Linked from
The 20 pages that link to Red algae, each with the reason it gives.
AgarNarrower topic: Agar is extracted from the cell walls of several red-algal groups.
CarrageenanNarrower topic: Carrageenan is extracted from the cell walls of particular red algae.
NoriNarrower topic: Nori-producing species belong to this broader algal group.
Edible seaweedNarrower topic: Nori and dulse belong to this broad algal group.