Many marine invertebrates have complex life histories that begin with a planktonic larval stage. These larval invertebrates often possess protruding body extensions, but their function beyond predator deterrence is not well documented. For example, the planktonic nauplius larval form of Crustaceans have both spines and setae. Using the epibiotic pedunculate barnacle Octolasmis spp. as a model species, we investigated the hydrodynamic consequences of the dorsal thoracic spine on swimming nauplii. Video motion analysis was used to compare Octolasmis spp. naupliar swimming behaviors with their spines intact and with their spines removed. Our analysis showed that Octolasmis spp. without spines swam more slowly than those with spines. Larvae whose spines had been removed showed an alteration in limb beat pattern suggesting compensation for the loss of the spine. Nonetheless, nauplii without spines showed more backward motion during the recovery stroke and had jerkier trajectories than individuals with spines. The dorsal spine appears to affect feeding as well as locomotion. Preliminary particle image velocimetry of flow around dynamically-scaled physical models of nauplii of Octolasmis spp. with and without the dorsal thoracic spine indicated greater flux of prey-carrying water through the capture zone of the model with a spine. Thus, body extensions in planktonic larvae can affect key ecological functions such as locomotion and feeding.