Discovery of complemental males in a Pliocene accumulation of Chelonibia testudinaria (LINNAEUS, 1758), with some notes on the evolution of androdioecy in turtle barnacles

While mostly hermaphrodite, thoracican cirripedes (stalked and acorn barnacles) are remarkable in presenting also androdioecy and dioecy. With few androdioecious exceptions, the majority of the acorn barnacles of the order Balanomorpha are hermaphroditic. One of the balanomorphs in which androdioecy has been verified is the turtle barnacle Chelonibia testudinaria (LINNAEUS, 1758) (Coronuloidea: Chelonibiidae), an epibiont represented by various morphs found on several hosts, including sea turtles, crabs, horseshoe crabs, sirenians, sea snails, sea snakes and crocodylians. An accumulation of shells of C. testudinaria from the late Pliocene of Italy is here reappraised, and complemental males are discovered among them. These complemental males consist of miniature shells occurring largely on the radii of much larger hermaphrodites, a condition known for living C. testudinaria. To the author's knowledge, this record represents the first report of fossilized complemental males for a member of Balanomorpha worldwide. The evolution and persistence of miniature males in C. testudinaria is likely due to their insuring cross-fertilization when distances between neighbouring hermaphrodites are great and speeding the rate of reproduction in this relatively short-lived species. Chelonibia testudinaria is known from fossil specimens as old as the Pliocene and the record herein demonstrates that androdioecy evolved in the lineage of C. testudinaria not later than the Piacenzian. It is thus tempting to propose that a combination of high phenotypic plasticity, diverse host habits, and androdioecy is responsible for the persistence of C. testudinaria, a refugial species.