- Kingdom: Plantae
- Phylum: Ochrophyta
- Class: Ulvophyceae
- Order: Fucales
- Family: Fucaceae
- Scientific name: Fucus vesiculosus
- Norwegian: blæretang
This brown seaweed is also known as black tang, rockweed, bladder fucus, sea oak, cut weed, dyers fucus and rock wrack. The fronds grow to a length of 90 cm. The almost spherical bladders are usually paired, one on either side of the midrib. On exposed locations the bladders are sometimes missing. It comes in different shades of olive-green and brown.
It thrives on hard substrate in the tidal and subtidal zones. It thrives on exposed as well as sheltered locations.
This species seems to be confined to the North Atlantic Ocean.