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.
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.
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."
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