The fabulous taxonomic adventure of the genus Geoplana

by Piter Kehoma Boll
ResearchBlogging.org

Freshwater planarians are relatively well-known as those cute arrow-shaped cockeyed animals. Land planarians are far away from having all the fame of their aquatic cousins and most people do not even know that they exist. Maybe in part it is because deeper studies of the natural world began in Europe, a continent were land planarians are almost non-existent. The first of those little animals to be known was described in 1774 by the Danish naturalist Otto Friedrich Müller. He named the small worm Fasciola terrestris, because he thought it was a terrestrial version of the parasitic worm. It was a small cyllindrical worm with a dark back and two small eyes at the anterior region.

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In 1788, the naturalist Johann Friedrich Gmelin transfered the species to the genus Planaria, described in 1776 by Müller. The worm was, therefore, now called Planaria terrestris. The genus, at this time, included everything that is currently known as planarian: worms with a ventrally located mouth, close to the middle part of the bod. The term most likely became popular by this time and so it continues until today as a general name for these animals.

planaria_terrestris

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Planarians with many eyes were transfered in 1831 by the naturalist Christian Gottfried Ehrenberg to a new genus, Polycelis. The term means “many dots” and refers to the dark dots that the eyes represent on the body.

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During Charles Darwin’s voyage around the world aboard the Beagle, he spent some time in the Brazilian Atlantic Forest and found several species of land planarians. He classified them in the genus Planaria, but highlighted that they formed a section within the genus because of their terrestrial habits, convex bodies and often colorful stripes. The first new species listed by him was called Planaria vaginuloides and was collected in the forests of Rio de Janeiro. The epithet vaginuloides was chosen because Darwin found them to be similar to slugs of the genus Vaginulus.

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One year later, in 1845, the naturalist Émil Blanchard found a species in Chile and named it Polycladus gayi. The name of the genus, Polycladus, refers to the highly branched gut of these animals, while the epithet gayi honors the naturalist Claudio Gay. But Blanchard made a terrible mistake: he mistook the anterior end for the posterior end and so thought that the genital opening was in front of the mouth!

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In 1850, the naturalist Karl Moriz Diesing transferred Darwin’s land planarians to the genus Polycelis because they have many eyes. Planaria vaginuloides was now Polycelis vaginuloides.

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In 18851, the zoologist Joseph Leidy found another land planarian species in Europe that was also small, cylindrical and with two eyes. He named it Rhynchodemus sylvaticus. The term Rhynchodemus means something like “bill-shaped body”. He also suggested the transference of Planaria terrestris to the new genus, so that it was now Rhynchodemus terrestris. Darwin and Blanchard’s species remained as Polycelis and Polycladus.

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Then in 1857 something funny happened. A new revision of land planarians was done by William Stimpson. He, for the first time, separated land planarians from freshwater ones and divided them in two families:

  1. Polycladidae: having a single genus, Polycladus, because it was still thought, at this time, that the genital opening was before the mouth.
  2. Geoplanidae: the rest of land planarians. Species in this family were divided into three genera:
  • Rhynchodemus: species with two eyes;
  • Bipalium: a genus for recently discovered species that have a hammer- or crescent-shaped head. The name comes from Latin bi-, two and pala, shovel.
  • Geoplana: species with many eyes. The name comes from geo, earth, and plana, flat, because of the flat body of those animals, as well as a direct reference to the genus Planaria, now restricted to aquatic species. The species Polycelis vaginuloides became Geoplana vaginuloides. So we arrive to the central genus of this story.
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By a huge coincidence, in this very same year of 1857, the naturalist Max Schultze, based on information from literature and new species collected in Brazil by the naturalist Fritz Müller, also decided to separate land planarians into another genus and also chose the name Geoplana! What are the chances? The papers of Stimpson and Schultze had only some weeks between them and everything seems to indicate that Schultze was unaware of Stimpson’s paper. The main difference between both papers is that Schultze ignored the discovery of the species classified as Bipalium. He also transfered all land planarians to Geoplana, so that Polycladus gayi, Rhynchodemus sylvaticus and Rhynchodemus terrestris were now Geoplana gayi, Geoplana sylvatica and Geoplana terrestris.

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Stimpson’s system, however, prevailed, and the four genera remained in use: Rhynchodemus, Bipalium, Geoplana and Polycladus. Among the species described by Schultze and Müller was Geoplana subterranea, an albine and eyeless species found underground and that feeds on earthworms. In 1861, Diesing decided to put this species into its own genus, Geobia. We had now 5 genera: Rhynchodemus, Bipalium, Geoplana, Geobia and Polycladus.

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In 1877, Henry Nottidge Moseley described a series of species from Australia, the Pacific and southeast Asia. A good amount of them were included in the genus Geoplana, but some of them were put in two new genera:

  1. Dolichoplana (“long flat”): very long and narrow species with two eyes as in Rhynchodemus;
  2. Caenoplana (“recent flat”): species considered by him to be intermediate between Geoplana and Dolichoplana because the body was longer and the eyes were restricted to the sides of the body.
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Ten years later, in 1887, J. J. Fletcher and A. G. Hamilton studied Australian land planarians and concluded that there was no need for the species named Caenoplana by MOseley to be in a separate genus and united them to Geoplana.

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During the following decade, the naturalist Arthur Dendy described several new species from Australia and New Zealand, classifying all in the genus Geoplana. The genus was growing, having tenths of species. In the last years of the 19th century, several new genera were created, many of them in the works of the zoologist Ludwig von Graff. These new genera were erected to species with very peculiar anatomical features, such as a differentiated head, for example. Anyway, the genus Geoplana kept growing. Any flat and many-eyed land planarian without a distinct feature was thrown into this genus. This system continued throughout most of the 20th century. Tenths of new species were described by the zoologist Libbie Hyman and by two zoologist couples: the Marcuses – Ernst Marcus and Eveline du Bois-Reymond Marcus – and the Froehlichs — Claudio Gilberto Froehlich and Eudóxia Maria Froehlich. These new species were mostly put in Geoplana. At that time the genus was widely distributed in South America and Australia.

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During this time, E. M. Froehlich determined that Geoplana vaginuloides should be the type species of the genus Geoplana. Perhaps you are now asking yourself “what is a type species?”. Well, in taxonomy, when a new genus is created, one of its species has to be considered the type species, the species that serves as a “model” to the genus. It is the type species that defines what the genus is. Once a species becomes the type species of the genus, it can never pass to another genus, except if the entire genus ceases to exist. Afterall, it is the species in which the existence of the genus is based. The fact is that in the 18th and 19th centuries there was no policy of type species, which was only later introduced in the rules to give names to organisms. Therefore Stimpson, when he created the genus Geoplana, did not define a type speciees. E. M. Froehlich chose Geoplana vaginuloides as the type-species because it was the first species listed by Stimpson and the proposal was accepted by the scientific community.

Let’s go back to the main subject. As it was said, the genus Geoplana was gathering more and more species throughout the 20th century, becoming huge. Then in 1990 the zoologists Robert Ogren and Masaharu Kawakatsu decided to clean the mess. By examining the inner anatomy of land planarians, they excluded from Geoplana all species from Australia and nearby areas because they have testes placed in the ventral region of the body, diferently from South American species, that have them in the dorsal region. However, letting all the South American pack inside Geoplana would still be a mess. So, they broke the genus into several smaller genera. The four main genera were defined based on two features of the copulatory apparatus: 1) the presence or absence of a penis papilla, i.e., a penis. Some planarians have a penis and some don’t. 2) The position of the oviducts, i.e., the canals that carry the eggs from the ovaries to a cavity named female atrium. The oviducts may enter the female atrium at the dorsal side or the ventral side. The classification of this two features allows four combinations:

  1. Species with a penis papilla and with oviducts entering dorsally. These species continued in the genus Geoplana, because this is the combination that occurs in Geoplana vaginuloides.
  2. Species with a penis papilla and with oviducts entering ventrally. These species passed to the genus Gigantea.
  3. Species without a penis papilla and with oviducts entering dorsally. These species were named Notogynaphallia.
  4. Species without a penis panilla and with oviducts entering ventrally. This is the opposite of what is found in Geoplana. These species were transfered to a genus named Pasipha.
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Things were starting to get more organized. Despite still having more than a hundred species, the genus Geoplana was a little more homogeneous now. But by the beginning of the 21th century, more detailed studies on the internal anatomy of planarians demonstrated that other parts of the body also had taxonomic importance. Furthermore, molecular studies were now available and the genus was challenged by the molecular phylogeny. The already expected result was confirmed. A study of molecular phylogeny by Fernando Carbayo and colleagues in 2013 revealed that the genus Geoplana, as defined by Ogren and Kawakatsu, was still a mess. Species were separated in several groups that needed to received their own genera. These new genera created from Geoplana were: Barreirana, Cratera, Matuxia, Obama and Paraba.

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At the end the type species, Geoplana vaginuloides, remained almost alone. The only other species that grouped with it was Geoplana chita. The genus Geoplana, once with hundreds of species throughout the whole world, now has only two species restriced to the Atlantic Forest between the Brazilian states of Rio de Janeiro and Paraná. And guess which of the new genera received most of the species one in Geoplana?

 

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Yes, we can!

Yes, we can!

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References:

Carbayo, F., Álvarez-Presas, M., Olivares, C., Marques, F., Froehlich, E., & Riutort, M. (2013). Molecular phylogeny of Geoplaninae (Platyhelminthes) challenges current classification: proposal of taxonomic actions Zoologica Scripta, 42 (5), 508-528 DOI: 10.1111/zsc.12019

Darwin, C. 1844. Brief description of several terrestrial planariae, and of some remarkable marine species, with an account of their habits. Annals and Magazine of Natural History, Annales de Sciences Naturelles, 14: 241-251.

Diesing. K. M. 1850. Systema helminthum. Academia Caesareae Scientiarium.

Fletcher, J. J.; Hamilton, A. G. 1887. Notes on Australian land-planarians, with descriptions of some new species. Part I. Proceedings of the Linnean Society of New South Wales, 2: 349-374.

Froehlich, E. M. 1955. Sôbre espécies brasileiras do gênero GeoplanaBoletim da Faculdade de Filosofia, Ciências e Letras da Universidade de São Paulo, Série Zoologia, 19: 289-339.

Gay, Claudio. 1849. Historia fisica y politica de Chile. Vol. 3.

Gmelin, O. F. 1788. Systema Naturae. Moseley, H. N. 1877. Notes on the structure of several forms of land planarians with a description of two new genera and several new species, and a list of all species at present known. Quarterly Journal of Microscopical Sciences, 17: 274-292.

Ogren, R.; 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.

Schultze, M.; Müller, F. 1857. Beiträge zur Kenntnis der Landplanarien. Abhandlungen der Naturforschenden Gesellschaft zu Halle, 4: 61-74.

Stimpson, W. 1857. Prodromus descriptionis animalium evertebratorum quae in expeditione ad Oceanum, Pacificum Septentrionalem a Republica Federata missa Johanne Rodgers Duce, observavit et descripsit. Pars I. Turbellaria Dendrocoela. Proceedings of the Academy of Natural Sciences of Philadelphia, 19-31.

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Filed under Molecular Biology, Systematics, taxonomy, worms, Zoology

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