The effect of gregarine (Apicomplexa) colonisation on the functional response of the amphipod host

Gregarines are a notably understudied but widespread group of protists that colonise aquatic and terrestrial invertebrates. This limited understanding of gregarines and their interactions with their hosts results partly from the absence of established culturing techniques and our understanding therefore has heavily relied on field collections. This study utilised for the first time cultured Gammarus pulex populations and comparative functional response models to explore the effects of gregarine colonisation on the host's consumption of Chironomid prey. This study shows that both positive and negative G. pulex displayed a Type II functional response. There were no statistical differences in the functional response parameters between the two groups. These results suggest that, under the study conditions, gregarines may function as commensal symbionts within their G. pulex host. This is consistent with growing evidence for gregarines acting across a range of symbiotic roles within their hosts. These findings provide insight into the role of gregarines in G. pulex, an invertebrate species frequently used for field- and lab-based experiments, contributing to the evidence of the complex and varied gregarine host-symbiont interactions.