Spinyhead blennies occupy abandoned worm holes in coral heads on Caribbean reefs. We conducted a series of short-duration field experiments off St. Thomas, US Virgin Islands, which indicated that such holes are a limiting resource and that larger fish competitively dominate smaller fish for access to holes. Few fish that were added to coral heads lacking vacant holes were able to secure holes, and did so only by displacing larger residents after severe combat. However, when vacant holes were added to coral heads before adding fish, transplanted fish readily occupied the new holes. When holes were added to coral heads without also adding fish, the new holes were colonized by immigrants from the surrounding habitat. Similarly, when resident fish were removed from coral heads, the emptied holes were colonized. Immigrants were smaller than removed residents, and, in cases where resident fish changed holes following removals of neighbors, they moved to sites previously occupied by larger fish. Fish displaced up to 5 m returned to their original holes, consistent with the possibility that spinyhead blennies may occasionally leave their holes and search for sites of better quality. We conclude that intraspecific competition for shelter holes may limit the number of spinyhead blennies occupying a coral head.
