Two vastly dissimilar organisms turn out to be the same species upon gene sequencing. In this hypothetical animal species one or more components of the life cycle is a fungus, algae, plant, lichen or other vegetative organism. The species may transition from animal to vegetative or vice versa between instars, it might alternate between vegetative and animal generations, or one of the sexes is vegetative.
How feasible would this be to evolve? What selection pressures would promote this adaptation? The ancestral organism doesn't need to be an animal.
(Inspired by Descolada from Speaker for the Dead and Orks from Warhammer 40,000.)
EDIT: here are some more detailed examples.
- In Speaker for the Dead, the capra is a ruminant that grazes exclusively on the capim grass. These are the female and male of the same species, respectively. When the capra graze they are fertilized by the capim pollen.
- Another species in Speaker for the Dead, the suckflies, hatch from the tassels of river reeds. These are the male and female of the same species, respectively.
- The most extreme example in Speaker for the Dead are the pequeninos, in which juvenile males molt into trees to reach maturity.
- In Warhammer 40,000 the orks produce spores which grow into fungi. These fungi produce underground uteri in which new orks gestate.