After creating ‘Chlorophyll B’, you can use more of the light spectrum and grow faster.

At this point, you’re quite large, and living in clusters in the sea and in freshwater rivers. Your flagellum-growing genes are turned off, as it’s a waste of energy to grow one when you live in a cluster. The genes are still there though - they just don’t do anything for now.

Some of you happen to be in tidal areas or pools where you dry out when the water goes away.

This is mostly acceptable - the benefits of lots of light are balanced by the damage done by drying out - except for your reproductive cells. Currently, when you release new cell clusters which will grow into new group organisms, they sink to the bottom and grow.

Now that you are invading tidal zones and small pools, they get thoroughly dried out and irradiated by ultra-violet light before the water comes back. This is enough to damage the important DNA inside them and prevent the new plants from growing.

If you want to hang out in the tidal zone and shallow waters, you’ll need to keep the sun off them. Or you could stay in deeper water?