No evidence for quorum sensing during egg hatching in the cestode Schistocephalus solidus

Schistocephalus solidus, a parasitic cestode with a multi-host life cycle, reproduces in its terminal host either by outcrossing with similarly sized individuals or selfing. Previous work found that selfing greatly depresses egg hatching rates, presumably as a result of inbreeding depression. We designed an experiment to test whether S. solidus evolved quorum sensing (QS) during hatching as a mechanism to facilitate synchronized infection, thereby increasing the opportunity for outcrossing in its terminal host. We also performed exploratory analyses to test whether QS varies across parasite populations and cross type (i.e., whether progeny were produced via outcrossing or selfing), though these had limited statistical power due to low sample sizes across treatments. We predicted that if QS was present, it would be common across all populations and that higher egg density within a small area would result in higher hatching rates. We also expected that outcrossed eggs would hatch at higher rates than those produced via selfing. While we found different hatching rates between populations, there was no evidence for QS. We also observed that selfed eggs hatched at lower rates than outcrossed eggs, replicating previous findings. Although we failed to find density dependent hatching within the scope of our sample size, we discuss the conditions that may either favor or disfavor QS evolution across S. solidus and other helminth populations.