What Is the Evidence for the ‘20% Body Size Rule' for Mating As a Speciation Mechanism Among Lampreys (and What are the Implications for Conservation and Management)?

Lampreys are interesting because of their ancient lineages, unique life histories, and disparate management scenarios.  For example, although the Sea Lamprey Petromyzon marinus is a well-known pest in the Great Lakes, many species worldwide are targets of conservation.  Body size has been hypothesized to be an important factor in speciation of the approximately 40 extant species of lampreys.  If this hypothesis is true, then information on body size could be one essential tool to estimate and predict the mating potential among sympatric species in general, and ‘paired’ parasitic (anadromous or adfluvial) and nonparasitic (brook) species in particular.  We reexamine Beamish and Neville's work (1992 Copeia) that led to the 20% 'rule' on body size as a reproductive isolating mechanism between Western River Lamprey (Lampetra ayresii) and Western Brook Lamprey (L. richardsoni).  Through laboratory breeding trials, they suggested a difference in total body length >20% precluded successful reproduction between the species.  We extend examination of this phenomenon to comparisons of body size data across other lamprey species to look for potential universal trends.  We provide a brief summary from other work on genetics and mating trials to supplement our survey of data on body sizes among lampreys.