Star Trek: 10 Crazy Facts About The Trill
6. Symbionts
What makes Trills unique is their symbiotic relationship with the symbionts they share their planet with. Both species evolved on the same planet, eventually joining together to survive. It was an evolutionary quirk quite specific to their home planet.
Very few symbionts live outside of a Trill host. The few that do so live in the Caves of Mak'ala in milky water where they communicate through electric impulses. They still rely on Trills to survive as several of them commit their lives to caring for the un-joined symbionts. Those symbionts stay in the Caves to propagate the species.
The process of joining benefits both sides incredibly. Symbionts have been known to live up to 550 years but cannot do so without a Trill host. The Trill host gains access to the lives, memories and experiences of their predecessors, which can be invaluable. Jadzia often spoke of the incredible impact joining with the Dax symbiont had on her.