by Piter Kehoma Boll
One of the big questions without an answer in our knowledge of the world is “How many species are there on the Earth?” and we are far from having even an approximation of the real number. There are, for sure, thousands of speculations, varying hugely between them, but we could say that a mean value would be about 10 million species, while we currently know only about 1.5 million. And that’s exactly the point I’m interested in focus here: the number of species we currently know. Are we sure that all we consider species are in fact so?
Probably everybody has already heard about a situation where a group of organisms once considered a single species where in fact two distinct ones, like the African elephants Loxodonta africana and L. cyclotis, where the latter was only classified as a distinct species by 2010.
As for people in general, if you ask them to say the name of an animal, they would probably tell the name of a mammal, or perhaps a bird, reptile or if you are lucky, they will say “butterfly” or “spider” and that’s all. Well, that’s nothing wrong with it, but I think people should realize that those animals, like lions or elephants, are just a small particle in the entire world of species.
I currently hold an undergraduate research at Unisinos (Universidade do Vale do Rio dos Sinos / University of the Sinos Valley), Brazil, working at the IPP (Instituto de Pesquisa de Planárias / Planarian Research Institute) with land planarians’ ecology, physiology and behavior.
For those who don’t know (and I know there are a lot of people who don’t), land planarians are terrestrial flatworms, belonging to the phylum Platyhelminthes, in the group Tricladida. They are usually found under rocks or logs in forested areas, but also in gardens or elsewhere. Many of them are very sensible to light, high temperatures or extremes of drought and moisture, so that their presence may indicate a more conserved area.
Land planarians are a group still poorly known and, despite a high number of species having been described in the last decades, many more are yet to be, and those already described are not well understood in what concerns their ecology and behavior.
The first described land planarians were defined based only on external features, mainly their body shape, color and eyes arrangement. But Ludwig von Graff, in his 1896 work “Über die Morphologie des Geschlechtsapparates der Landplanarien” (On the Morphology of the Reproductive Apparatus of Land Planarians) already noticed the importance of internal morphology, mainly that of the copulatory apparatus, for a more precise identification at the species level.
Despite that, the following years were still marked by publications concerning only external features, like the work of Schirch (1929). Only by the 1950’s a real focus was started to be given in the structure of male and female organs. Most of the following works on land planarians’ descriptions, like those of the Marcus and Froehlich couples, focused on the copulatory apparatus together with external features, so giving a more trustful description of new species. By this moment, the reproductive structures became essential for the classification of new species and eventually led to the creation of new genera.
In 1990, Ogren and Kawakatsu published an index of all known species of land planarians in family Geoplanidae by that time. They noticed that many species still classified in the genus Geoplana, like those described by Schirch, were never reviewed and were still only known by external features, so that their position within Geoplana may not be correct. To avoid this misclassification, they created a new “temporary genus”, which they called Pseudogeoplana (false Geoplana) and put all those dubious species in it to stay there until someone reviewed them and could place them in the correct genus, either the original Geoplana or other one.
But guess what? Ogren and Kawakatsu did that in 1990 and now we are in 2012 and the situation remains the same. Those poor planarians species are still waiting in that taxonomic shelter until someone moves them to the place they belong to.
So how can we be sure about anything from those species? They were described in 1929, almost a century ago, and no one cared about them since then. And I guess the same occurs in other less cute and attractive groups, so while we describe hundreds or thousands of new species every year, other hundreds or thousands are left behind, forgotten inside dusty glasses in the zoological museums worldwide.
I just hope it will change someday.
Thanks for reading.
For more about systematics, you might want to see:
- A Brief History of the Kingdoms of Life
- Why I Don’t Trust Jack Horner 1: The Holes in the Old Triceratops Idea
Baptista, V. & Leal-Zanchet, A. 2005. Nova espécie de Geoplana Stimpson (Platyhelminthes, Tricladida, Terricola) do sul do Brasil. Revista Brasileira de Zoologia, 22 (4), 875-882 DOI: 10.1590/S0101-81752005000400011
Du Bois-Reymond Marcus, E. 1951. On South American Geoplanids. Boletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo, Série Zoologia, 16, 217-256.
Froehlich, E. M. 1955. Sobre Espécies Brasileiras do Gênero Geoplana. Boletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo, Série Zoologia, 19, 289-339.
Graff, L. v. 1896. Über die Morphologie des Geschlechtsapparates der Landplanarien. Verhandlungen der Deutschen Zoologischen Gesellschaft, 73-95.
Marcus, E. 1951. Turbellaria Brasileiros. Boletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo, Série Zoologia, 16, 5-215.
Mora, C., Tittensor, D., Adl, S., Simpson, A. & Worm, B. 2011. How Many Species Are There on Earth and in the Ocean? PLoS Biology, 9 (8) DOI: 10.1371/journal.pbio.1001127
Ogren, R. E. & Kawakatsu, M. 1990. Index to the species of the family Geoplanidae (Turbellaria, Tricladida, Terricola) Part I: Geoplaninae. Bulletin of Fuji Women’s College, 28, 79-166.
Rohland, N., Reich, D., Mallick, S., Meyer, M., Green, R., Georgiadis, N., Roca, A. & Hofreiter, M. 2010. Genomic DNA Sequences from Mastodon and Woolly Mammoth Reveal Deep Speciation of Forest and Savanna Elephants. PLoS Biology, 8 (12) DOI: 10.1371/journal.pbio.1000564
Schirch, P. F. 1929. Sobre as planarias terrestres do Brasil. Boletim do Museu Nacional, 5, 27-38.