Osmundea pinnatifida is a small seaweed, typically up to 8 to 10 centimeters in length.  It is tough and cartilaginous with branched and flattened fronds growing from a discoid holdfast.  Branching is alternate in a single plane, with branches becoming shorter and more rounded towards the top.  Color and size of Osmundea pinnatifida varies depending on the overall sunlight available.  Higher shore plants are generally dwarfed and yellow-green to green with strong sunlight, but it ranges from purple to red to reddish brown under shady conditions on the lower shore, and can even be a near-black shade of red.  It can be common on exposed to moderately sheltered middle and lower rocky shores, often covering large areas with a greenish-yellow turf like growth in pools and on rocks but never subtidal. 

Source: https://seaweedindustry.com/seaweed/type/osmundea-pinnatifida