Reproductive ecology and evolution in the invasive terrestrial planarian Bipalium adventitium across North America

Comparisons between invasive species and their relatives can help in identifying traits that facilitate dispersal and colonization, and comparisons among populations within invasive species may highlight the roles of plasticity and evolutionary adaptation. Bipalium adventitium Hyman 1943 is an exotic, earthworm-eating soil flatworm that has become widespread and abundant in North America. As a first step in better understanding the ecology and evolution of this invasive planarian, we studied its reproductive traits in populations across its extensive geographic range and conducted an experiment on the effects of food availability. Colonization by B. adventitium is facilitated by hermaphroditism, sperm storage, and tough egg capsules containing multiple offspring, traits present in most Terricola. The egg capsules produced by B. adventitium were large (22.5 mg), hatched in 7-37 days (mean=23 days), contained 1-8 offspring (mean=3.4), and represented about 21% of a parent's predeposition mass. Many individuals produced multiple egg capsules. Although considerable intraspecific variation was detected for most reproductive parameters, significant differences among geographic populations were noted for egg capsule mass, mean offspring mass, and measures of total reproductive output. Intraspecific correlations between parent mass and the masses of egg capsules and offspring, and the results of our experiment showing that food availability affected egg capsule production, together suggest that thus far intra-population variation and plasticity have been more important than local evolutionary adaptation in the invasion of this species.