All animals live in symbioses with microbes, and one interesting association is between animals and algae. The breakdown of coral-algal symbiosis resulting in bleaching is a reminder that there is still much to learn about these important associations. Acoel worms hosting symbiotic alga are infamous to aquarium hobbyists as “pests” that are hard to kill. However, some Acoela, like corals, host dinoflagellate symbionts, so the qualities that make Acoela “pests” also indicate incredible resilience to environmental changes. They may hold answers both to fundamental questions surrounding symbiosis, but also the very practical need to understand bleaching events.

 

In my PhD work, we make the surprising discovery that symbionts are highly mobile in Waminoa sp. (an acoel worm). Using in-toto imaging while tracking single cells within living animals, we discover an organismal-scale mobility of hundreds of dinoflagellate symbionts (Amphidinium sp,) inside “transport tunnels” within the tissue of an acoel worm (Waminoa sp.). We find that this host-mediated mobility of large 20μm dinoflagellate cells inside ultra-thin parenchymal cells (35nm thin) is enabled by host actin polymerization. Although highly confined in the unique parenchymal tissue, symbionts in Waminoa sp. move at a remarkable pace. We find this long-range mobility enables a highly dynamic spatiotemporal distribution of symbionts throughout the animal’s body, which manifests itself in cyclic clustering and in clump formation during regeneration. This discovery of host-mediated symbiont mobility provides a behavioral phenotype that invites future work 1) on a physiological timescale, asking if and how symbionts transition between mobile and non-mobile states, and 2) on an evolutionary timescale, asking how mobile symbionts may eventually become immobile.