In general, epiphytic organisms usually only derive physical support from their host and not nutrition.  In Soranthera’s case, this is not well documented and is hard to know whether or not this alga is actually utilizing some resource from Odonthalia or Rhodomela.  One hypothesis explaining the epiphytic interaction could be based on the morphological structure of the stipe.  Soranthera’s stipe is very small and attached to the host plant narrowly at the base.  The general morphology of Rhodomela and Odonthalia is similar suggesting a possible shared characteristic that allows Soranthera to successfully grow and reproduce only on this structure of host plant.  Soranthera has evolved particular stipe morphology specific to each host plant. These observations tend to support the reasoning for physical support as the basis for this epiphytic interaction. (Abbott, 1976)

Abbott, Isabella A., Hollenberg, George J.. Marine Algae of California.  Stanford, California: Stanford University Press, 1976.