Yersa was an Argelian exobiologist in the 23rd century. In 2267, she traveled to Sherman's Planet on a freighter carrying an emergency replenishment of quadrotriticale. After assisting Cyrano Jones in his investigations into tribble reproduction, she transferred to space station Deep Space Station K-7, and from there began to conduct field studies on planet Iota Geminorum IV. Her research discovered how tribbles were affected by their natural environment, in particular their reproduction rate. (TOS short story: "The Last Tribble")