Bio-luminescent squid or other such animal leads to an ink pod that leaves a light trail. While it lasts, the ink provids minor illumination, but acts as a breadcrumb trail for longer sojourns or cave exploration. Customize color based on the player…
I apologize preemptively how jazzed I am about this discussion.
If you have the tech to get close to a deep sea vent without dying, some eel or microorganism could unlock research for heat resistance or hydroponic farming (alternative so…