Editor’s Note – This essay was subsequently published as: Dillon, R.T., Jr. (2019c) The classification of the hydrobioids. Pp 205 - 210 in The Freshwater Gastropods of North America Volume 3, Essays on the Prosobranchs. FWGNA Press, Charleston.
I confess that simply composing a header for the present blog post was a minor challenge for me. How should I entitle an essay about a great big group of little tiny snails that really isn’t even a group? The noun “hydrobioid” (without taxonomic status) has found widespread application in the literature , but if “tiny little odd-lot leftovers” could be Latinized into something that sounded fancy enough, that’s the word I’m fishing for.
The non-group of snails that will be the subject of this month’s blog post are the freshwater representatives of the Superfamily Rissooidea, vanilla gastropods bearing cusps around the base of their median radular tooth. They give the impression of being smaller-bodied than is typical for the Class Gastropoda, although I’m not sure that’s true, and seem to include an unusually high proportion of shallow water, intertidal, or amphibious taxa. Sexes are separate, as is true for most gastropods, the penis arising from behind the head.
And I should hasten to add the disclaimer that hydrobioids are not “leftovers” in the American West. Throughout the Great Basin and Pacific drainages, hydrobioids can be the dominant (sometimes only) element of the freshwater gastropod fauna. Their endemicity is legendary, and conservation concerns legitimate . But here in The East, we naturally tend to focus on the pulmonates and the pleurocerids and the viviparids, and maybe, at the end of the afternoon, we might find a couple sullen little hydrobioids sucked onto a stick.
Through most of the 20th century, classifications of the hydrobioid taxa have typically recognized no more than five families worldwide. Essentially all workers have always recognized the (huge, diverse) Hydrobiidae of Troschel (1857), to which Thiele (1929) added the Old World Micromelaniidae and the marine Rissoidae, for example. Wenz (1939) also recognized three families: the Hydrobiidae, the Micromelaniidae, and the Truncatellidae, including Pomatiopsis. Any of my readership with a taste for taxonomic arcana are referred to the 1993 monograph of Kabat and Hershler  for a tabulation of 16 different, and often strikingly discordant, 19th and 20th century hydrobioid classification systems.
|Table 1 of Kabat & Hershler (1993)|
The FWGNA project, at our birth in 1999, adopted the broad-sense definition of the Hydrobiidae that arose from Kabat & Hershler’s scholarly review. So for the last 18 years we have recognized here in North American freshwaters the Bithyniidae (Gray 1857) with just one species, the Pomatiopsidae (Stimpson 1865) with just a couple species, and a huge, diverse Hydrobiidae with a zillion tiny little odd-lot leftovers.
But the dawn of molecular phylogeny was rendering Kabat & Hershler’s broad-sense concept of the Hydrobiidae obsolete even as we were adopting it. In 1998 an international team of researchers headed up by Tom Wilke and George Davis began publishing the first gene trees with hydrobioid taxa at their tips . The analysis of the Wilke team suggested that Troschel’s old family Hydrobiidae was polyphyletic, implying that many of the taxa previously considered subfamilies underneath the Hydrobiidae deserved promotion to the full family rank.
In retrospect, we might have updated our hydrobioid classification back there as early as 2001, and probably at least once or twice again in the mid-2000s as well. I confess that I’ve just been letting the pot simmer. But at some point one must serve.
So by 2013, the Wilke team, now including our good friends Bob Hershler, H-P Liu, and Winston Ponder among others, had “pushed their short DNA fragments to the limit” . Using a concatenation of two mitochondrial genes and one nuclear gene (CO1, 16s, and 18s) Wilke and colleagues classified individual representatives from 90 mostly  genus-level rissooidean taxa into approximately 21 family-level groups . The most important result, from our standpoint in the FWGNA kitchen, is that the old Troschel concept of a vast, inclusive Hydrobiidae has been boiled away.
From this day henceforth the old subfamily Amnicolinae (Amnicola, Lyogyrus) stands promoted to the Amnicolidae, the old Lithoglyphinae (Gillia, Somatogyrus, Holsingeria) is the Lithoglyphidae, and the old Cochliopinae (Littoridinops, Pyrgophorus) is the Cochliopidae. Our unwelcome guest from New Zealand, Potamopyrgus, is henceforth segregated into the separate-but-equal Tateidae. Left behind in the Hydrobiidae pot (s.s.) is the subfamily Nymphophilinae, which includes Marstonia, Floridobia, Notogillia and Spilochlamys. An updated FWGNA website went online this morning to reflect all this taxonomic churn.
The subfamily Fontigentinae has also been left behind in the old Hydrobiidae pot, but this ingredient should be considered especially volatile. The single Fontigentine species analyzed by Wilke and his colleagues, a Fontigens nickliniana sample from Michigan, actually clustered more closely with the European Bithyinellidae and Emmericiidae than the Hydrobiidae (ss). But I get the impression that the Wilke team really didn’t feel as though they had enough information to deal with little, local exceptions such as Fontigens seems to be, and ran short of patience. So the pot probably isn’t quite off the stove just yet.
I shouldn’t fail to mention that, although the Wilke classification system was based entirely upon molecular evidence, their 2013 paper did feature an extensive Appendix B tabulating 50 morphological and anatomical characters for 13 “selected” rissooidean families. All of the freshwater families mentioned above are included in this useful resource – the old Pomatiopsidae and Bithyniidae as well as the new Amnicolidae, Lithoglyphidae, Cochliopidae and Tateidae, and the condensed Hydrobiidae (ss). It will be helpful to have the 1998 review work by Hershler & Ponder  open on your desk if you want to dig through Wilke’s Appendix B.
As is universally expected for studies of this sort, Wilke and colleagues concluded their 2013 paper by calling for “additional analyses based on more and/or longer gene fragments.” Additional samples of tiny little odd-lot leftovers are not (apparently) wanted.
 I originally thought that the term “hydrobioid” was first proposed in 1979 by my mentor, G. M. Davis, in the same monograph in which he proposed elevating the subfamily Pomatiopsinae (Stimpson 1865) to the family level. But my good buddy Gary Rosenburg has more recently called my attention to several much earlier uses of the term, including Pilsbry, H. A. (1896) Notes on new species of Amnicolidae collected by Dr. Rush in Uruguay. Nautilus 10: 86 - 89.
 I have previously published several posts on the conservation of western hydrobioids, most recently:
- Megapetitions of the Old West [14July09]
I also did a 2005-06 series on Pyrgulopsis robusta, culminating with:
- FWS Finding on the Idaho Springsnail [4Oct06]
 Kabat, A. R. & R. Hershler (1993) The prosobranch snail family Hydrobiidae (Gastropoda: Rissooidea): Review of classification and supraspecific taxa. Smithsonian Contributions to Zoology 547: 1 – 94.
 Davis, G.M., Wilke, T., Spolsky, C., Zhang, Y., Xia, M.Y., Rosenberg, G. (1998) Cytochrome oxidase I-based phylogenetic relationships among the Hydrobiidae, Pomatiopsidae, Rissoidae, and Truncatellidae (Gastropoda: Prosobranchia: Rissoacea). Malacologia 40, 251–266. Wilke, T., Davis, G.M., Gong, X., Liu, H.-X. (2000) Erhaia (Gastropoda: Rissooidea): phylogenetic relationships and the question of Paragonimus coevolution in Asia. Am. J. Trop. Med. Hyg. 62, 453–459. Wilke, T., Davis, G.M., Falniowski, A., Giusti, F., Bodon, M., Szarowska, M. (2001) Molecular systematics of Hydrobiidae (Mollusca: Gastropoda: Rissooidea): testing monophyly and phylogenetic relationships. Proc. Acad. Natl. Sci. Phila. 151, 1–21.
 Wilke T., Haase M., Hershler R., Liu H-P., Misof B., Ponder W. (2013) Pushing short DNA fragments to the limit: Phylogenetic relationships of “hydrobioid” gastropods (Caenogastropoda: Rissooidea). Molecular Phylogenetics and Evolution 66: 715 – 736.
 Our buddy Tom and his extensive network of colleagues apparently all subscribe to the U1S2NMT3 rule – each genus usually being represented by one species, sometimes two, never more than three.
 The Wilke team listed 21 rissooidean nomina in the column labelled “classification” of their Appendix 1. Of these, 18 ended with the “idae” suffix suggesting that the team endorses family-level status, two nomina ended in “inae,” and one was simply called a “group.” The Wilke team also excluded several obscure family-level rissooidean taxa from their analysis entirely, for various reasons.
 Hershler, R. & W. F. Ponder (1998) A review of morphological characters of hydrobioid snails. Smithsonian Contributions to Zoology 600: 1 – 55.