Yellow-stained oikopleurid appendicularians are caused by bacterial parasitism

Abstract

While planktonic tunicates of the genus Oikopleura (Appendicularia) are generally quite colorless and transparent, specimens with an opaque, strong yellow colour have been reported occasionally for more than a century. Light and electron microscopy of 2 such yellow individuals; one O. vanhoeffeni , and one O. dioica , revealed large concentrations of rod-shaped bacteria in all interstitial and blood-filled spaces. In otherwise identical controls these spaces were filled by a completely transparent and acellular fluid. The 2 yellow appendicularians were actively feeding inside their jelly-like houses and expanded new houses in captivity. Thus they behaved like their healthy colorless congeners. The functional and pathological significance of this bacterial parasitism remains obscure.