SIZE SHIFT IN THE MEXICAN EARTHWORM SPECIES BALANTEODRILUS-PEARSEI (MEGASCOLECIDAE, ACANTHODRILINI) - A POSSIBLE CASE OF CHARACTER DISPLACEMENT

The earthworm genus Balanteodrilus includes three well-differentiated species. One of the species (B. pearsei) is common to eastern and sout h-eastern Mexico. All but one population of B. pearsei are medium-size d earthworms (6.4-11 cm in length). The smallest morph of B. pearsei w as found sympatrically coexisting at La Mancha forest with another spe cies of the genus. In this locality, resources are scarce, the two spe cies present different vertical distributions and their abundances are relatively low. A change of size in this population was not the resul t of random variation. Three possible explanations are proposed: (1) c haracter displacement resulting from past long-term competition; (2) r esponse to poor soil nutrients; or (3) reproductive character displace ment or reinforcement. (C) 1997 Elsevier Science Ltd.