Invertebrates do not have blood with three lineages of blood cells. Instead, they have hemolymph, and a single blood cell type called a hemocyte (functions mostly in host defense). Leukemias (clonal disorder of the hemocyte) have been observed in edible bivalve mollusks such as clams, mussels, cockles, and oysters. A study in the journal Cell showed that clam leukemia genotypes are distinct from their hosts and nearly identical to each other, suggesting. that the transmissible cancer clone likely arose in a single individual and that clam leukemia is transmitted horizontally between animals as contagious cancer cell. Wow!