Clionid sponges, most commonly Cliona spp. In shells of mussels from Scandinavia and bottom-growing mussels from North America as well as other species of bivalves that live on the surface of the substrate including oysters and scallops . Host species Mytilus edulis . Impact on the host Penetrates the periostracum forming holes in the outer surface and a tunnel network throughout the shell.
Severe penetration reduces the commercial value of the mussel and weakens the shell.
Sponge tunnels may become inhabited by other organisms, such as polychaete worms, which can also reduce market value. Inner shell surface may show the presence of dark spots or bumps, depending on the degree of penetration by the sponge. References Lauckner, G. 1983. Diseases of Marine Animals. Introduction, Bivalvia to Scaphopoda.