Ren (1980) described a new species of barnacle, Cylindrolepas sinica, collected from green sea turtles, Chelonia mydas (Linnaeus, 1758). Hayashi (2009) subsequently recorded Cylindrolepas sinica from three species of sea turtle: the green sea turtle, Chelonia mydas, loggerhead, Caretta caretta (Linnaeus, 1758), and hawksbill, Eretmochelys imbricata (Linnaeus, 1766). Subsequently, 13 species of the superfamily Coronuloidea, including Cylindrolepas sinica, were recorded during a 10-year (2002–2011) survey of epibionts attached to marine vertebrates from Japanese waters (Hayashi 2012). As shown in previous studies (Hayashi 2009, 2012), Cylindrolepas sinica is a common species on Japanese sea turtles.

This species was described as a cylindrical and rounded barnacle in previous studies (as shown in Fig. 1A and 1B). This study describes the intraspecific variation occurring in Cylindrolepas sinica and emphasises the morphological differences between Cylindrolepas sinica and related species. Brief comments on host selectivity are also presented.

Epibiotic barnacles were sampled from sea turtles on breeding beaches, in bycatch, and from strandings in Japanese waters (see Hayashi 2012). Specimens of Cylindrolepas sinica were collected from the turtle (skin and plastron) and preserved in 99% ethanol. The specimens were dissected and their soft parts mounted on slides with a drop of glycerine. The specimens examined have been deposited in Fujukan, the Museum of the University of Ryukyus, under accession numbers RUMF-ZC.

The rounded form (Fig. 1A and 1B) of this species (described by Ren 1980; Hayashi 2012) is found in the soft skin of sea turtles and forms colonies in proximity to other individuals, as well as in aggregations (Fig. 2A). The hexagonal variant of this species burrows into the hard parts of the turtle body (plastron) and is often found as isolated individuals (Fig. 2B). Therefore, the shell morphology of Cylindrolepas sinica exhibits phenotypic plasticity through habitat selection.

In a rare case, one individual was collected from a plastic tag attached to a loggerhead sea turtle (Fig. 2C, D). The tagged turtle was captured on June 24, 2010 in a set net at Yomitan, Okinawa, Japan, and recaptured in the same net on August 26, and finally found as a floating stranding nearby on September 26. These records indicate that this turtle was a resident in this coastal area, as reported in Hayashi (2009), and the presence of Cylindrolepas sinica is consistent with the previous report. In addition, this is the first record of the occurrence of Cylindrolepas sinica on an artificial object. This case indicates that Cylindrolepas sinica can attach to hard substrates, as well as living soft tissue, and the host selectivity of Cylindrolepas sinica is not the substrate material. Nogata and Matsumura (2006) reported the larval development and settlement of the whale barnacle, Coronula diadema (Linnaeus, 1767), which settled in a polystyrene Petri dish containing a small, isolated piece of skin tissue from the host whale. They suggested the involvement of a chemical cue from the host whale tissue in inducing larval settlement. In light of these findings, our finding of Cylindrolepas sinica settling on a plastic tag, and not directly on the body of the turtle, suggests that this settlement was triggered by a similar chemical cue. More information is necessary to clarify the settlement mechanism of epibiotic barnacles.