Osmotrophy is a feeding mechanism involving the movement of dissolved organic compounds by osmosis. Organisms that use osmotrophy are called osmotrophs. Osmotrophy is used by diverse groups of organisms.^[1] Organisms that use osmotrophy include microorganisms like bacteria, many species of protists and most fungi. Invertebrate animal groups like molluscs, sponges, corals, brachiopods and echinoderms may use osmotrophic feeding as a supplemental food source.

Osmotrophy, as a means of gathering nutrients in microscopic organisms, relies on the cellular surface area to ensure that proper diffusion of nutrients occurs in the cell.^[2] In other words, an osmotroph is an organism that has their "stomach" outside of their body. Some osmotrophs may have an internal digestive system, while still using osmosis as a way to gain supplemental nutrients. With bigger organisms, the surface-area-per-volume ratio drops and osmotrophy becomes insufficient to meet nutrient demands. Larger, macroscopic organisms that rely on osmotrophy, compensate with a very flat, thin body. A tapeworm is an example of such an adaptation.

Fungi^[1] are the biggest osmotrophic specialist since they are major degraders in all ecosystems. For organisms like fungi, osmotrophy facilitates the decomposition process. This is a result of the osmotrophy resulting in metabolites that continue growth.

Some mixotrophic microorganisms use osmotrophy to acquire some of their energy.

• Autotrophy
• Heterotrophy
• Mixotrophy
• Phagotrophy
• Phototrophy