Introduction
Sea anemone invasions appear to be increasingly common around the world (Glon et al., Reference Glon, Daly, Carlton, Flenniken and Currimjee2020; Gimenez and Brante, Reference Gimenez and Brante2021; Gimenez et al., Reference Gimenez, Rivera and Brante2022). In recent years, for example, the South American anemone Phymactis papillosa (Lesson, 1830) has invaded Europe (Pereira et al., Reference Pereira, Silva and Mateus2022), a European Sagartia species has invaded South Africa (Robinson and Swart, Reference Robinson and Swart2015), the Asian Diadumene lineata (Verrill, 1869) has continued to spread to become one of the most widespread anemones worldwide (González Muñoz et al., Reference González Muñoz, Lauretta, Bazterrica, Puente Tapia, Garese, Bigatti, Penchaszadeh, Lomovasky and Acuña2023), and the southeastern United States anemone Aiptasiogeton eruptaurantia (Field, 1949) has arrived in southern New England (Pederson et al., Reference Pederson, Carlton, Bastidas, David, Grady, Green-Gavrielidis, Hobbs, Kennedy, Knack, McCuller, O'Brien, Osborne, Pankey and Trott2021). To these records we now add the invasion on the Atlantic coast of North America of the European sea anemone Actinia equina Linnaeus, 1758, whose distribution is currently held to be from northern and western Europe to the Mediterranean (Perrin et al., Reference Perrin, Thorpe and Solé-Cava1999). Actinia equina is a common species on rocky shorelines throughout its distribution, being found across the intertidal zone from the high-water neap level down to the upper subtidal (Manuel, Reference Manuel1988). Although A. equina has been reported to be gonochoristic, whether it reproduces sexually remains moot (Perrin et al., Reference Perrin, Thorpe and Solé-Cava1999; Wilding et al., Reference Wilding, Fletcher, Smith, Prentis, Weedall and Stewart2020). Actinia equina certainly does reproduce through somatic embryogenesis, with clonal individuals internally brooded until release from the parental coelenteron. Although not all individuals are brooding at any one time, those that are typically have up to 20 clonal individuals in their coelenteron and in rare cases more than this (Brace and Quicke, Reference Brace and Quicke1986). Consequently, this species has the capacity to rapidly colonize suitable habitat through the release of these clones.
Here, we present data on Actinia found recently at multiple sites on man-made rocky structures of the beaches of Monmouth County, New Jersey, on the Mid-Atlantic Coast of the United States. The anemones found match the characteristic A. equina morphology, and we confirm identification through DNA barcoding.
Materials and methods
Specimens were examined and collected in 2023 at low tide from man-made rocky groynes on the New Jersey shoreline. Photographs of anemones in situ were taken at Roosevelt Ave beach in the town of Deal (latitude 40.2578306, longitude −73.9871418; site A on Figure 1). Twenty-six anemones were collected at Philips Ave beach (latitude 40.2532037, longitude −73.988791; site B in Figure 1), also in Deal, and four anemones were collected from Ocean Grove, Neptune Township (latitude 40.2103212, longitude −74.0020381; site C in Figure 1). To facilitate further laboratory studies, anemones were carefully peeled from rocks and transported individually in glass jars with seawater to a laboratory at Monmouth University.
Figure 1. Map of sampling sites alongside iNaturalist records for Actinia equina. Sites A, B, C: sites sampled for this study. (A) Roosevelt Avenue, Deal, New Jersey; (B) Phillips Avenue, Deal NJ; (C) Ocean Grove, Neptune Township, NJ. Sites 1–11: iNaturalist records (iNaturalist observation number given). (1) 176354922 (Long Branch Beach, August 2023); (2) 162967884 (Ocean Grove, May 2023); (3) 132054972 (Ocean Grove, May 2022); (4) 121145126 (Ocean Grove, June 2022); (5) 179718648 (Ocean Grove, August 2023); (6) 125436946 (Bradley Beach, July 2022); (7) 127216915 (Ocean Grove, July 2022); (8) 163414767 (Ocean Grove, May 2023); (9) 92531365 (Bradley Beach, August 2021); (10) 129471163 (Belmar, August 2022); (11) 186565245 (Manasquan Inlet, Manasquan, October 2023). For a global picture of the distribution of A. equina, see https://www.inaturalist.org/taxa/130085-Actinia-equina.
In addition to our observations reported here, we considered observations of Actinia equina reported on iNaturalist (https://www.inaturalist.org/).
Four specimens collected from Ocean Grove (site C) were analysed through barcoding of both the standard mitochondrial barcoding locus cytochrome c oxidase I (CoI) (Hebert et al., Reference Hebert, Cywinska, Ball and deWaard2003) and the nuclear rDNA–internal transcribed spacer (ITS) cluster for which ITS1 has been recommended as an nDNA barcode (Santamaria et al., Reference Santamaria, Fosso, Licciulli, Balech, Larini, Grillo, De Caro, Liuni and Pesole2017). DNA was extracted using the extraction technique described in Wilding et al. (Reference Wilding, Fletcher, Smith, Prentis, Weedall and Stewart2020). We then amplified and sequenced the mitochondrial CoI locus using the protocol of Wilding and Weedall (Reference Wilding and Weedall2019) with the primers of Folmer et al. (Reference Folmer, Black, Hoeh, Lutz and Vrijenhoek1994) and the rDNA locus containing ITS1 and ITS2 following Reimer et al. (Reference Reimer, Takishita, Ono, Tsukahara and Maruyama2007).
To examine whether these anemones contained clonal individuals, ten anemones were stressed with gentle agitation and irrigation of the anemone's coelenteron to extract clones. Purged clones were observed under a microscope and counted per individual.
Results
Distribution and habitat
Anemones were observed by DB and JEA at three sites in 2023 on the New Jersey shoreline (Figure 1). Animals were found near the base of the seaward end of groynes, in a zone typically associated with blue mussel (Mytilus edulis) beds and surrounded by macroalgae (Ulva spp.). They were observed on both high-shore open rock and under rocks on the lower shore, although most of them are found in the latter habitat. Anemones were frequently seen in dense aggregations wherein individuals were immediately adjacent to and often in contact with each other (F–J in Figure 2).
Figure 2. Images of individual Actinia specimens with extended tentacles in observation tank (A–E). Note the blue limbus on these individuals. Images of anemone aggregations on rocky groynes (F–J). Image (F) captured in Ocean Grove, (G–J) on Philips Ave, Deal.
On iNaturalist (https://www.inaturalist.org/) we found 11 records (1–11 in Figure 1) where descriptions and photographs match those of Actinia equina. The northernmost record is from Long Branch Beach (latitude 40.2946367, longitude −73.9783467; site 1 in Figure 1) found in August 2023; the southernmost record is from Manasquan Inlet, Manasquan (latitude 40.1029159, longitude −74.03307; site 11 in Figure 1) found in October 2023. Six of the records are centred around Ocean Grove. The earliest record is from August 2021, when the anemones were first discovered in Bradley Beach (latitude 40.2023959, longitude −74.0059152; site 9 in Figure 1).
As of July 2024, no records of A. equina have been reported on iNaturalist from neighbouring states (New York, Delaware, and Pennsylvania).
Morphology
Anemones collected from the New Jersey shoreline had a pale green colouration in the column with a blue limbus and grey pedal disc. Tentacles had a pale green/yellow colour with blue tips (Figure 2). Individual animals have typical sizes of 10–40 mm, the upper end of which is similar to the maximum size of 40–50 mm disc width reported previously for A. equina (Davenport et al., Reference Davenport, Moloney and Kelly2011; Carling et al., Reference Carling, Gentle and Ray2019).
Following gentle agitation and irrigation of the coelenteron of ten individuals, 3.16 (±1 SEM) clonal individuals were released per individual (range 0–10).
Genetic analysis
All four samples genetically analysed had a CoI haplotype found previously in A. equina (GenBank accession number MH636618) and a rDNA–ITS haplotype also found previously in A. equina (GenBank accession number PP349608). Haplotypes were identical across all the samples studied.
Discussion
Actinia equina is here reported from rocky structures on the New Jersey shore. The limited geographical extent of sites where Actinia has been observed (approximately 22 km) and the recent records (only since 2021) suggest this anemone may be a relatively recent introduction. The mitochondrial DNA CoI and nuclear ITS haplotypes identified in these samples are identical to those found in other A. equina from widespread geographic locations (CSW, personal observation) providing DNA barcoding confirmation of species identity.
Wherever A. equina occurs it can be patchily distributed or occur in aggregations. What appears to be unusual about the New Jersey populations is the higher density aggregations with animals abutting each other. Anemones at these sites are aggregated in multiple clusters, some on the order of 1 m2. Although other species of intertidal anemones which reproduce asexually form tightly packed clonal aggregates (Francis, Reference Francis1988), aggregations of A. equina on European shores are invariably less dense (e.g. Figure 3 of Brace (Reference Brace1990) for an example).
Although individuals can be more aggregated during the winter (Brace, Reference Brace1990), this does not explain the tightly packed distribution seen in the New Jersey populations. Sea anemones, including A. equina (Brace and Pavey, Reference Brace and Pavey1978; Brace, Reference Brace1990) are by nature territorial (Williams, Reference Williams1991; Turner et al., Reference Turner, Lynch, Paterson, León-Cortés and Thorpe2003), protecting their territory through aggressive fighting encounters in which nematocysts, chiefly those of the acrorhagi, are used in combat. Fine-scale distribution is mediated by this aggression which ensures individuals can compete for space through territorial disputes. However, relative aggression levels in these territorial disputes varies dependent upon relationships between individuals. Where this aggression can be considerable towards non-relatives, they are more tolerant of, and less aggressive towards, neighbouring clonemates (Ottaway, Reference Ottaway1978; Brace et al., Reference Brace, Pavey and Quicke1979; Ayre, Reference Ayre1982, Reference Ayre1983) in A. equina and Actinia tenebrosa, both of which have been confirmed to reproduce through asexual viviparity through genetic analysis of brooded individuals (Black and Johnson, Reference Black and Johnson1979; Orr et al., Reference Orr, Thorpe and Carter1982). For A. tenebrosa, Ayre (Reference Ayre1983) describes densely clumped clusters of adult anemones in which disproportionately more neighbouring adults were genotypically identical. Anemones also exhibit size-related hierarchies of aggression (Brace and Pavey, Reference Brace and Pavey1978). In the populations described here, the aggregations are formed of anemones of relatively uniform size. A combination of size homogeneity and clonal expansion from an introduction of one or few individuals may explain the extremely tight clustering of individuals seen in these populations which appears to be different from that seen in their typical range.
The studied locations are approximately 25 km from the entrance to New York Harbour, one of the largest ports in the United States, and around 150 km from the port of Philadelphia. Ocean plumes from New York/New Jersey flow as a constrained coastal current southwards along the New Jersey shore (Choi and Wilkin, Reference Choi and Wilkin2007), towards the area in which A. equina was found. We thus suggest that shipping from Europe is the most probable vector for the introduction of this anemone to the American coast. Sea anemones are known from both vessel hull fouling and from ballast water (see Glon et al., Reference Glon, Daly, Carlton, Flenniken and Currimjee2020). Although there are questions as to whether A. equina produces planula larvae (Perrin et al., Reference Perrin, Thorpe and Solé-Cava1999; Wilding et al., Reference Wilding, Fletcher, Smith, Prentis, Weedall and Stewart2020) which could be transported in ballast water, small anemones could be taken into ballast tanks and ballasted cargo holds attached to bits of debris. We also note that Actinia have long been popular aquarium animals (Friese, Reference Friese1972; Bellomy, Reference Bellomy1975).
We suggest that given the small scale of these populations of anemones, they likely represent a relatively recent introduction and are composed either completely or largely of clones of a single or a few immigrant animals. We suggest three reasons for this conclusion. First, these animals contain clonal offspring, and, at least in the congeneric A. tenebrosa, settlement and recruitment to new sites is largely through asexual clonal proliferation (Ayre, Reference Ayre1983). The ability to reproduce asexually is particularly important for invasion success in anemones (Glon et al., Reference Glon, Daly, Carlton, Flenniken and Currimjee2020). The homogeneity of body size within the aggregations fits with a single introduction with subsequent spreading through release of clones. Ayre (Reference Ayre1983) showed that in populations of A. tenebrosa, single clones can dominate an established habitat. Although there is little comparable data for A. equina colonizing new habitats, Brace and Quicke (Reference Brace and Quicke1986) examined colonization and resettlement by A. equina of experimentally cleared surfaces. However, such experimentation differs from the situation described here in that neighbouring territory in their study had established long-term, genotypically diverse, adult A. equina populations which could move into the newly cleared substrate. Second, the concentrated aggregation with little spacing between individuals is consistent with clonality since these individuals must be less aggressive to clonemates to tolerate this close proximity. Third, the haplotypes at both loci (mitochondrial and nuclear) were the same across all four samples strongly consistent with a clonal origin (although we recognize the caveat of having studied just N = 4).
Integrative approaches combining both morphological and molecular studies are vital for understanding Cnidarian diversity (Yap et al., Reference Yap, Mitchell, Quek, Tan, Tan and Huang2023). Here, we have combined these to identify Actinia specimens from New Jersey. However, further sampling is recommended to determine the geographic extent of A. equina on the US Atlantic coast in order to determine whether these small-scale stable populations are spreading. Recent work has developed environmental DNA technologies for the identification of sea anemones around New Jersey shores (Lockwood et al., Reference Lockwood, Vastano, Zoccolo and Dickey2023) and integrating this technique into future surveys would be prudent.
Data
The authors confirm that the data supporting the findings of this study are available within the article.
Acknowledgements
Joe Rowlett was the first to suggest that the New Jersey populations were Actinia equina, and noted the significance of this invasion, on iNaturalist. The authors thank Ron M. L. Ates for helpful discussions. D. B. thanks Tahlia Botha and Tyler Barkey for field assistance and aquarium expertise to maintain live specimens in tanks. We are grateful to the many citizen scientists who have contributed their observations to i-Naturalist. We also thank two anonymous reviewers for useful suggestions.
Author contributions
D. B. and J. E. A. examined specimens in situ and collected specimens for laboratory observations. C. S. W. undertook molecular analysis. All authors contributed to writing the manuscript.
Financial support
This research received no specific grant from any funding agency, commercial, or not-for-profit sectors.
Competing interests
None.
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