Nearly all marine animals harbor epibionts, organisms living on their body surfaces. The positive or negative effects that epibionts have on their hosts depend on many factors, including the size and location of the epibionts on their host. The present study examined the effects of epibionts on gas exchange, locomotion, and drag of three species of Antarctic sea spiders (pycnogonids). Sea spiders are a cosmopolitan group of marine arthropods that lack gills and rely instead on the diffusion of oxygen directly across their cuticle. Encrusting epibionts, such as bryozoans and algae, had only minor effects on surface oxygen levels, but they reduced the functional diffusion coefficient of oxygen through the cuticle by about half. Although these effects are significant locally and may be severe in individuals with high coverage by epibionts, the total coverage on most individuals was not high enough to significantly alter oxygen fluxes into the animal. Macroepibionts, such as barnacles, had no effect on host walking speeds, but they increased by two-to-threefold the drag experienced by host sea spiders. This likely increases the energetic costs of walking and increases the chance of being dislodged by high currents. These results suggest that epibionts can impose diverse costs to their hosts but only in subtle ways that depend on total epibiont coverage of the host and rates of water flow.