Essays On the Prosobranchs

The rivers, lakes, and streams of North America are inhabited by a diverse assortment of gill-bearing freshwater snail populations collectively called "prosobranchs." Prosobranchs are typically longer-living and slower-growing than the more familiar pulmonate snails, inhabiting more permanent and stable bodies of water. There is reason to think that some prosobranch populations from Southern Appalachian drainages may have evolved hundreds of millions of years ago and remained morphologically unchanged ever since.

Collected in this volume are 37 essays on the systematics and evolutionary biology of North American prosobranch gastropods. The tension between gene trees and species trees, explored at some length using pulmonate models in Volume II, is aggravated in prosobranch populations by a phenomenon here described as "mitochondrial superheterogeneity." The challenge introduced by ecophenotypic variation in shell morphology, also evident in populations of pulmonate snails, is further explored. A new term, "Goodrichian Taxon Shift," is coined to describe intrapopulation variance so extreme as to prompt an erroneous hypothesis of speciation and generalized to "Cryptic Phenotypic Plasticity" in later chapters.

Some North American populations of freshwater prosobranch snails have been listed as objects of conservation concern. This introduces a new subtheme into Volume III, the relationship between science and public policy. The taxonomic revisions prompted by improvements in our understanding of the evolution of prosobranchs have sometimes met resistance both from natural resource agencies and from conservation biologists dependent upon such agencies for funding. The overarching theme of the present volume becomes, then, one of a struggle to understand the evolution of the freshwater prosobranch gastropod fauna of North America, both finely scientific and clumsily human.

Volume I in this series reported the scientific results from our survey of the freshwater gastropods of United States Atlantic drainages, Georgia to the New York line.  In the preface to that volume, I mentioned that during the 20 years over which the FWGNA Project has unfolded, I developed the habit of sending regular emails to an expanding list of collaborators, a practice which ultimately evolved into an internet blog.  Those emails and blog posts have sometimes contained important supplementary information on the biology of the fascinating organisms toward which the FWGNA Project has been directed, as well as historical background, context, and rationale for various methodological and taxonomic decisions made in Volume I.

Volume II collected 29 essays on the freshwater pulmonate gastropods.  Here in Volume III we have assembled 37 essays on the evolutionary biology and systematics of the prosobranchs, published online between 2006 and 2019.  These essays have been edited rather heavily from the form in which they were originally posted, in many cases, and re-ordered into subthemes.  Essays 1 – 27 focus on the pleurocerid gastropods, my favorite group of organisms in the world, and essays 28 – 37 the hydrobioids, with topics of general evolutionary and systematic interest sprinkled throughout.  Each essay opens with its date of publication, which is important to notice, because single subthemes can span multiple years, and my own thoughts have often evolved over time.

We have deferred discussion of the Viviparids and the Ampullariids to Volume IV, which will be organized around ecological and biogeographical themes.  This is partly due to space considerations, and partly because the most prominent elements of both families are invasive.  It is probably unfair to tar higher taxa by the reputations of their most notorious species.  For this I apologize.