Fucus is a genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of brown algae found in the intertidal zone

Intertidal zone

The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...

s of rocky seashore

Seashore

-Landform:* Coast* Intertidal zone, between high and low water lines* National seashore, a special designation in the United States* Shore-Other:* Seashore , an open source image editor, based on GIMP written in Cocoa for Mac OS X...

s almost throughout the world.
The thallus is perennial with an irregular or disc-shaped holdfast

Holdfast

A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. ...

 or with haptera. The erect portion of the thallus is dichotomous or subpinnately branched, flattened and with a distinct midrib. Gas-filled pneumatocyst

Pneumatocyst

In phycology, a pneumatocyst is a large float containing gas found in brown algae. An organism may have more than one. They provide buoyancy to lift the blades toward the surface, allowing them to receive more sunlight for photosynthesis....

s (air-vesicles) are present in pairs in some species, one on either side of the midrib.