A green spotted Japanese sillago (Sillago japonica) was caught by a fisherman and brought to the laboratory
for pathological inspection. The green spots were abundant on the lateral line and more extensively so within
the mouth cavity. In both sites, green spots were embedded within the fish flesh and formed 2–3 mm
dome-shaped colonies. SEM revealed these colonies to harbor numerous unknown cells with small, surface
warts (ornamentations). Molecular analysis showed the cells were Desmodesmus (D. komarekii), a common
freshwater coccoid green alga found in ponds and rivers worldwide. It is uncertain how the host fish came
to be infected with the alga which was not merely attached externally but embedded within the flesh and
inside the mouth cavity. This is the first case of parasitic form of coccoid green algae in marine fish and provides
new insights into the variable nature of green algae.