Light micrograph (darkfield) of a symbiotic relationship between microalgae and copepods. The microalgae are epibionts living on the surface of the copepod. Copepods are tiny crustaceans, about 1.5 mm in size. Normally, these species live separately, but in times when resources such as nutrition and oxygen levels are low, both benefit from a symbiotic relationship. Overgrown copepods, com計ared to those free from microalgae, are believed to have advantages in oxygen supply provided by the photosynthetic algae. At the same time, the copepod is the source of ammonium nitrogen the algae need. Magnification: x55 when printed at 10 centimetres wide at its longest edge.