9th Grade Biology SOL Practice Test

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Which description matches commensalism?

One species benefits and the other is harmed.

Both species benefit.

One species benefits while the second is neither helped nor harmed.

Commensalism is a relationship where one organism benefits and the other is unaffected. The benefiting partner gains things like food, transport, or a place to live, while the other organism doesn’t gain or lose anything significant. A classic example is barnacles on a whale: the barnacles get a place to live and access to waterborne nutrients, while the whale isn’t helped or harmed in a meaningful way. So describing it as one species benefits while the second is neither helped nor harmed matches this relationship. The other possibilities involve harm to one or both partners or benefits to both, which do not describe commensalism.

Both species are harmed.