First discovery of a barnacle (Balanus crenatus) on the critically endangered flapper skate (Dipturus intermedius)

Abstract

Charles Darwin, who devoted many years to the study of barnacles, once remarked that “Cirripedes now abound so under every zone, all over the world, that the present period will hereafter apparently have as good a claim to be called the age of Cirripedes as the Palaeozoic period has to be called the age of Trilobites” (Darwin, 1851). Thoracican barnacles are among the most versatile colonizers in the marine realm, occupying a niche breadth that spans from high-intertidal rocks to deep-sea hydrothermal vents. Their success is rooted in an extraordinary plasticity for substrate selection (e.g., Figure 1); they encrust everything from driftwood and ship hulls to the living tissue of crustaceans, sea turtles, and cetaceans (Chan et al., 2009; Pilsbry, 1916).

While marine mammals, namely, whales and dolphins, are described as “moving islands” commonly colonized by epibionts including barnacles (Geraci & Aubin, 1987). Fishes, however, are among the few animal groups on which barnacles do not commonly occur, possibly due to the protective mucus layer of fish skin (Balakrishnan, 1969). Species of Conchoderma, the stalked barnacle typically associated with floating substrates and crustaceans (including parasitic copepods and isopods), have nonetheless been recorded from several fish species, where it reportedly attaches to spines, areas of damaged skin, or in the mouth of fishes (Anislado-Tolentino et al., 2021; Hastings, 1972). Similarly, Lepas anatifera has been documented on the damaged skin of a stranded sunfish, Mola mola (Barreiros & Teves, 2005). The occurrence of Platylepas decorata on the scales of a garpike remains the only documented case of a sessile barnacle recorded from a fish host (Ryder, 1879). Seilacher (2005) postulated that the acorn barnacle species evolved specialized anchoring mechanisms and host-specific adaptations that allow them to settle and colonize living hosts. Similarly, Anelasma squalicola, a naked stalked barnacle, represents a highly specialized parasitic form that attaches to deep-sea sharks that predominantly attach to the dorsal or pectoral fins in an area where the skin is thinner, or turbulence may facilitate attachment (Eliassen, 2016; Ste-Marie et al., 2023). While ectoparasites are commonly recorded as occurring on the skin of skates in the Atlantic (superorder: Batoidea) (Boxshall, 1974), to our knowledge, there is no documented observation of any barnacle species colonizing skates (Family: Rajidae) to date.

The flapper skate, Dipturus intermedius (Parnell, 1837) is the largest skate recorded in the North Atlantic, reaching over approximately 2.5 m in total length (Last et al., 2016). Flapper skate is a conservation-listed demersal species that exhibits K-selected life history traits that make it vulnerable to extinction. While it has a constrained distribution (Garbett et al., 2023), mark-recapture data suggest individuals are capable of dispersal across the North Sea (Little, 1995) and the North Channel (between Northern Ireland and southwestern Scotland; Orrell et al., 2025). Although there are published records of ectoparasites, including copepods, isopods, and marine leeches on the common skate complex (Boxshall, 1974; Isler et al., 2022), no barnacle has been reported to be attached to D. intermedius. Flapper skates possess a median row of 12–18 predorsal thorns that extends along the median disc and tail (Last et al., 2016). In the present study, we report for the first time a case where a barnacle has colonized a flapper skate (D. intermedius), which was captured in shallow water off the southwest coast of Ireland. Several specimens of boreo-Arctic barnacle Balanus crenatus were collected from an enlarged and abnormal median thorn on the skate's tail (Figure 1).

We identified the barnacle as B. crenatus based on morphological examination. BLAST analyses showed that the newly generated cytochrome c oxidase subunit I (COI) sequence clustered within available sequences of B. crenatus, exhibiting a maximum similarity of 99.51% to a B. crenatus sequence from the North Sea (GenBank accession number PQ739635.1), supporting the results from morphological identification. In the haplotype network (Figure 2), the new sequence (H2) stays close to a common haplotype (H1), with two mutation steps between them. The COI network shows that the most common haplotype (H1) is shared between Atlantic and Pacific populations.

B. crenatus was originally described from the coast of England and the North Sea and is widely distributed in the northern Atlantic, northern Pacific, and Arctic seas (Darwin, 1854; Pilsbry, 1916), as well as within the study area in the Celtic Sea (GBIF, 2023). This species occupies a variety of substrates, and while it is not considered parasitic, it has been found to occupy crustacean carapaces and molluscan shells, from the subtidal zone to deeper waters of approximately 70 m (Darwin, 1854; Pilsbry, 1916). It has also occasionally been reported from floating objects and pebbles on the seafloor, but notably not from rocky substrates (Darwin, 1854). B. crenatus is an obligate cross-fertilizing hermaphrodite with high fecundity (over 10,000 embryos in large individuals) and peak settlement in spring and summer. It exhibits rapid juvenile growth, with basal diameter increasing by several millimeters per month (Barnes & Powell, 1953; Rainbow, 1984).

Considering the high abundance of larvae during the peak season, and the short period of planktonic naupliar stages (10 days, Weiss, 1948), B. crenatus can settle wherever suitable surfaces are encountered. Flapper skates occupy a range of depths, overlapping with that of B. crenatus, from shallow water coastal areas with a core depth range of 20–200 m but also recorded at depths of up to 600 m (Ellis et al., 2024). Flapper skates have been observed to rest for extended periods of time on the seabed, which could enable colonization by epibionts (Wearmouth & Sims, 2009). The lack of documented batoid–barnacle interactions to date is likely due to the abrasive microtopography of placoid scales, which are highly effective deterrents under normal physiological conditions (Damodaran & Murthy, 2016), reducing larval attachment success and preventing secure cementation. In this case, host pathology likely created a suitable surface for attachment, enabling colonization by opportunistic barnacle cyprids. The settlement of B. crenatus on D. intermedius represents a significant departure from known cirripede–elasmobranch interactions. In this skate specimen, settlement was localized to a site where the median thorn was enlarged and diffusely hard (mineralized) relative to the other thorns. Rajidae caudal thorns have been identified as long-standing structures and have therefore proven useful in aging studies (Gallagher et al., 2005). The surface in this case could provide a longitudinal attachment point for opportunistic settlement. Hitchhiking events on rafting flotsam or mobile megafauna could provide a stepping stone to maintain genetic connectivity across large spatial scales (Schiffer & Herbig, 2016; Shahdadi, 2023). We posit the host-size and predictability hypothesis, whereby large, long-lived, mobile hosts provide a sufficiently stable substratum for epibionts, with movement facilitating reliable water flow for continuous filter feeding. While B. crenatus has been documented as being attached to floating debris that moves passively in ocean currents, flapper skates have the potential to actively disperse over large distances. For example, two skates tagged in Scotland traveled 240 km (581 days at liberty) and 900 km (~1949 days at liberty), respectively. Our finding underscores the importance of rare and opportunistic behavior in the evolutionary success of sessile invertebrates in factors including biogeographic dispersal and population connectivity and highlights the potential for elasmobranchs to act as potential vectors for epibiotic dispersal, which may be more significant than currently recognized. While current efforts to study large-scale movements of rare skates include employing mark-recapture and electronic tagging (i.e., satellite tags), epibionts such as barnacles offer a unique biological “passport” that can help piece together regional movements, space use, and connectivity (Detjen et al., 2015). Future work should include monitoring for epibiotic and parasitic life on flapper skates to determine their frequency of occurrence and assess potential factors that drive host-associations. Additionally, investigation of epibiont tolerances, that is, thermal preferences, is important in the context of climate change and connectivity across remaining hot spots for critically endangered host species, like the flapper skate.

Adnan Shahdadi: Writing—review and editing; investigation; data analysis. Benny K. K. Chan: Conceptualization; data analysis; writing—review and editing. Danielle L. Orrell: Conceptualization; investigation; project administration; visualization; resources; writing—review and editing. Alfonso Cohuo: Investigation; review and editing. Min-Hsien Lee: Data analysis.

The authors declare no conflicts of interest.

The Health Products Regulatory Authority of Ireland approved experimental procedures under project license AE19130/P189. Animal handling followed best practice guidelines for the focal species.

Details of the material examined and the methods used in this study are provided in Appendix S1: Section S1. All DNA sequences analyzed in this study are available in the National Center for Biotechnology Information (NCBI) GenBank database, with accession numbers and corresponding links provided in Appendix S1: Section S2. The sequence generated in this study is available in GenBank under accession number PX988570 at https://www.ncbi.nlm.nih.gov/nuccore/PX988570.