A Global Synthesis of the Correspondence Between Epizoic Barnacles and Their Sea Turtle Hosts.

Barnacles are the epitome of a sessile animal, so it seems incongruous that some are among the widest-roving invertebrates on the planet, maybe beyond even what Mr. Quammen has conceived. All members of the barnacle superfamily Coronuloidea have acquired the borrowed ability to travel many hundreds, or thousands of kilometers over a lifetime as epibionts of their mobile hosts or basibionts. These epizoites of sea turtles, sea snakes, crabs, whales, and other mobile fauna (Darwin 1854; Zann 1975; Hayashi 2013a), utilize their live substratum as a platform for feeding and dispersal, but not as a source of nutrition. The greatest number specializes on sea turtles but their host repertoires and specificity for particular turtle species have not previously been rigorously evaluated. I endeavor to fill this gap to gain greater understanding of these associations and insight on how barnacles select a mobile home. The nature of barnacle–turtle relationships falls somewhere between parasitism and mutualism. Not a phoretic symbiosis, whereby one animal attaches to another temporarily for conveyance (White et al. 2017), the association is most frequently considered a commensalism. The term refers to feeding at a common table (van Beneden 1876) and results in the commensal deriving benefit from the arrangement while the host remains unaffected. However, the view that turtle hosts are not negatively affected by barnacles is, depending on the situation and the symbiont involved, often plausible, in some cases debatable, and occasionally untenable. Commensalisms are rarely demonstrated as obligate and specific (Wahl and Mark 1999) and many are not stable, tipping toward the benefit or detriment of the host depending on circumstances; indeed, the argument has been made that the concept of commensalism, in the narrow sense, is a theoretical state that cannot be empirically demonstrated (Zapalski 2011). Thus, multiple states apply for epizoic barnacles. Whatever the designation, advantages certainly accrue to barnacles by living on mobile hosts, including access to reliable currents for passive feeding and protection from benthic predators (Foster 1987). The obligate nature of coronuloid barnacles on their hosts, especially as associates of whales and turtles (Scarff 1986; Hayashi 2013a), is well-demonstrated, with several taxa known from only a single species of cetacean or chelonian (Marloth 1900; Monroe and Limpus 1979). Of the more than 200 species of epibionts documented from sea turtles (Frick and Pfaller 2013), most are facultative associates, but barnacles are the most common obligate taxa. Yet, despite their affinity for sea turtles, a one barnacle–one turtle paradigm is likely the exception rather than the rule. For example, the common “turtle” barnacle, Chelonibia testudinaria (Fig. 1A), has the most elastic host use, regularly utilizing manatees, various crabs, and all species of sea turtles as basibionts (Zardus et al. 2014) and exceptionally, other kinds of reptiles and crustaceans (Ross and Jackson 1972; Badrudeen 2000; Ortiz et al. 2004; Nifong and Frick 2011), even various synthetic substrata (Edmondson and Ingram 1939; Frazier and Margaritoulis 1990; Sloan et al. 2014). Lacking an all-encompassing label to describe the relationship of barnacles with sea turtles, it is perhaps most easily summarized as an obligate association of generally intermediate specificity with neutral to negative consequences for the host. Open in a separate window Fig. 1 Barnacle exemplars that attach to sea turtles by: (A) cementing, Chelonibia testudinaria; (B) clinging, Platyleaps hexastylos; (C); embedding, Stomatolepas elegans [the shell is right side up!]; (D) penetrating, Stephanolepas muricata; and (E) boring, Chelolepas cheloniae (scale bars = 5 mm).