EFFECT OF ALGAL EPIPHYTES ON THE MUSSEL MYTILUS-CALIFORNIANUS

The effects of facultative epiphytes on a bivalve host were studied on an islet near Santa Catalina Island, California. The primary cover in the mid-intertidal zone was a mosaic of red algal turf (Corallina officinalis, Gigartina canaliculata, and Gelidium coulteri) and clumps of the mussel Mytilus californianus. In certain circumstances the algae attached to and overgrew the mussels. In other marine habitats, facultative epibionts benefit bivalve hosts by masking them from predators. At Catalina, spiny lobsters (Panulirus interruptus) and oystercatchers (Haematopus bachmani) preyed on intertidal mussels. However, in tagging studies, overgrown mussels displayed a nonsignificant trend towards lower survivorship. A factorial experiment manipulating exposure to predators and the cover of epiphytes demonstrated that over-growth significantly reduced survivorship. There was no significant statistical interaction between treatment effects, indicating that the presence of algae did not protect the mussels from predators. Overgrown mussels also had significantly lower rates of growth and reproduction than naturally clear mussels of similar size and location. Experimental removal of the epiphytes caused significant increases in mussel growth and reproduction relative to overgrown controls. In the community circumstances chosen for this study, the epiphytic habit is a part of the processes in which otherwise freeliving algae become dominant on the rock surface.