Surface interactions of the epiphytic macroalga Hincksia mitchelliae (Phaeophyceae) with the shoalgrass, Halodule wrightii (Cymodoceaceae)

Meadows of Halodule wrightii (Cymodoceaceae) underwent a decline in a tidal flat located at Paran- ́ agua Bay (Parana, SE Brazil). This decline appeared to be related to an overgrowth of the epiphytic mac- roalga Hincksia mitchelliae (Harv.) P. C. Silva (Phaeo- phyceae). In order to characterize the type of epiphytism between the alga and its plant host, we compared two samples from the beginning and end of the algal overgrowth via electron and optical microscopes. The investigation revealed that at both sampling periods, there was an epiphytism of type II, which is due to an infection of epiphytes strongly attached to the surface of the host but not associ- ated to any apparent direct host-tissue damage. The presence of plasmodesmata between the cells of Hincksia only in the late stage of the host–epiphyte interaction indicated a change in the vegetative orga- nization of Hincksia in relation to its host to improve nutrient absorption and distribution through the epiphyte cells. This is the first report on plasmodesmata in H. mitchelliae. The proposed mechanisms with which the algal epiphytes lead seagrasses to death are shadowing by adhesion on Halodule surface and disruption of its osmoregula- tory system. Our findings have implications for the conservation and management strategies of seagrass ecosystems.