In the shallow

The giant ciliate Zoothamnium niveum, covered with symbionts, loses them when deprived of sulfide. The propagules (called swarmers) seek for sulfide and if they stay two days swimming in water without it, they lose the symbionts too. Strikingly they still can found colonies that grow without symbionts and develop a different shape, similar to the ancestral shape of the genus. Sulfide starvation provokes an additional effort in the ciliate colonies to reproduce, so that they keep pace and send each a median of one swarmer to find better conditions. Symbionts during sulfide starvation become larger and more rod-shaped, while they vanish progressively from the ciliate surface.

All these results and more can be found in the following two papers:

Bright, Monika; Espada-Hinojosa, Salvador; Volland, Jean-Marie; Drexel, Judith; Kesting, Julia; Kolar, Ingrid; Morchner, Denny; Nussbaumer, Andrea; Ott, Jörg; Scharhauser, Florian; Schuster, Lukas; Zambalos, Helena Constance, and Nemeschkal, Hans Leo (2019). Thiotrophic bacterial symbiont induces polyphenism in giant ciliate host Zoothamnium niveum. Scientific Reports 9(1):15081

Espada-Hinojosa, Salvador; Drexel, Judith; Kesting, Julia; Kniha, Edwin; Pifeas, Iason; Schuster, Lukas; Volland, Jean-Marie; Zambalos, Helena C.; and Bright, Monika (2022). Host-symbiont stress response to lack-of-sulfide in the giant ciliate mutualism. PLOS ONE 17(2):e0254910