Navigating captivity: Intercellular motility of a dinoflagellate symbiont in an acoel host

Biological individuality has long intrigued philosophers and scientists. Symbiosis - often marked by persistence of the community, but reproduction of the individual constituents - offers a unique lens to this concept. Important aspects of agency are the abilities to move, and to control one’s motion. In symbiosis with multicellular hosts, inhibition of symbiont motility and host-controlled symbiont mobility are common in order for the host to achieve community goals such as prescribed symbiont distribution within the host. Self-controlled motility inside a tissue is usually associated with parasitism. We introduce Waminoa sp., an acoel worm - without a gut, flat in morphology, and hosting symbiotic dinoflagellates - as an ideal platform to probe symbiont motility and individuality inside a host tissue. By using tracking microscopy and quantitatively analyzing motility patterns of symbionts within the host tissue, we discover and characterize impressive motility of dinoflagellate symbionts in Waminoa sp., and find that the observed motility may be mediated by host tissue architecture. We use electron microscopy and perturbation experiments to further probe the potential roles of symbiont motility mechanisms and host tissue structure in the observed motility behavior. Our findings raise intriguing questions about the interplays of motility mechanisms and tissue environment, and contribute to our understanding of agency in symbiosis.