Ascidian tunic cells: morphology and functional diversity of free cells outside the epidermis

Tunic cells are free cells distributed in the tunic, the integumentary matrix of tunicates. In ascidians, various types of tunic cells have been described both in solitary and in colonial species. Many of them are functionally specialized and are related to the protection of the animal, such as phagocytosis to prevent infection, acid storage to avoid predation, and pigmentation to protect against solar radiation. While some tunic cells are known to play a role in colonial allorecognition, bioluminescence, and algal symbiosis, the functional roles of many cell types still remain to be determined. The composition of tunic-cell types varies among ascidian species, most likely reflecting the functional requirements of the tunic in each species. Although some cell types, e.g., tunic net cells and tunic bladder cells, are restricted to particular taxa of ascidians, tunic phagocytes are found in all known ascidians. Therefore, tunic phagocytes are hypothesized to be basal and shared with ancestral tunicates. In some ascidians, phagocytic cells are involved in other functions, such as pigmentation, intracellular photosymbiosis, and bioluminescence. These specialized phagocytic cells are hypothesized to be derived from tunic phagocytes, suggesting that tunic cells have a high potential to diversify and evolve a wide variety of cellular functions.