Presence of epiphytic algae, Vertebrata lanosa, positively impacts host algae, Ascophyllum nodosum, by reducing desiccation rates.

This study explicitly aims to understand the potential positive effects of the intertidal red algal epiphyte Vertebrata lanosa on its brown algal host Ascophyllum nodosum via reductions in desiccation and herbivory rates. To determine the effect of this epiphyte on the desiccation rates of its host, desiccation rates of both the host and epiphyte were measured when allowed to dry in contact with or in isolation from each other in high and low light conditions. To test if V. lanosa might reduce herbivore pressure on A. nodosum by providing an alternative and preferred food source, the feeding rates of two common herbivorous snail congeners ( Littorina littorea and L. obtusata ) on both algae were measured when offered each alga simultaneously or in isolation. Specimens used were collected from Bailey Island, Maine, USA (43.721249, -70.001740) in July 2024 and May 2025. The preferences and consumption rates of our herbivores were fixed regardless of whether they had a choice of which algae to eat, indicating that this epiphyte does not benefit its host by altering the behavior of these dominant herbivore species. However, the epiphyte significantly reduced the hosts’ rate of water loss but only in low light conditions. As most intertidal Fucoid algae photosynthesize fastest at low to intermediate levels of desiccation, the presence of V. lanosa is likely to allow A. nodosum to have greater photosynthetic rates for longer, during periods of emersion, allowing for greater overall growth.