Research Article |
Corresponding author: Maxim V. Vinarski ( radix.vinarski@gmail.com ) Academic editor: Matthias Glaubrecht
© 2015 Maxim V. Vinarski.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Vinarski M (2015) Conceptual shifts in animal systematics as reflected in the taxonomic history of a common aquatic snail species (Lymnaea stagnalis). Zoosystematics and Evolution 91(2): 91-103. https://doi.org/10.3897/zse.91.4509
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Lymnaea stagnalis (L., 1758) is among the most widespread and well-studied species of freshwater Mollusca of the northern hemisphere. It is also notoriously known for its huge conchological variability. The history of scientific exploration of this species may be traced back to the end of the 16th century (Ulisse Aldrovandi in Renaissance Italy) and, thus, L. stagnalis has been chosen as a proper model taxon to demonstrate how changes in theoretical foundations and methodology of animal taxonomy have been reflected in the practice of classification of a particular taxon, especially on the intraspecific level. In this paper, I depict the long story of recognition of L. stagnalis by naturalists and biologists since the 16th century up to the present day. It is shown that different taxonomic philosophies (essentialism, population thinking, tree thinking) led to different views on the species’ internal structure and its systematic position itself. The problem of how to deal with intraspecific variability in the taxonomic arrangement of L. stagnalis has been a central problem that made systematists change their opinion following conceptual shifts in taxonomic theory.
Animal taxonomy, historical development, species, Historia naturalis , malacology, great pond snail, Lymnaea stagnalis
The development of science as a whole, as well as the progress of a particular scientific discipline, is a complicated and diverse process with many separate aspects (Hull 1988) that permit several distinct ways to represent the history of science. In my opinion, at least three approaches are imaginable in this case. Firstly, any history is a story telling us about a temporal sequence of events. It means that a narrative approach, which is merely an account of persons and their discoveries, is inevitable. Secondly, one may focus on conceptual shifts in scientists’ minds that reflect the theoretical rather than temporal development of science. This approach presupposes a study of continuous changes in scientific concepts and ideas as well as in the methodological foundations of the art of doing science. It may well be a non-linear process since the development of theories does not always run parallel with the progressive sequence of events. Lastly, any description of the practical aftermath of these conceptual shifts may be considered as the third approach to the history of science. It encompasses the “external” manifestations of scientific activity, including modes of representing of knowledge, scientists’ social interactions, university curricula, working classifications of studied objects, and so on.
Biological systematics is, probably, the oldest of the branches of life sciences. Its roots may be traced back to the pre-scientific epoch, since so-called “ethnotaxonomy” was just the first attempt to capture biological diversity by using more or less implicit categories and vernacular names (
The aim of this paper is to outline the taxonomic history of a widespread and commonly known invertebrate species in Europe and North America – the great pond snail, Lymnaea stagnalis (Linnaeus, 1758). The material for the study was obtained during my work with malacological collections of the Zoological Institute, Russian Academy of Sciences (Sankt-Petersburg; ZIN hereafter), Göteborgs Naturhistoriska Muséet (GNM hereafter), Vienna Museum of Natural History, Austria (NHMW hereafter), Naturhistoriska riksmuseet, Stockholm (Sweden), and Zoological Museum of the Copenhagen University, Denmark (ZMUC hereafter). These collections contain a large number of samples of L. stagnalis collected and identified by prominent malacologists of the end of the 18th – the first half of the 20th centuries, and examination of these materials helped me to understand how the taxonomists’ views changed with time and to trace these changes by analysis of the information available from museum labels. The extensive search through old taxonomic literature has been carried out as well. I used the books kept in ZIN and NHMW libraries and utilized those fantastic facilities provided by electronic archives such as Biodiversity Heritage Library (http://www.biodiversitylibrary.org/).
It is a freshwater pulmonate snail (Fig.
Though the accepted scientific name of this snail should be credited to
The only (and rather curious) alleged mention of the pond snail in Antiquity belongs not to a naturalist but to an anonymous poet who was the author of the mock poem Batrachomyomachia (“Battle of Frogs and Mice”) intended to mimic the Homer’s masterpiece The Iliad. The poet describes the armour of the Frogs preparing to battle with the Mice:
”…their bucklers were
Good thick-leaved cabbage, proof ’ gainst any spear;
Their spears sharp bulrushes, of which were all
Fitted with long ones; their parts capital
They hid in subtle cockleshells from blows”
(
The German malacologist Menke (quoted after
Medieval descriptive zoology, being deeply dependant on the works of Aristotle and other Ancient naturalists, overlooked L. stagnalis as well. One need only envisage the image of this snail in the very crude drawings of aquatic snails which appeared at the end of the fifteenth century (see
The first naturalists of early Modern Europe interested in aquatic animals, namely Pierre Belon (1517–1564), Guillaume Rondelet (1507–1566), and Konrad Gessner (1526–1565), did not mention the great pond snail either. Though these authors were not slavish commentators of the Greek and Roman texts and added their own observations on aquatic creatures, they were more interested in marine mollusks than in freshwater ones.
The first record of the great pond snail in the European scientific literature I managed to find is that by Ulisse Aldrovandi (1522–1605), a junior contemporary of Belon and Gessner. His posthumous treatise De reliquis animalibus (
The essence of a taxon was an Aristotelian category not seen directly by the eyes, but being a mental construction based on the subjective weighting of animal characters aimed to distinguish between essential and secondary (accidental) ones. A modern scientist would say the essential diagnosis is a hypothesis since different authors may come to different views on which characters are essential and which are accidental. Therefore there was no commonly accepted scientific name for the great pond snail before
Polynomial taxonomic names proposed for the great pond snail in the 17th and 18th centuries.
Author | Latin name | English translation |
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Turbo levis item in stagnis degens | Turbo with smooth shell, living in stagnant waters |
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Buccinum longum 6 spirarum, omnium & maximum & productius, subflavum, pellucidum, in tenue acumen ex amplissima basi mucronatum | Buccinum with long [shell] having six whorls, whole, large, oblong, yellowish coloured, transparent, [apex] sharp and narrow, [shell] basis very ample |
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Longior antecedenti Turbo, levissimus, colore atro cum nitore | Turbo longer than the antecedent [species], smoothest, [shell] dark colored, glossy |
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Buccinum fluviatile, testa tenuissima, & fragillissima, prima spira notabiliter ventricosa, & elongata, in mucronem aculeatum statim definens, subflavum, pellucidum | Buccinum riverine, shell very narrow and fragile, the first whorl notably inflated and oblong, [shell] ends with sharp apex, yellowish, transparent |
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Cochlea testa producta cuminata opaca, anfractibus senis subangularis, apertura ovata | Cochlea with elongated dark coloured shell having six subangulate whorls and ovale aperture |
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Cochlea fluviatilis, indigena, ex oblongo acuminata, lineolis veluti taeniata | Cochlea riverine, native [= European], with oblong and pointed shell, covered by thin lines |
Klein 1755: 54–55 | Auricula stagnorum – subflava, pellucida, in tenue acumen ex amplissima basi mucronata |
Auricula stagnorum – [shell] yellowish and transparent, with pointed apex and very wide [shell] basis |
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Turbo fluviatilis major, corpore oblongo ampullaceo definente in mucronem acutissimum & limacem continente fuscum | Turbo riverine, large, body [= shell] oblong, inflated [in its base] and ending by a sharpest apex; it contains soft body of contunuosly dark coloration. |
Geoffroy 1767: 72 | Buccinum testa oblonga, fusca, anfractibus senis | Buccinum with oblong shell of brownish black colour, having six whorls |
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Buccinum fluviatile, testa tenui et fragili, forma oblonga, ventricosa; sex spiris exertis parum convexis in apice acuto definentibus compositum; colore corneo, pellucido, apertura spatiosa, elongata, integra, et labio expanso distinctum | Buccinum riverine, its shell is narrow and fragile, oblong and inflated; it consists of six slightly convex whorls ending with a sharp apex; [shell] horny-coloured, transparent, aperture ample, elongated, whole, differs by a wide lip |
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Helix <…> testa albida, pellucida, superne turrita, inferne ventricosa, apertura effusa seu ampliata, columella sinuosa | Helix <…> [with] whitish pellucid shell, in its upper part it is turreted; the lower part is inflated. Aperture ample or wide; columella folded |
The next step in the study of the great pond snail was undertaken almost 70 years later, in England. Martin Lister (1639–1712) was an English physician and naturalist, and vice-president of the Royal Society. He contributed extensively to many branches of science, including arachnology (
In Lister’s Historiae animalium Angliae tres tractatus (
The picture of the great pond snail shell given by
Later on,
Most subsequent authors, whose works were published between the works of Lister and Linnaeus, were mere collectors of deposited shells of L. stagnalis and other freshwater species in their private museums and shell “cabinets”. This form of hobby was extremely popular among European noblemen and the educated part of the middle class in the 17th and 18th centuries (
Carolus Linnaeus, the Swede, was a great botanist and reformist of biological taxonomy, but his malacological (or, more correctly, conchological) works received rather low esteem among next authors. For example, Maton and Rackett (1804: 175) stated that “there has been a very general belief that less attention was devoted by Linnaeus to the history and arrangement of the Testacea than to any other order of the animal kingdom, and that he even thought their external coverings, or shells, scarcely worthy of becoming subjects of scientific distribution”.
The examination of several descriptions of L. stagnalis found in Linnaeus works (
Linnaeus and his immediate follower Johann Friedrich
As a rule, animal taxonomists of the second half of the seventeenth century were still not aware of the existence of intraspecific variation (
At the end of the 19th century, the practice of recognition of varieties in L. stagnalis reached its summit in the works of
Having searched through the old malacological literature, I compiled a synonymy of L. stagnalis s. lato that contains nearly 80 varieties of this species described between the 1820s and 1920s (Electronic Appendix 1) A closer examination of this „zoo“ reveals a plethora of causes serving as grounds for establishing new varieties. I had an opportunity to examine many of these varieties using samples of L. stagnalis identified by malacologists of the 19th century (Fig.
Those reflecting variation in shell size. Example: L. stagnalis var. major Moquin-Tandon, 1855. Sometimes, shells of knowingly juvenile individuals were described as a distinct variety (L. stagnalis var. junior Nilsson, 1822).
Sinistral mutants: L. stagnalis var. sinistrorsa Jeffreys, 1862.
Variants of the shell surface colouration: L. stagnalis var. bicolor Hartmann, 1840; L. stagnalis var. roseolabiata Beck, 1837; L. stagnalis var. fasciata Merkel, 1908.
Varieties based on shell proportions: L. stagnalis var. ampliata Clessin, 1876; L. stagnalis var. producta Colbeau, 1859.
Varieties based on peculiarities of shell structure, including structure of aperture and umbilicus, whorls’ shape: L. stagnalis var. umbilicata Hutton, 1905.
Varieties based on ecological preferences: L. stagnalis var. lacustris Studer, 1820; L. stagnalis var. alpicola Gredler, 1859
Geographical races: L. stagnalis var. bottnica Westerlund, 1884; L. stagnalis var. gallica Bourguignat, 1864.
Varieties based on conchological similarity with other lymnaeid taxa: L. stagnalis var. palustriformis Kobelt, 1870.
Varieties of L. stagnalis as they were identified by malacologists of the 19th century. A. L. stagnalis var. typica (det. S. Clessin; ZIN). B. L. stagnalis var. media (det. C.A. Westerlund; GNM). C. L. stagnalis var. producta (det. C.A. Westerlund; GNM). D. L. stagnalis var. rosea (identified by a unknown person; ZMUC). E. L. stagnalis var. colpodia (det. C.A. Westerlund; GNM). F. L. stagnalis var. variegata (det. C.A. Westerlund; GNM). G. L. stagnalis var. turgida (det. C.A. Westerlund; GNM). H. L. stagnalis var. raphidia (det. C.A. Westerlund; GNM). I. L. stagnalis var. palustriformis (det. A. Fuchs, NHMW). Scale bars 5 mm.
This short review shows how vague and indeterminate was this common practice of the discrimination of varieties.
Some authors attempted to reduce this vagueness by making demand that only those varieties that are found to have a hereditary basis are real. For instance,
Nevertheless, the most peculiar approach to the systematization of pond snails was realized by a group of French malacologists of the second half of the nineteenth century known under the name “Nouvelle École” (
A seemingly more productive approach to the L. stagnalis group taxonomy was proposed in Germany by Wilhelm Kobelt (1840–1916), a prominent zoologist who was also a strong critic of the “Nouvelle École” methods.
“Normalform” (a typical morph, see Fig.
“Hungerform” (a starvation morph) – dwarf phenotype of L. stagnalis arising allegedly as a result of food shortage.
“Seeform” – a phenotype of large lakes.
“Kanalform” – a phenotype produced in canals.
Kobelt’s idea helped to reduce the mammoth number of varieties to a few comprehensible entities with relatively clear content. Its influence may be traced in the works of David
The advent of population genetics in the 1910–1920s brought to malacologists a new way of thinking capable of explaining the great pond snail variation. The futility of establishing an endless number of obscure varieties was demonstrated by
This idea was a consequence of a quite novel form of biological thought known under the label “population thinking” (
Further progress in biometric studies and conchometry based on measurements of large samples for drawing statistical inferences led to the complete renunciation of the local race concept in “freshwater” malacology. Eventually it became a trivial fact that any well isolated population of a given species should differ phenotypically from other populations of the same species. A total number of local races of L. stagnalis would be positively immense, so that makes the concept itself impracticable. Another cause of neglect of local races and similar intraspecific entities were anatomical studies focused mainly on the reproductive morphology of lymnaeids. Though the first information on L. stagnalis anatomy was obtained as early as the 17th century (M. Lister, J. Swammerdam), its taxonomic relevance was not acknowledged until the 1910s (
In the twenthienth century, the Synthetic Theory of Evolution (STE) absolutely dominated over biologists’ minds. One of the most influential STE constituents was the so called ‘biological species concept’ (BSC) that is universally known and needs no detailed exposition here. I wish only to remind that BSC sees species as isolated gene pools able to keep their integrity due to complete or almost complete absence of gene flow among them (
The first attempt to apply BSC in lymnaeid systematics was undertaken in the USSR in the 1970s and 1980s by N.D. Kruglov and Ya.I. Starobogatov. They adopted two main methods of systematization: artificial crossing experiments (
As a consequence,
Taxonomy of the Lymnaea stagnalis species complex according to
Subgenus | Section | Species |
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Lymnaea | Lymnaea s.str. | 1. L. doriana (Bourguignat, 1862) 2. L. fragilis (L., 1758) – L. fragilis fragilis – L. f. producta (Colbeau, 1859) 3. L. stagnalis (L., 1758) – L. stagnalis stagnalis – L. s. turgida (Hartmann, 1840) |
Kobeltilymnaea Kruglov & Starobogatov, 1993 | 4. L. araratensis Kruglov & Starobogatov, 1985 | |
Stagnaliana Servain, 1881 | 5. L. media (Kobelt, 1877) 6. L. bodamica (Miller, 1873) |
The fate of
I think this question could not be resolved in principle before the modern “revolution” in taxonomy that was triggered by introduction of the molecular genetic techniques in the 1980s.
The ‘brave new world’ of animal taxonomy relies heavily on inferences based on molecular studies and the quasi-cladistic way of bringing up and testing phylogenetic hypotheses (
Already the first attempts to examine the internal diversity of L. stagnalis by molecular methods (
In 2008, a group of Ukrainian malacologists (
Having used the DNA sequencing technique and four gene markers (two nuclear and two mitochondrial),
However, a thorough analysis of morphological variation in L. stagnalis s. l. shows that the internal structure of the species cannot be restricted to molecularly defined groups. Though numerous morphs and varieties of the great pond snail lack the genetic support, the reality of some morphologically distinct entities within it has been proved by statistical methods (
These results represent a clear example of drastic incongruence between molecular and morphological data. Though the validity of “minor” species of the great pond snails accepted by
Nevertheless, this brave new view on L. stagnalis will, possibly, change in the nearest future since new, more powerful methods of molecular study (next generation sequencing, transcriptomics) are coming. The application of these methods may bring essentially new results concerning the great pond snail – this long studied but still not completely understood species.
I would like to thank museum curators for their help during my work: Pavel Kijashko and Lidiya Yarochnovich (Sankt-Petersburg), Anita Eschner (Vienna), Tom Schiøtte (Copehnagen), Ted von Proschwitz (Gothenburg) and Anders Warén (Stockholm). I thank also Peter Glöer (Hetlingen) for translation from the