The Lymnaeidae 2012: A clue

Editor's Note. This essay was subsequently published as Dillon, R.T., Jr. (2019b)  The Lymnaeidae 2012: An important stipulation.  pp 45-50 in The Freshwater Gastropods of North America Volume 2, Essays on the Pulmonates.  FWGNA Press, Charleston.

Plasticity of shell phenotype is such a pervasive phenomenon in our area of research that the topic has become a regular category label on this blog, with nine posts to date and climbing.  We've seen quite a few excellent laboratory studies over the years, beginning with Arthur (1982) on Lymnaea stagnalis and including notable contributions by Lam & Calow (1988) on L. peregra,  DeWitt (1998) and Auld & Relyea (2011) on Physa, Hoverman & Relyea (2007) on Helisoma, Holomuzki & Biggs (2006) on the hydrobiids, and Urabe (1998) and Krist (2002) on the pleurocerids [1].   I've reached a point that I now take the phenomenon for granted.

So, looking back on my posts of April and May regarding the “stagnicoline” lymnaeids of North America [2], I skipped as lightly through the "5 nominal species in the dark-skinny elodes group and 16 nominal species in the pale-broad catascopium/emarginata group" recognized by Burch [3] as I did through the “14 dark, skinny marsh-dwellers and 25 pale, fat river & lake-dwellers” recognized by Baker [4], taking for granted that essentially all of the shell and body color variance upon which this elaborate taxonomy has been hung since the 19th century must be attributable to phenotypic plasticity.  The only question in my mind was whether there might be some cryptic species hiding underneath the phenotypic noise.  About which, I concluded in May, "I just don't think we here in North America have ever caught a clue."

That was hyperbole. 

Of course there must be a genetic component to shell morphology, and of course shell morphology must give us some information about the evolution of the snails bearing them.  Reared in a common environment, lymnaeids, planorbids and physids do not converge on a single dextro-sinistro-planospiral axis of coiling.  Indeed, natural selection is only effective on the component of the variance that is heritable.  If all that shell variance Bengt Hubendick so treasured in the lymnaeid populations of Europe were simply ecophenotypic responses to current or substrate or predation, selection would grind to a halt just as surely as if no variance existed at all.

Thus the paper published late last year by Christer Brönmark and his colleagues at the University of Lund [5] arrives as an especially welcome addition to the worldwide research program on freshwater gastropod evolution.  For here we see an elegant demonstration not just that the shell morphology of lymnaeid snails is subject to great phenotypic plasticity, but also that after all the environmental component of the variance is boiled away, at least some genetic component remains.

Brönmark opened his paper with a demonstration that Swedish populations of Lymnaea peregra/balthica [6] wild-collected from ponds with molluscivorous fish tend to bear lower spired, more rounded shells than populations inhabiting fishless ponds.  He then sampled snails from four ponds with fish and five fishless ponds and reared their three-week hatchlings (20 hatchlings per population) in two treatments for 12 weeks, with fish and without.  When measured on the first principal component of shell morphology (71% of the variance) the progeny of all nine populations responded identically – slender shells in the absence of fish predation and rounded shells in the presence [Figure (a) above, Note 7].  But on the second PC (8% of the variance) the two groups responded differently to the absence of fish; populations originating in ponds with fish (filled diamonds in figure b) developing significantly wider second whorls than populations from fishless ponds (open squares). 

I wish that Brönmark’s paper had come to my attention a couple months ago.  He and his colleagues have offered us a lovely demonstration of apparently heritable and adaptive differences in shell morphology among conspecific populations of lymnaeids separated not by any great distances or barriers, but simply by their selection regimes.  Such a phenomenon can easily explain the observations of Turner & Brady I featured in May, without resorting to a hypothesis of cryptic speciation.  There may be something to that Darwin stuff, after all.

Notes

[1]  These are some of my favorite laboratory studies of shell phenotypic plasticity in freshwater snails.  By no means an exhaustive list:  Arthur, W. (1982) Control of shell shape in Lymnaea stagnalis. Heredity 49:153-161.  Auld, J. & R. Relyea (2011) Adaptive plasticity in predator-induced defenses in a common freshwater snail: altered selection and mode of predation due to prey phenotype.  Evol. Ecol. 25:189-202.   DeWitt, T. (1998) Costs and limits of phenotypic plasticity: Tests with predator-induced morphology and life history in a freshwater snail.  J. Evol. Biol. 11:465-480.  Holomuzki, J. & B. Biggs (2006) Habitat-specific variation and performance trade-offs in shell armature of New Zealand mudsnails.  Ecology 87:1038-1047.  Hoverman, J. & R. Relyea (2007) The rules of engagement: how to defend against combinations of predators.  Oecologia 154:551-560.  Krist, A (2002) Crayfish induce a defensive shell shape in a freshwater snail.  Invert. Zool. 121:235-242.  Lam, P. & P. Calow (1988) Differences in the shell shape of Lymnaea peregra (Muller) (Gastropoda: Pulmonata) from lotic and lentic habitats; environmental or genetic variance?  J. Moll. Stud.  54:197-207.  Urabe, M. (1998) Contribution of genetic and environmental factors to shell shape variation in the lotic snail Semisulcospira reiniana (Prosobranchia: Pleuroceridae).  J. Moll. Stud. 64:329-343.

[2] The Lymnaeidae 2012: Tales of L. occulta [23Apr12]

The Lymnaeidae 2012: Tales from the cryptic stagnicolines [10May12]

[3] Burch, J. B. (1989) North American Freshwater Snails. Hamburg, MI: Malacological Publications.

[4] Baker, F. C.  (1911) The Lymnaeidae of North and Middle America, Recent and Fossil. Special Publication, no. 3. Chicago: Chicago Academy of Natural Sciences.

[5]  Brönmark, C., T. Lakowitz, and J. Hollander. 2011. Predator-induced morphological plasticity across local populations of a freshwater snail. PLoS ONE 6 (7):e21773.  [Open access]

[6]  Some European authorities have recently resurrected the Linnean nomen “balthica” to apply to the snail everybody for 200 years has called L. peregra, despite the fact that Linnaeus (1758) gave the habitat of his Helix balthica as the littoral zone of the Baltic Sea.  Since Linnaeus used the genus Helix for land snails, my guess would be that balthica was probably what we today call a Littorina.   Hubendick simply dismissed the nomen as, “Probably no Lymnaea.”   But you be the judge:  "H. testa imperforata ovata acuminata, rugis elevatis, apertura ovata amplicata."  (Systema Naturae pg. 775).

[7] This is the left half of Figure 2 from Brönmark et al. (reference above).  "Reaction norms of shell morphology in Radix balthica from the common garden experiment."