A recent paper (Christensen, listed below) not only is the shortest paper with the longest title I’ve seen in awhile, but has reminded me of the confused state of the taxonomy and classification of freshwater snails. The paper suggests that a freshwater limpet named in 1900 is actual another species, named in 1863, which is…wait for it, actually the correct name for another species also named in 1863. These ‘species’, maybe we should call them ‘nominal species’, are thus synonyms of each other.
Summary: What was once was thought to be three distinct species is actually one species.
This (condensing nominal species into fewer species) appears to be a recent trend, and one, I hope, that will continue in the coming years. Before molecular data came into the mix it was thought that North America housed a dizzying array of over 600 species of freshwater snail (I suspect that it is more like 287 species (yes, I’m just making that number up)). For casual aquatic ecologist this typically meant that snails were not identified to species but instead were listed just by genera or even family with a loss of detail in the ecological data.
Take a look at this plate from the go-to identification guide of freshwater snails for aquatic scientist for nearly 30 years (I don’t want to seem too critical of this guide, it has served me well, and, is such a monumental work it makes me tired just thinking about completing such a project; BUT lets not lets reverence for a publication arrest changing things when they need changing). Imagine you’re a aquatic biologist, not a snail expert, trying to figure out the species you just collected and you find your way to this plate. How many species are listed here? Are they actually different species?
The family illustrated in the plate, Physidae, has been recently reduced in number of species by molecular and other work that suggest that a very large number of species, more than 50, is actually just 10-12 species (the largest study reduced 28 taxa to 5 or 6. A few notable papers (listed at the end of this post) illustrate the trend of combining nominal species. This is just a small sample of recent publications. This problem - more species names used than their are actual species, is not confined to snails but snails have a broad history of misidentification. The winner of multiple identifications is the Rough Periwinkle, a marine snail, that has been described as a separate species or subspecies 113 times.
Why so many nominal species? A least a few reasons come to mind.
1. Terse species descriptions. Here is the description for a species of Lymneaidae, Lymnea bulimoides: The publication also included a single tiny image of the snail that provides little help.
Testa ovato-conica, subtenui, laevi, nitida, diaphana, fuseo-lutea, minute perforata; spira breviuscula ; suturis parvis; anfractibus quinis subconvexis; apertura ovata.
Translation:
Shell ovate-conical, thin, smooth, clean, transparent, dark-yellow, minute holes in them; base rather short; small sutures; whorls five subconvex; aperture oval.
2. Morphological variation is common among snails. This is another trend, that of describing the huge morphology array (phenotypic plasticity) displayed by a single species - perhaps due to where it lives (river vs lake), or other factors (parasitized, predator rich environment, pH). I’ll go more into morphology of snails in a latter blog - much to say on this :-)
3. Desire to describe a species. Scientist are people after all.
(Quick note: Issac Lea in 1864 paper described 24 ‘new’ species in the family Physidae: None of these species (NONE) are considered valid today.
It is apparent that species have been described and named multiple times and thus the merging of species seems to me to be a very good thing. Fewer species means it will be easier to identify species, especially helpful for the casual snailologist (that’s not actually a word). However, didn’t I mention that scientist are people. People don’t like to change their minds. The data trend of merging nominal species does not mean everyone follows the changes (or apparent changes, after all good science means maintaining a healthy skepticism). Lets add another twist, people have the name they like, and so even if we successfully merge some species names, the name used varies among authors. There are actually rules, the basic rule; the first to name it gets priority. But again, people don’t like to change their minds (or use a name when they like the ‘other’ name. I’m guilty of this.
For an interesting read (super short paper) the limpet paper, mentioned above and listed below, by Christensen summarizes his following of the rules pretty well. However, I suspect most scientist will not follow the name suggested by Christensen (Ferrissia californica). Some recent papers failed to follow the results concerning the family Physidae (the left-handed pond snails from the plate above), even though multiple studies have shown that there are far fewer species than originally thought, and keep listing a huge array of species for this family. Similar problems concerning the Lymneidae (right handed pond snails) persist. Scientist/people/humans have a hard time changing their minds. The Lymnaeids suffer not only from species confusion but also generic confusion. For the same species the genera Lymnaea, Stagnicola, Fossaria, and Galba have been used. Another name, one I wasn’t familiar with, has been added to my array, that of Ladislavella. (That name, and the chart below, via works by Dr. Maxim Vinarski out of St Petersburg State University).
Here is one brief look at how different authors have used the genera in this family. From Vinarski 2013.
and clearly takes priority over other names if the powers that be determine the genera are not separate.
Dr. Dillon has this to say about the many studies of phenotypic plasticity in this group:
"Evidence suggest that the traditional taxonomy of the
North American stagnicolines (medium sized Lymnaeids) may have
been based on shell characters largely ecophenotypic in their origin”
Rob’s blog for those interested in more snailology: http://fwgna.blogspot.com.
Confusion reigns and it is no wonder that I get contacted by aquatic ecologist attempting to name the species they have found and collected. Confusion reigns but there is hope; work continues.
For now I’ll continue doing what I have done recently, include a long list of species names I consider synonyms in my publications. I’m also attempting to condense species into a short identification list for aquatic biologist in my region. I’m just getting started on that: (http://fwgmb.weebly.com)
Further reading and notes:
Answer to the plate question: This guide list four species on this page. Next challenge, which images are the same species?
Burch Guide: North America Freshwater Snails. Published in various years/formats.(PDF here: http://molluskconservation.org/PUBLICATIONS/WALKERANA/Vol2/walkerana%20vol2%20no6%201-80.PDF)
Limpets:
Christensen, CC. 2016. Change of status and name for a Hawaiian freshwater limpet: Ancylus sharpi Sykes, 1900, is the invasive North American Ferrissia california (Rowellm 1863), formerly known as Ferrissia fragile (tyron, 1863) (Gastropod: Planorbidae: Anycylinae). Records of the Hawaii Biological Survey for 2015. Bishop Museum Occasional Papers 188: 5-8.
Walther, A.C, J.B Burch and D.Ó. Foighil. 2010. Molecular phylogenetic revision of the freshwater limpet genus Ferrissia (Planorbidae: Ancylinae) in North America yields two species: Ferrissia (Ferrissia) rivularis and Ferrissia (Kincaidilla) fragilis. Malacologia 53: 25-45.
Walther, A.C., T. Lee, J.B. Burch, and D.Ó. Foighil. 2006. E Pluribus Unum: A phylogenetic and phylogeographic reassessment of Laevapex (Pulmonata: Ancylidae), a North American genus of freshwater limpets. Molecular Phylogenetics and Evolution 40: 501-516.
Lymnaeidae:
Correa, A.C., J.S. Escobar, P. Durand, F. Renaud, P. David, P. Jarne, J.P. Pointier, and S. Hurtrez-Boussès. 2010. Bridging gaps in the molecular phylogeny of the Lymnaeidae (Gastropoda: Pulmonata), vectors of Fascioliasis. BMC evolutionary biology 10 (1): 381.
Vinarski, MV. 2013. One, two, or several? How many lymnaeid genera are there? Ruthenica 23 (1): 41-58.
Physidae:
Dillon, R.T., J.D. Robinson, T.P. Smith, and A.R. Wethington. 2005. No reproductive isolation between freshwater pulmonate snails Physa virgata and P. acuta. The Southwestern Naturalist 50(4): 415-22.
Lea, I. 1864. Descriptions of twenty-four new species of Physa in the United States and Canada. Proceedings of the Academy of Natural Sciences of Philadelphia 16 (2): 114-116.
Wethington, A.R., and C. Lydeard. 2007. A molecular phylogeny of Physidae (Gastropoda: Basommatophora) based on mitochondrial DNA sequences. Journal of Molluscan Studies 73(3): 241.