Back in June of 2009, when last we touched on the systematics of ancylid limpets in North America, we were standing at a crossroads (1). Paul Basch’s monograph, which has formed the basis of our understanding of the group for many years, lists five species in the widespread genus Ferrissia: rivularis, fragilis, parallela, mcneilli and walkeri (2). But the DNA sequence data of Andrea Walther (at that time unpublished) suggested that only F. rivularis and F. fragilis were at all genetically distinct, subsuming parallela under the former nomen, and mcneilli and walkeri under the latter.
Then in the spring of 2009 came the freshly-published allozyme data of Dillon & Herman (3) demonstrating that South Carolina populations of Ferrissia were reproducing entirely by self fertilization, “voiding the biological species concept, and necessitating a retreat to the morphological.” And along with our allozyme data came the results of common-garden experiments suggesting that the morphological criteria by which F. rivularis and F. fragilis had previously been distinguished were ecophenotypic in origin. So in the absence of evidence that any morphological distinction might have a heritable component, Dillon & Herman synonymized the nomen F. fragilis under F. rivularis, leaving North America with but a single species of Ferrissia.
I am now pleased to report that Andrea Walther, together with her colleagues Jack Burch and Diarmaid O’Foighil, has cast additional light on this situation (4). Writing in the issue of Malacologia currently on the newsstands, the team from Ann Arbor has been able to correlate apparently reliable features of the Ferrissia shell apex with their DNA sequence data, pulling fragilis back out from synonymy under rivularis.
Populations of F. rivularis, in our newly clarified understanding of that taxon, bear shells in which the apex is unambiguous – the cap of the earliest (juvenile) shell remains at the tip of the conical shell of the adult – generally at the midline or very near it [photo at left above - click for larger]. But in populations of F. fragilis, the juvenile shell cap is not at the apex of the adult shell, but rather is located slightly below and to the right of the midline [photo right - click for larger].
Under the older (Basch) concepts of the species (5), populations of F. rivularis were understood to inhabit rocky streams throughout the Blue Ridge ecoregion east into the upper Piedmont of all four southern Atlantic states. Ferrissia fragilis populations were restricted to vegetation and debris in calmer rivers, ditches and swamps in the lower Piedmont and Coastal Plain.
In our newly clarified understanding, however, almost all the Ferrissia populations inhabiting southern Atlantic drainages appear referable to F. fragilis alone, including those bearing quite robust shells inhabiting high-gradient streams in the Blue Ridge.
The only populations of bona fide F. rivularis in southern Atlantic drainages appear to inhabit tributaries of the Potomac River in Northern Virginia, ranging south up the Great Valley into the upper James and Roanoke drainages. This much more restricted range for F. rivularis becomes rather strikingly similar to that of Physa gyrina, another pulmonate snail more characteristic of the American interior, especially in northern latitudes.
Ancylid limpets are among the most common and familiar elements of the North American freshwater macroinvertebrate fauna. It is oddly reassuring to see our understanding of such fundamental aspects of their biology shift in just a few years; indeed, in a matter of months. Our science is an active one. For that, we should be thankful.
(1) Just One Species of Ferrissia [10June09]
(2) Basch, P.F. (1963) A review of the recent freshwater limpet snails of North America (Mollusca: Pulmonata). Bull. Mus. Comp. Zool. Harvard Univ. 129: 399–461.
(3) Dillon, R. T. and J. J. Herman (2009) Genetics, shell morphology, and life history of the freshwater pulmonate limpets Ferrissia rivularis and Ferrissia fragilis. Journal of Freshwater Ecology 24: 261-271. [PDF]
(4) Walther, A. C., J. B. Burch and D. O’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.
(5) To be fair, Basch did notice differences in the apex of his five Ferrissia species. For F. rivularis, his couplet specified "apex in midline or slightly to the right." He attributed "apex subacute, often far in the right posterior quadrant" to F. walkeri. Regarding the apex of F. fragilis, however, he was silent.