Ectocarpus siliculosus

Plants forming filamentous tufts to 30 cm high and, in sheltered waters, entangled mats over 1m long; epiphytic, epizoic, or saxicolous, attached by prostrate filaments; Vegetative cells are 15 um in diameter and of variable length due to an active intercalary meristem. Plurilocular sporangia are the most distinctive character and are about 12.5 um in diameter and 200 um in length. They are also born on branchlets (pedicells) of 2-3 cells.

(In: - http://chrsl.chem.lsu.edu/~wwwpb/Chapman/Ecto_sil_Ch.html)