The Silver Lamprey and the Paddlefish

Ichthyomyzon unicuspis and Polyodon spathula in the Mississippi River drainage provide opportunities to investigate an ancient predator-prey relationship. How do they coexist?  First, paddlefish are large relative to silver lamprey.  One lamprey is unlikely to remove enough blood from a paddlefish to exceed its regenerative capabilities.  Second, paddlefish tend to bear more wounds than attached lampreys, and breaching by paddlefish may limit a lamprey’s feeding time. Because paddlefish often aggregate, detached lampreys need not spend much time or energy seeking new hosts.  However, growth by lampreys in the Wisconsin River, where they coexist with paddlefish, is less than in Green Bay, where they do not, and natural selection should favor traits that reduce detachment during breaching events.  One example is the relatively large oral disc, especially in newly transformed lampreys beginning the parasitic phase.  Disc diameter relative to total length is significantly greater in the Wisconsin River than in Green Bay.  Another possible trait is nonrandom selection of attachment sites within the paddlefish branchial cavity, where lampreys may be more protected from physical effects of breaching.  Another hypothesis consistent with the tendency for paddlefish to bear many more lamprey wounds than attached lampreys reflects high lipid concentrations in paddlefish skin.