Final answer:
The relationship between spider crab and algae represents commensalism; the algae benefits from mobility and access to light while the spider crab is not significantly affected.
Step-by-step explanation:
The symbiotic relationship between the spider crab and algae falls under commensalism, which is a type of symbiosis where one species benefits and the other is not significantly affected. In this relationship, algae may attach themselves to the spider crab's shell.
This provides the algae with mobility and access to light, which can increase their photosynthetic ability and thus their growth. The spider crab, however, remains largely unaffected by the presence of the algae.
This relationship differs from mutualism, where both organisms benefit, and from parasitism, where one organism benefits at the expense of the other.