Animal-chlorophyte photosymbioses: evolutionary origins and ecological diversity.

Photosynthetic symbiosis occurs across diverse animal lineages, including Porifera, Cnidaria, Xenacoelomorpha and Mollusca. These associations between animal hosts and photosynthetic algae often involve the exchange of essential macronutrients, supporting adaptation to a wide range of aquatic environments. A small yet taxonomically widespread subset of animals host photosymbionts from the core chlorophytes, a phylogenetically expansive clade of green algae. These rare instances of 'plant-like' animals have arisen independently across distantly related lineages, resulting in striking ecological and physiological diversity. Although such associations provide valuable insights into the evolution of symbiosis and adaptation to novel ecological niches, animal-chlorophyte photosymbioses remain relatively understudied. Here, we present an overview of photosymbioses between animals and chlorophytes, highlighting their independent evolutionary origins, ecological diversity and emerging genomic resources. Focusing on Porifera, Cnidaria and Xenacoelomorpha, we review shared and lineage-specific adaptations underlying these associations. We also contrast them with dinoflagellate-based systems to demonstrate their distinct ecological and cellular features. Our work sets the stage for elucidating the molecular mechanisms underlying these associations, enhancing our understanding of how interspecies interactions drive adaptation to unique ecological niches through animal-chlorophyte symbiosis.