webs suggest that ≈75% of the links in food webs involve a parasitic species; these links are vital for regulation of host abundance and potentially for reducing the impact of toxic pollutants. This implies that parasite extinctions may have unforeseen costs that impact the health and abundance of a large number of free-living species.

/pdf/homage-to-linnaeus-how-many-parasites-how-many-hosts-44vz8rxglf.pdf

Homage to Linnaeus: How many parasites? How many hosts?

We review studies that have used molecular markers to address ecological and microevolutionary processes in parasites. Our goal is to highlight areas of research that may be of particular interest in relation to the parasitic lifestyle, and to draw attention to areas that require additional study. Topics include species identification, phylogeography, host specificity and speciation, population genetic structure, modes of reproduction and transmission patterns, and searching for loci under selection.

/pdf/molecular-ecology-of-parasites-elucidating-ecological-and-2fv2n85wk3.pdf

Molecular ecology of parasites: elucidating ecological and microevolutionary processes

We examined the relative merits of mitochondrial DNA loci and ribosomal DNA internal transcribed spacers for their use in prospecting for cryptic species of platyhelminth parasites. Sequence divergence at ITS1 and ITS2 was compared with divergence at 2 mtDNA loci (NADH dehydrogenase-1 and cytochrome c oxidase I) between closely related species of trematodes and cestodes. Both spacers accumulated substitutions substantially more slowly than mtDNA, which clearly shows a higher level of divergence among species relative to intra-specific variation. Besides a slow rate of substitution, other caveats that may be encountered when using ITS sequences as a prospecting marker are discussed. In particular, we note recent studies that suggest the existence of substantial levels of intra-individual variation in ITS sequences of flatworms. Because it is likely that closely related species share this phenomenon, it may confound the detection of cryptic species, especially if small sample sizes are studied. Although potential limitations of mtDNA are also recognized, the higher rate of evolution and smaller effective population size of this marker increases the probability of detecting diagnostic characters between cryptic species.

/pdf/a-comparison-between-mitochondrial-dna-and-the-ribosomal-279elq7b3v.pdf

A comparison between mitochondrial DNA and the ribosomal internal transcribed regions in prospecting for cryptic species of platyhelminth parasites

Molecular-genetic analyses reveal cryptic species of trematodes in the intertidal gastropod, Batillaria cumingi (Crosse).

We conducted an extensive literature review on studies that have used DNA sequences to detect cryptic species of parasites during the last decade Each literature citation that included the term "cryptic" or "sibling" species was analyzed to determine the approach used by the author(s) Reports were carefully filtered to retain only those that recognized the existence of cryptic species centered on the use of DNA sequences Based on analysis of these papers, we comment on the different ways that parasite cryptic species are discovered in studies focusing on different aspects of the host-parasite relationship, or disciplines, within parasitology We found a lack of methodological and theoretical uniformity in the discipline for finding and delimiting cryptic species, and we draw attention to the need for standardizing these approaches We suggest that cryptic species, in the strict sense, are always provisionally cryptic, in that the possibility does exist that new morphological studies or techniques will reveal previously unknown diagnostic structural differences which will permit rapid and practical morphological diagnosis To avoid future taxonomic confusion, we recommend that parasitologists describe (and formally name) cryptic species following standard taxonomic practice

https://www.jstor.org/stable/info/40606804

What We Don't Recognize Can Hurt Us: A Plea for Awareness About Cryptic Species

In the north-western (NW) Mediterranean, the teleosts Diplodus sargus, D. vulgaris and D. annularis coexist in infralittoral habitats. These fishes are infected by two species of the Digenea (Platyhelminthes, Trematoda): Macvicaria crassigula (Opecoelidae) and Monorchis parvus (Monorchiidae) for which we obtained Internal Transcribed Spacer rDNA sequences. Each parasite species represents a complex of two cryptic species, one restricted to D. annularis, and the other shared by D. sargus and D. vulgaris. Cytochrome b mtDNA sequences were used to infer host phylogenetic relationships which showed that the distribution of parasites in Diplodus hosts is not a consequence of coevolutionary interactions. We used diet analyses available for the fish hosts to assess the degree of overlap in the use of food among the three species. The feeding overlap was significant only between D. sargus and D. vulgaris, but not for the other fish pairs. The possible mechanisms involved in the speciation of the digenean fauna of Diplodus fishes are discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1420-9101.2000.00221.x

Cryptic speciation among intestinal parasites (Trematoda: Digenea) infecting sympatric host fishes (Sparidae)

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Cryptic speciation among intestinal parasites (Trematoda: Digenea) infecting sympatric host fishes (Sparidae)