On two barrier islands of Georgia, U.S.A., three species of sandspur (genus Cenchrus) occupy different types of habitats: Dune sandspur (C. tribuloides) occurs in newly formed dunes; southern sandspur (C. echinatus) occurs in established dunes; and coast or field sandspur (C. incertus) in areas associated with high levels of human disturbance (“waste places”). All three species have seeds with spines. I investigated the roles of spines in each species' natural history, giving particular attention to the contrasting habitats occupied by each. Seeds from C. tribuloides typically float more than twice as long as seeds of the other two species, as might be expected for this coastal species, yet experimental removal of spines did not decrease the floating duration for any species. In a wind tunnel, unaltered seeds of C. echinatus dispersed at lower wind speeds than did the other species, but experimentally removing spines gave its seeds wind-dispersal properties similar to the other two species. This suggests that spiny seeds increase wind dispersal for that species occupying habitats of open sand flats, precisely where wind dispersal is common. Tethered spineless seeds of C. tribuloides (the only species with seeds large enough to tether) were also more susceptible to predation by crabs, the most common granivore of the sand dunes. Therefore, spines serve to protect against predation in open areas. This study demonstrates the multiple roles that spines can play in seed performance and how these roles may differ among species according to the habitats in which they occur.