Binucleusphotoedo rami branched off from B. stringi as a group drifted southward. B. stringi formed long strings, each a single cell thick, growing as individual cells split in two. B. rami has developed the ability to branch; when a cell with neighbors on either side divides in two, it may split off in a new direction. Thus, they can spread out in two dimensions rather than one. This increases their intake of sunlight, which is low and spread out in these polar waters.
The overall colonies retain a flat shape, allowing them to float on the surface of the water, forming a thin greenish film.
For the most part, cells reproduce by binary fission, thereby spreading and branching the colony. Likewise, the colonies reproduce by fragmenting into smaller pieces, which will themselves grow further, as waves and such break the colonies apart. However, when cells are too crowded, with nowhere to grow out to, they've developed a new trick. Then, a cell will split off two new cells, each with only one of the original nuclei. These single-nucleus cells drift off through the ocean until they encounter another of their kind, at which point they merge into a single two-nucleus cell, exchanging genetic material and founding a new colony.