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 south-eastern Mexico. All but one population of B. pearsei are medium-sized earthworms (6.4–11 cm in length). The smallest morph of B. pearsei was found sympatrically coexisting at La Mancha forest with another species of the genus. In this locality, resources are scarce, the two species present different vertical distributions and their abundances are relatively low. A change of size in this population was not the result of random variation. Three possible explanations are proposed: (1) character displacement resulting from past long-term competition; (2) response to poor soil nutrients; or (3) reproductive character displacement or reinforcement.