Introduction
Marine environments have undergone rapid and profound changes during the Anthropocene, among which the global accumulation of plastic debris is one of the most conspicuous. Floating plastics now act as persistent artificial substrates, facilitating the transport and colonization of a wide range of marine organisms and altering natural dispersal pathways (Therriault et al., Reference Therriault, Nelson, Carlton, Liggan, Otani, Kawai, Scriven, Ruiz and Murray2018). Numerous studies have documented biofouling assemblages on marine plastic debris, ranging from microbial communities to diverse macrofouling organisms such as barnacles, bryozoans, molluscs, and other sessile invertebrates (Zettler et al., Reference Zettler, Mincer and Amaral-Zettler2013; Subías-Baratau et al., Reference Subías-Baratau, Sanchez-Vidal, Di Martino and Figuerola2024). However, reports involving host-associated epibionts that are typically restricted to living marine vertebrates remain exceptionally rare.
Epibiotic barnacles of the genus Chelonibia differ fundamentally from typical fouling organisms. They are predominantly associated with living hosts, particularly sea turtles, crabs, and manatees (Cheang et al., Reference Cheang, Tsang, Chu, Cheng and Chan2013; Zardus et al., Reference Zardus, Lake, Frick and Rawson2014), and are highly adapted to mobile biological substrates exposed to strong hydrodynamic stress.
Recent studies have shown that a variety of marine organisms may modify their reproductive or settlement behaviour in response to the increasing availability of artificial substrates. Marine litter has been reported as an oviposition substrate for cephalopods, fishes, skates, and sharks, potentially providing novel nursery habitats when natural substrates are scarce or unpredictable (Gündoğdu et al., Reference Gündoğdu, Çevik and Karaca2017; De-la-Torre et al., Reference De-la-Torre, Valderrama-Herrera, Reyes and Walker2022; Valderrama-Herrera et al., Reference Valderrama-Herrera, Cardenas, Calvo-Mac, Celi-Vértiz, Chumpitaz-Levano, Flores-Miranda, Lopez-Tirado, Molina-Alvarez, Rubio-Cheon, Trucios-Castro, Fernández Severini, Forero López, Ramos, Pretell, Castro, Ribeiro, Dobaradaran, Espinoza-Morriberón, Ben-Haddad, Dioses-Salinas and De-la-Torre2023; Figuerola et al., Reference Figuerola, Ruiz-García, Subías-Baratau, Maceda-Veiga, Sanchez-Vidal and Barría2024). These observations suggest that anthropogenic materials can influence habitat selection and settlement processes across diverse marine taxa.
Here, an unusual case of the turtle barnacle Chelonibia testudinaria firmly attached to floating plastic debris is reported. Though not the first record of this barnacle attaching to anthropogenic material, this observation provides insight into the potential ecological flexibility of a host-associated epibiont in response to the increasing availability of artificial substrates in the modern ocean.
Materials and methods
Field observations were conducted on 27 December 2025 on Hasaki Beach, Kamisu City, Ibaraki Prefecture, eastern Japan (Figure 1a). The study was carried out during an opportunistic survey of epibiotic organisms associated with a stranded loggerhead sea turtle, Caretta caretta (Figure 1b). A fragment of stranded plastic debris was visually inspected, and all attached macrofouling organisms were recorded (Figure 1c). The plastic substrate was examined in situ and subsequently collected for further observation. Specimens of C. testudinaria and associated barnacles were carefully removed from the substrate and preserved for morphological examination. Morphological identification was conducted based on external shell characteristics, including overall shape, plate arrangement, and opercular structures, following published descriptions (e.g., Cheang et al., Reference Cheang, Tsang, Chu, Cheng and Chan2013; Hayashi, Reference Hayashi2013a). Measurements of rostro–carinal length were taken using digital callipers to the nearest 0.1 mm.
Multiple specimens of the turtle barnacle Chelonibia testudinaria on an artificial substrate. (a) The specimens were collected from the Pacific coast of Hasaki, Kamisu City, Ibaraki Prefecture, eastern Japan (35.760847° N, 140.824181° E); (b) The turtle barnacles C. testudinaria firmly cemented to a stranded turtle carapace; (c) The specimen firmly cemented to plastic debris fragment stranded on the beach; (d) Apical, basal, oblique, and lateral views showing the conical profile of the collected specimens of turtle barnacle C. testudinaria (NMNS-Cr-33588); (e) The detached individual with an attached coastal acorn barnacle, Amphibalanus amphitrite (NMNS-Cr-33589). Scale bars = 5 mm.
Results
Two individuals of C. testudinaria were found on a fragment of floating plastic debris during a coastal observation. The plastic fragment appeared to be a rigid polymer component of anthropogenic origin, with no visible remains of biological substrates such as turtle scutes. One individual was firmly cemented to the plastic surface, with the basal plate conforming closely to the substrate (Figure 1d). A second specimen was found detached within the debris, likely dislodged during the stranding event. Though C. testudinaria is capable of translocation on turtle carapace (Moriarty et al., Reference Moriarty, Sachs and Jones2008) and acrylic panels in the laboratory (Chan et al., Reference Chan, Wong, Robinson, Lin, Yu, Dreyer, Cheng, Høeg and Zardus2021), no movement traces were visible on the plastic surface. The shell morphology, including the arrangement of plates and overall outline, was consistent with diagnostic characteristics of C. testudinaria (Figure 1d–e).
The specimen (Figure 1d) was small but intact, suggesting successful settlement. The rostro–carinal length of the attached individual was approximately 19.4 mm, and it was estimated to be several months old, based on growth models (Doell et al., Reference Doell, Connolly, Limpus, Pearson and van de Merwe2017). A second specimen of C. testudinaria was found detached adjacent to the plastic fragment (Figure 1e). This specimen carried an attached Amphibalanus amphitrite and was likely dislodged from the same plastic debris, as A. amphitrite was also abundant on the reverse side of the fragment.
Discussion
Attachment of C. testudinaria to plastic debris represents a striking deviation from its typical host-associated lifestyle. Unlike obligate fouling barnacles, C. testudinaria has evolved in close association with mobile vertebrate hosts, where hydrodynamic conditions, surface chemistry, and host behaviour differ markedly from those of inert substrates (Hayashi et al., Reference Hayashi, Chan, Simon-Blecher, Watanabe, Guy-Haim, Yonezawa, Levy, Shuto and Achituv2013; Hayashi, Reference Hayashi2013a).
From a broader perspective, this observation highlights how anthropogenic materials can blur traditional ecological boundaries between host-associated and free-living organisms. A comparable but equally rare case was reported for another turtle-associated barnacle, Cylindrolepas sinica, which was found attached to a plastic identification tag affixed to a loggerhead sea turtle in the wild (Hayashi, Reference Hayashi2013b). Plastic debris may serve as a novel vector not only for invasive species (Therriault et al., Reference Therriault, Nelson, Carlton, Liggan, Otani, Kawai, Scriven, Ruiz and Murray2018) but also for organisms previously considered host-restricted, thereby reshaping epibiotic assemblages and dispersal dynamics.
Previous records of C. testudinaria attached to artificial or inanimate substrates are uncommon but not unprecedented. Under the name Chelonibia patula, field occurrences have been reported from floating plastics (Stubbings, Reference Stubbings1967; Frazier and Margaritoulis, Reference Frazier and Margaritoulis1990), a human femur (Monroe, Reference Monroe1981), and a dolphin scapula (Collareta and Bianucci, Reference Collareta and Bianucci2021). Additional records include settlement on artificial test panels in Hawaii (Edmondson and Ingram, Reference Edmondson and Ingram1939), buoys in the South China Sea (Chen, Reference Chen, Wei and Chen1989), and acrylic and slate panels in South Carolina (Sloan et al., Reference Sloan, Zardus and Jones2014). Furthermore, laboratory studies demonstrated that cyprids of C. testudinaria readily settled, metamorphosed, and grew on artificial substrates including glass and PVC (Zardus and Hadfield, Reference Zardus and Hadfield2004; Lane et al., Reference Lane, McElroy, Kendrick and Zardus2021).
Subsequent molecular studies based on mitochondrial COI haplotypes suggested that C. patula is conspecific with C. testudinaria (Cheang et al., Reference Cheang, Tsang, Chu, Cheng and Chan2013), and that C. testudinaria, C. patula, and C. manati form a single genetic lineage (Zardus et al., Reference Zardus, Lake, Frick and Rawson2014), although this interpretation remains debated (Hyatt and Dunbar, Reference Hyatt and Dunbar2024).
The present observation can be interpreted as a rare but genuine case of C. testudinaria settling on artificial plastic debris under natural conditions. Unlike earlier reports, which predate the current understanding of species boundaries in Chelonibia, this record documents substrate use by C. testudinaria sensu lato under contemporary marine plastic pollution scenarios. While Monroe (Reference Monroe1981) hypothesized a preference for calcium matrices based on the bone attachment, the present observation on synthetic polymers suggests that surface physical properties (e.g., hardness, roughness) may be more critical than chemical composition for the settlement of this species. Several mechanisms may explain this unusual settlement. First, surface roughness and microtopography of weathered plastics may provide sufficient cues for cyprid attachment. Second, increasing scarcity or spatial unpredictability of suitable biological hosts in certain environments may promote settlement on suboptimal substrates, potentially functioning as an ecological trap.
The concept of the ‘plastisphere’ has attracted increasing attention over the past decade, largely focusing on microbial communities colonizing plastic debris and their ecological and health risks (Zettler et al., Reference Zettler, Mincer and Amaral-Zettler2013; Li et al., Reference Li, Gillings, Zhang, Chen, Zhu, Wang, Zhao, Xu, Leung, Li, Liu and Jin2024), whereas responses of larger sessile invertebrates remain poorly documented. This observation indicates that plastic pollution provides an abundant, artificial niche that may fundamentally alter the settlement dynamics of this ‘host-specific’ species, potentially turning a rare ecological accident into a common phenomenon.
Acknowledgements
The author gratefully acknowledges Tomoya Hayashi and Kumi Hayashi for discovering the plastic debris bearing the turtle barnacles and Ever Lasting Nature of Asia for providing an opportunity to be involved in stranded sea turtle research. Constructive comments and suggestions from Dr. Alberto Collareta and two anonymous reviewers substantially improved this manuscript.
Declaration of interest
The author declares no conflicts of interest.
Data availability statement
Voucher specimens are deposited in the National Museum of Nature and Science, Tokyo (NMNS- Cr-33588-33593).
Ethics and approach
Not applicable.
Funding statement
Not applicable.
