Ectosymbiosis

The ectosymbiotic species, or ectosymbiont, is generally an immobile (or sessile) organism existing off of biotic substrate through mutualism, commensalism, or parasitism.

[3] Ectosymbiosis has evolved independently many times to fill a wide variety of ecological niches, both temperate and extreme.

Additionally, in the case of mutualism, the evolution improved the fitness of both species involved, propagating the success of ectosymbiosis.

[7] Ectosymbiosis adds to the biodiversity of the environment, whether on land, in freshwater, in deserts, or in deep sea vents.

This niche specialization between species also leads to stabilization of symbiotic relationships between sessile and motile organisms.

Although ectosymbiosis is typically an evolutionary stable behavior, the different host and parasite dynamics independently vary in their stability.

[5] Groups of organisms – greater than a single pair of a host and parasite – can also form mutualistic ectosymbiotic interactions.

Additionally, mature Branchiobdellida bacteria act as a nutrient thief in the gut of crayfish species to exist.

European mistletoe is an example of an ectosymbiotic parasite that lives on top of trees and removes nutrients and water.
Sea urchins , with their many spines, provide protection for the ectosymbiotic parasites that live on them.
Remora fish form ectosymbiotic commensal interactions with lemon sharks in order to scavenge food and travel long distances.
Branchiobdellid annelids are mutualistic parasites. They will attach to a signal crayfish and feed on diatoms , bacteria , and protozoans that accumulate on the exoskeleton.
The head louse is an ectosymbiotic parasite that feeds off of the blood of humans by attaching itself to the scalp.