Comparing Species Concepts in Lamprey “Paired Species”

In North America, at least seven lamprey species pairs are recognized in which the filter-feeding larvae are morphologically similar but the adults differ dramatically, becoming either parasitic or non-parasitic (i.e. non-trophic) following metamorphosis.   It is widely accepted that non-parasitism arose independently in each pair.  Conspicuous morphological differences (particularly adult body size) distinguish non-parasitic adults from parasitic forms, and most lamprey taxonomists (applying the Morphological Species Concept) consider each a distinct species.  Due to size assortative mating, it is assumed that reproductive isolation between parasitic and non-parasitic lampreys would be absolute and immediate; thus the Biological Species Concept would also dictate that each be recognized as distinct species.  However, a growing body of molecular data is demonstrating that wide-ranging non-parasitic species are invariably polyphyletic (supporting an insightful suggestion by Carl Hubbs and Milton Trautman in 1937 that non-parasitism has evolved repeatedly within individual pairs) but are frequently genetically indistinguishable from their sympatric parasitic counterpart.  Thus, according to the Phylogenetic Species Concept, paired species are NOT distinct species.  Furthermore, in at least one pair, even high-resolution genetic markers have been unable to distinguish between parasitic and non-parasitic lampreys when sympatric, questioning the previous assumptions of reproductive isolation.  What’s a taxonomist to do?