Siphon regeneration capacity is compromised during aging in the ascidian Ciona intestinalis

► The oldest animals in wild Ciona intestinalis do not regenerate the oral siphon. ► Lack of regeneration is accompanied by dramatically decreased cell proliferation. ► Stem/precursor cells for siphon pigment organs over-differentiate in old animals. ► Overdifferentation is due to excessive stem/precursor cell accumulation.