Noburu Sensui and Euichi Hirose (2018) In didemnid ascidians with cyanobacterial symbionts, the tunic has a specific peak absorbing ultraviolet radiation (UV-R) due to the presence of ultraviolet (UV)-absorbing compounds, which probably include mycosporine-like amino acids (MAAs). The UV-R absorbing tunic is supposed to protect the symbionts in the common cloacal cavity of the host colony. The histological distribution of UV-R absorption in the tunic was examined using a UV light microscope equipped with a digital camera, from which the low-pass filter of the UV-sensitive image sensor was removed. The cell peripheries of tunic bladder cells and cell-like objects were visualized with the trans-illumination of UV light, indicating UV-R absorption at that site. In contrast, tunic matrix and vacuolar content of tunic bladder cells appeared to lack of UV-R absorption, allowing damaging wavelengths to penetrate. Accordingly, UV-absorbing compounds are expected to be contained in the cytoplasmic matrix of tunic bladder cells and possibly other types of tunic cells.
Keywords: Colonial ascidian; Cyanobacterial symbiosis; Prochloron; Tunic, UV microscopy.