Discussion

Amphibalanine barnacles typically inhabit the intertidal zone, yet Amphibalanus pulchellus and A. zhujiangensis were initially found on floating objects (Ren, Reference Ren1989), and subsequent records indicate their prevalence on marine debris, such as buoys or ship hulls (Otani et al., Reference Otani, Oumi, Uwai, Hanyuda, Prabowo, Yamaguchi and Kawai2007; Wells and Booth, Reference Wells and Booth2012; present study). Phylogenetic analysis suggests a close relationship among these species (Figure 4), highlighting their affinity for colonizing marine debris or floating objects. Despite continuous shoreline observations along the Japanese coast, no established intertidal populations of these poorly documented species were detected. Their apparent association with floating substrates suggests an opportunistic occurrence pattern, which may account for their limited historical records.

Figure 4.

The phylogenetic trees in the neighbour-joining and maximum-parsimony analyses were inferred from 1000 bootstrap replicates, and both analyses were conducted using MEGA X (Kumar et al., Reference Kumar, Stecher, Li, Knyaz and Tamura2018). The sequences were aligned using MUSCLE, included in MEGA X. At each node, the numbers separated by a slash indicate the percentage NJ and MP bootstrap support, respectively (1000 replicates).

Although these rarely recorded species in the genus Amphibalanus have been sporadically reported from the Northwestern Pacific to Australian waters (Figure 1a), they have been discussed as potentially introduced or non-native taxa in Japanese and Australian waters (Puspasari et al., Reference Puspasari, Yamaguchi and Ross2002; Ahyong and Wilkins, Reference Ahyong and Wilkens2011; Wells and Booth, Reference Wells and Booth2012). However, determining whether such species are truly invasive or historically overlooked natives remains difficult, owing to the scarcity of reliable historical records. The closely related intertidal acorn barnacle Fistulobalanus albicostatus (Pilsbry, Reference Pilsbry1916), widely distributed in the western Pacific (from northern Japan to the South China Sea), comprises two major clades exhibiting a COI divergence ranging from 1.25% to 2.8% (Chang et al., Reference Chang, Chan, Hayashi, Shuto, Tsang, Chu and Chan2017). In contrast, despite the broad distribution of A. zhujiangensis in Indonesia, Japan, and Taiwan, the genetic distance within A. zhujiangensis populations is even lower (<0.01, Supplementary Table S1). This pattern is more consistent with recent dispersal or ongoing genetic connectivity than with long-term regional isolation. However, the native geographic origin of A. zhujiangensis remains uncertain because of the scarcity of historical records. Although A. zhujiangensis has been infrequently reported since its original description and could be considered a native species across subtropical regions including the East China Sea, this high genetic homogeneity suggests a different scenario. It indicates that the species’ distribution may have expanded rapidly in recent times, potentially associated with increasing availability of durable floating substrates in recent decades, although direct evidence for substrate specificity is lacking. In the Anthropocene, plastic debris has become an increasingly dominant component of marine surface waters, with long-term datasets documenting pronounced increases over the past decades (Ostle et al., Reference Ostle, Thompson, Broughton, Gregory, Wootton and Johns2019). Plastics in aquatic ecosystems serve as rich substrates for epiplastic organisms, forming what is referred to as the ‘plastisphere’ (Zettler et al., Reference Zettler, Mincer and Amaral-zettler2013). Okinawa Island, where the present study was conducted, has been reported to contain large amounts of marine debris (Ilechukwu et al., Reference Ilechukwu, Das, Jamodiong, Borghi, Manzano, Hakim and Reimer2024). Due to the limited records of plastisphere-associated fauna, the life history of these species remains largely unknown.

While our data suggest a recent expansion for A. zhujiangensis, the status of A. pulchellus remains even more obscure. Prior to this study, not a single genetic sequence had been publicly available for this species, rendering it one of the most poorly understood taxa within the genus. The lack of baseline data currently prevents assessment of its biogeographic status, whether A. pulchellus is a native species with a previously cryptogenic distribution in the subtropical Pacific or a newly introduced alien species. By providing the first publicly accessible COI and H3 sequences, this research establishes the necessary foundation for future comparative studies to clarify the origin or invasion history of these ‘forgotten’ species. Together with the recent discovery of Chelonibia testudinaria on plastic debris (Hayashi, Reference Hayashi2026), the present study suggests that anthropogenic floating objects may represent an overlooked source of distributional and ecological information for marine epibionts. Continued surveys of stranded marine debris may therefore reveal additional records of poorly known barnacle species and improve our understanding of their biogeography.