Many marine bacterial symbioses appear to be obligatory, thereby apparently defining the distribution of the host animal through the symbiont's requirements for energy. Whether the bacteria reside externally or endosymbiotically, in most cases we know little about their role in the association. Many of these symbioses are loosely classified as mutualistic, however in most cases this view has yet to be adequately tested.
Epibiotic associations appear the most widespread in marine systems.
Often epibiotic symbionts exist as a phenotypically mixed population making it impossible to decipher the role of independent members. Even with significant technological advances our ability to understand these associations has been severely limited because in most cases the symbionts cannot be cultured free of the host to enable metabolic characterization. A powerful means to describe this type of association is to use a molecular genetic approach.