The sea cucumber does not gain anything from the relationship, nor is it harmed means the relationship is commensalism.
what is commensalism ?
A long term biological relationship where one members species benefited from other in terms of nutrition while other species neither benefited nor harmed called as commensalism
The commensal get benefited in terms of nutrient, shelter, support, and movement.
Three types of commensalism, such as Inquilinism where host act as a permanent shelter for the commensal. Example, A woodpecker is a commensal where as saguaro cactus is a nesting place.
Second type is Metabiosis where dead organism make a habitat for another organism, such as bacteria growing in dead matter.
Thirdly, Phoresy where one organism transports another like
Arthropods live on other insects.
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