The occurrence of an endozoic green alga in the marine mollusc, Clinocardium nuttallii (Conrad, 1837)

Samples of mantle tissue of the Pacific common cockle, Clinacardium nuttallii (Conrad, 1837) which contained green inclusions were excised and examined using light and electron microscopy. Green tissue was found to contain intercellular aggregations of a green alga (considered a Chlorella sp.) apparently actively growing within the body of the mollusc. Algal cells were found in all stages of growth and division, and were morphologically similar to several other previously studied symbiotic Chlorella from other hosts. A thick amorphous outer layer was usually found beyond the outer cell walls of the alga, often forming a matrix enclosing several algal cells. This outer layer or matrix appeared to be of algal origin. Algal cells were also found within the amoebocytes of Clinocardium. It is suggested that the association may be considered parasitic, or at best, commensal, rather than mutualistic.