Algae: Reproduction in Spirogyra

In Spirogyra, sexual reproduction occurs by conjugation. The gametes are non-motile and look alike (morphologically similar), so morphologically the process is isogamous.
However, the gametes behave differently: the gamete of one filament (male) moves actively through the conjugation tube by amoeboid movement into the other cell, while the other gamete (female) remains passive.
So the sex cells are physiologically differentiated into male and female, although they are structurally identical (physiological anisogamy). This is considered an advanced feature compared with true isogamy.
There are no motile (flagellated) gametes in Spirogyra.
So sexual reproduction in Spirogyra shows physiologically differentiated sex organs (gametes).