Comparative Morphological Analysis of Two Species of Turtle Leeches Coexisting in North America (Hirudinea: Glossiphoniidae): Embryological Evidence for Character Displacement

Abstract

The genus Placobdella (Glossiphoniidae) has a centre of species concentration in North America. The type species P. costata is the only representative in the Palaearctic region. American Placobdella which feed on turtles are represented predominantly by two common species, P. parasitica and P. rugosa, which geographically overlap in eastern USA and southern Canada. The latter species is morphologically indistinguishable from P. multilineata of southeast USA. These two tuberculated forms are recognised herein as a clade and treated as a single ‘species’ for comparative purposes. Both P. parasitica and P. rugosa clade commonly coexist and feed on the same turtle species without host preference. This paper addresses morphological differences between them. An unexpected finding is that hatchlings of both species are very difficult to distinguish. A morphologically significant observation is that developmental divergence results in tubercle prominence in P. rugosa clade, but tubercle suppression in P. parasitica, the first example of character displacement in the Hirudinea. Morphological differences are interpreted as reflecting interspecific competition, a phenomenon not found in their Palaearctic counterpart. Why do the two American turtle leech species coexist rather than reduce competition by partitioning their food supply?

Keywords

• coexistence
• co-infection
• syntopic
• comparative morphology
• interspecific competition
• adaptive evolution