Using growth rates to estimate age of the sea turtle barnacle Chelonibia testudinaria

Sea turtles are known to host diverse communities of plants and invertebrate animals (Caine 1986; Kitsos et al. 2005; Robinson et al. 2017). Past analyses of the size, abundance, and/or species composition of the organisms present on an individual turtle have provided insight into the health (e.g. Flint et al. 2009), habitat use (e.g. Pfaller et al. 2014), and movement (e.g. Killingley and Lutcavage 1983) of the host turtle. Hitchhiking barnacles present a particularly interesting epibiont, since their presence or absence, as well as the chemical information stored in their shells may be useful to obtain knowledge about the biology, life history and movement patterns of the host turtle (e.g. Hayashi and Tsuji 2008). Yet limiting the use of these barnacles as indicator species is the lack of basic life history information, such as growth rates and age estimates. Understanding the growth of commensal barnacles could, for example, allow inferences about the length of stay in either neritic or oceanic habitats (e.g. Killingley and Lutcavage 1983). This could be achieved through the analysis of the size of commensal acorn barnacles (e.g. Chelonibia spp.), that tend to attach in coastal waters, versus the size of pedunculate barnacles (e.g. Lepas spp.), that are more oceanic in origin (Newman and Abbott 1980). Similar analyses of the relationship between size and lifespan of the barnacle could provide insights into the Abstract Epibionts can serve as valuable ecological indicators, providing information about the behaviour or health of the host. The use of epibionts as indicators is, however, often limited by a lack of knowledge about the basic ecology of these ‘hitchhikers’. This study investigated the growth rates of a turtle barnacle, Chelonibia testudinaria, under natural conditions, and then used the resulting growth curve to estimate the barnacle’s age. Repeat morphometric measurements (length and basal area) on 78 barnacles were taken, as host loggerhead turtles (Caretta caretta) laid successive clutches at Mon Repos, Australia, during the 2015/16 nesting season. Barnacles when first encountered ranged in size from 3.7 to 62.9 mm, and were recaptured between 12 and 56 days later. Fitting the growth measurements of these barnacles to a von Bertalanffy growth curve, we estimated the age of these barnacles as a function of their size. Length growth rate decreased over time in a nonlinear fashion, while basal area growth rate showed a linear relationship with age. The average estimated age of barnacles at first capture was approximately 6 months (barnacle length 30.3 ± 1.8 mm). The largest and oldest individual had a length of 62.9 mm when first measured, and was estimated to be 642 days old. The finding that C. testudinaria