Algae and Brown algae

Algae noun - Primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves.

Brown algae and algae are semantically related. In some cases you can replace phrase "Brown algae" with "Algae", this terms are similar.

Brown algae noun - Algae having the chlorophyll masked by brown and yellow pigments.

Algae and brown algae are semantically related. You can use "Algae" instead a noun phrase "Brown algae".