Ancestral reproductive structure in basal kelp Aureophycus aleuticus.

Laminarialean species (so-called kelps) are the largest photosynthetic organisms in aquatic environments, constituting significant ecological components of coastal ecosystems. The largest kelps such as Macrocystis exhibit differentiation between stipe and blade, as well as buoyancy to maintain the distal portion at the water's surface for photosynthesis, while bearing reproductive structures only near the base on special blades (sporophylls). There is a considerable gap between basic kelps such as Chorda and derived kelps, and the evolution of kelp specialization remains unclear.