The sunleecher split from its ancestor. It now has a radical new way of life compared to its ancestor. Instead of just sustaining itself solely on sunlight, the sunleechers also formed a symbiotic relationship with sagavermes.
First, they attach themselves to the gills or breathing apparatuses of the sagavermes with the hooks on the sides of their cell walls, and then break the skin. They use the blood to gain nutrients. They also get the added bonus of locomotion and protection from predators. They grow into thick mats that eventually cover the lining of the gills or breathing apparatuses entirely.
In exchange for this, they use their gas chambers to convert carbon dioxide from the water or air and turns it into oxygen through photosynthesis, which they injects directly into the blood stream of their hosts. This provides their hosts with a more efficient way of taking oxygen. Allowing air breathing organisms are now able to hold their breathe for a much longer time, and water breathing creatures to breathe out of water for a few minutes.