After One Year, Just the Tip of the Iceberg

Throughout this year we've met blood-feeders, mind-benders, parasitic castrators, brood usurpers, outrageous shape-shifters, skin-clingers, eye-invaders and deadly plagues, but this is only a minuscule fraction of the true biodiversity of parasites. For some perspective let's calculate the number of years it would take to feature all the known metazoan parasites (such as worms, lice, and other multicellular animals) at a rate of one per day:

Myxozoa >1350 = 3.70
Trematoda >18000 = 49.30
Monogenea >20000 = 54.95
Cestoidea >5000 = 13.67
Acanthocephala >1200 = 3.29
Nematoda >10500 = 28.77
Mollusca >5600 = 15.34
Arachnida >30800 = 84.38
Crustacea >5360 = 14.68
Insecta >9400 = 25.75

Even without the odds and ends with less speciose groups like the Nematomorpha and Pentastomida, it would take us a little over 295 years just to feature every known and described species of metazoan parasites. This number does not include the multitude of undescribed species out there; a recent study (Randhawa and Poulin 2010) estimate that 3600 species of tapeworms are yet to be described from elasmobranchs (sharks and rays) alone - so that's another 10 years worth of tapeworms, all undescribed. The number of species of monogenean yet to be described is greater still, with 21000 - 22000 species yet to be described (Whittington 1998) - that's more than another 57 years' worth. For the digeneans, parasitic flukes, in Australia alone, over 5500 species are yet to be described (Cribb 1998) - 15 years worth of worms. All undescribed and unknown to science.

And that is just from the flatworms, which form one phylum out of many. What about the arthropods? And the nematodes? There is no reasons to think why there would be any fewer undescribed parasitic arthropods and nematodes than there are undescribed parasitic flatworms, and if such trend holds, it is likely that it would take an entire millennium to feature all described and undescribed species of metazoan parasites.

But that in itself is merely the tip of a very, very large iceberg. Moving away from the animals, what about the many parasitic fungi and plants? Parasitism as a life-style is just as common in fungi and plants as they are in animals. What about the eukaryotic single-cell parasites? This include the apicomplexans and the trypanosomes, which include parasites which cause diseases like malaria and sleeping sickness.

On top of that, throughout the year, we also featured many pathogenic bacteria and virus, and their diversity readily dwarfs the diversity found in the eukaryotes. With the use of new technology such as metagenomics, we have only begun to scratch the surface of their mind-boggling diversity. For this blog, we have looked beyond the traditional definition of a "parasite" to also included phytophagous insects (which technically are merely insects that are parasitic on plants rather than animals), and animals which have some aspect of "parasitism" to their life-style, such as brood parasites and kleptoparasites.

The Year 2010 was named as the "International Year of Biodiversity" and this blog was our attempt at showing the amazing, cool, and sometimes gross diversity just in parasitic organisms. But, we have barely scratched the tip of the iceberg with this blog and if this iceberg represents all the species which have been described, then it is merely a small chunk from a much greater ice sheet. Any study of biodiversity that does not take parasites into account will be ignoring the elephant in the room...or should I say the lively colony of critters living in and on the elephant?

We hope that you have enjoyed your daily dose of parasites throughout 2010. We'll continue to add other posts on cool and new and interesting parasites, so please follow the blog if you want to be alerted when these are added.

Finally, a big thank you to the more than two dozen people who contributed over the year and a HUGE thank you to all of you for reading this, sharing this, and giving us your comments and questions.

References:

Cribb, T. H. 1998. The diversity of the Digenea of Australian animals. International Journal for Parasitology 28: 899-911.
Randhawa, H.S. and Poulin, R. 2010. Determinants of tapeworm species richness in elasmobranch fishes: untangling environmental and phylogenetic influences. Ecography 33: 866-877.

Whittington, I.D. 1998. Diversity "down under": monogeneans in the Antipodes (Australia) wih a prediction of monogenean biodiversity worldwide. International Journal for Parasitology 28: 1481-1493.

-- By Tommy Leung and Susan Perkins