 Some capsomers on the external shell have fibres extending away from the virus to aid in host attachment.Chloroviruses infect certain unicellular, eukaryotic chlorella-like green algae, called zoochlorellae, and are very species and even strain specific.These zoochlorellae commonly establish endosymbiotic relationships with the protozoan "Paramecium bursaria", the coelenterate "Hydra viridis", the heliozoon "Acanthocystis turfacea" and other freshwater and marine invertebrates and protozoans.