Sef

Sef was the very first Trill symbiont to be joined to a humanoid host. 25,000 years ago, Sef was one of many "symbiont" who broke the surface of the water. It used it's telepathic abilities to entice fourlegged creatures to the water's edge where Sef would attach itself to them and ride on land. It would then return to the water and share its experiences with it's fellow swimmers.

One day, a Trill healer named Hodak joined the Sef symbiont with an ill woman, after noticing symbionts (previously called riverworms) attach themselves to animals. The transformation appeared to give the animals strength and Hodak believed that the attachment of a riverworm to the sick woman could have healed her. This was the first symbiosis. Ezri Dax experienced the memory of this event during her visit through the Annuted's mindscape.