A case of probable drilling predation on a Pliocene turtle barnacle (Crustacea, Thoracica)

By attaching to motile, long-living substrates, the suspension-feeding turtle and whale barnacles (Coronuloidea; families Chelonibiidae, Coronulidae and Platylepadidae) enjoy a continuous flow of seawater and nutrient particles while also significantly reducing the risk of predation. Consequently, very little (if anything at all) is known about the potential predators of these cirripedes. Here, I show that available illustrations of a compound rostrum from the Pliocene of Gran Canaria, which is the lost type specimen and only known individual of Chelonibia hemisphaerica, feature the presence of a circular, non-penetrative boring on the exterior of the paries. This perforation is interpreted as a drill hole left by a predatory gastropod (most likely a muricid). Although drill holes are relatively frequent in both fossil and Recent acorn barnacle hardparts, this appears to be the first report of such a shell modification in a coronuloid barnacle. The common absence of drill holes on coronuloid shells is explained by the fact that the latter are usually excluded from interaction with gastropods and other predators due to their attachment preference for large, actively swimming hosts. Targeted predation on a living barnacle occurring on a dead/inanimate substrate (or slowly moving host) or erroneous “predation” on an empty dead shell would both seem reasonable explanations for the remarkable occurrence of a drill hole in the type compound rostrum of C. hemisphaerica.