This project explores the hidden world of microbes living inside planktonic animals called pteropods (“sea butterflies”)—specifically Limacina rangii, a key part of the Antarctic marine food web. These small, shelled planktonic snails help move energy and carbon through the ocean and are especially important in waters around the rapidly warming Western Antarctic Peninsula and the Ross Sea. Recent studies have revealed that a group of specialized bacteria called Mollicutes dominate the gut of L. rangii, but little is known about what these bacteria do or how they respond to environmental change. By analyzing samples collected over the past decade and partnering with international researchers, this project will study how the microbes inhabiting pteropods vary across time and space and how these microbial communities may help pteropods adapt to changing ocean conditions. Scientists will use modern genetic tools, including DNA sequencing and metagenomics, to uncover the diversity and function of these microbes and to understand the role they play in the health and ecology of their animal hosts. Beyond advancing science, the project serves the national interest by supporting early-career researchers, training graduate and undergraduate students, and engaging the public through hands-on educational events and classroom materials. By studying the relationship between marine animals and their microbes in one of Earth’s most vulnerable ecosystems, this work helps us better under