Calder’s expertise was in the biological sciences. Although serving in operations on stardate 3421.7 in the year 2267, she was part of a landing party sent to the wandering asteroid Tactis II with the assignment of studying any life found on the surface.
For the survey, she proposed that the landing party travel via shuttlecraft instead of by transporter, so as to allow larger bio-detection equipment to be brought down. She and James T. Kirk were assaulted by a lifeform on the surface which knocked them out and created duplicates of them. After Leonard McCoy revived them, she correctly assessed that the Tactisian was a parasite able to drain life forces, was shaped like protein molecule chains, had a central core living within a volcano, and was capable of deploying smaller pieces of itself to hunt down and absorb energy. She theorized that the lifeform hibernated to conserve energy and was capable of generating evidence of movement in order to lure passing energy sources to the surface. (TOS comic: "The Mimicking Menace")