Introduction

The Antarctic Circumpolar Current (ACC) is considered a primary force promoting events of dispersion, isolation and speciation in Southern Ocean (Patarnello et al. 1996; Beu et al. 1997). This current, originated about 25–23 million years ago (in the late Oligocene) (Lyle et al. 2007), flows around the Antarctic continent in a west–east direction. It is delimited by the sub-Antarctic Front in the north and the Southern ACC Front in the south, encompassing in between the Antarctic Polar Front (Orsi et al. 1995). These frontal zones have been frequently regarded as natural barriers to genetic exchanges between species located within and outside the ACC, as well as those occurring at both sides of the ACC (Barker et al. 2007; Hunter and Halanych 2008).

Historically, based on the overall morphological similarities, several invertebrate species have been reported as widely distributed in the Southern Ocean, showing circum-Antarctic or circum-sub-Antarctic patterns (e.g., Dell 1972). In this regard, several authors have suggested that the ACC could be responsible for this distributional patterns, either through the dispersion of larval stages (as members of plankton community) or of juvenile/adult specimens by rafting (e.g., Fell 1962; Dell 1972). Additionally, some authors (e.g., Castilla and Guiñez 2000) regarded the anthropogenic dispersal as an alternative way for explaining this distributional pattern in some taxa. However, these hypotheses were seldom supported by factual evidence. In the last decades, the usage of genetic techniques applied to the taxa occurring in the Southern Ocean revealed contrasting distributional patterns: although some species appear to be actually widely distributed (e.g., Nikula et al. 2010; Moon et al. 2017), several other species previously regarded as widely distributed actually proved to correspond to species’ complexes, with “cryptic” species occurring in different localities (e.g., Wilson et al. 2007; Allcock et al. 2011). This fact led some authors to conclude that few taxa actually show circum-Antarctic or circum-sub-Antarctic distributions (e.g., Clarke et al. 2007: 620). However, to date, it is difficult to determine what “few taxa” exactly means, as a reduced number of the taxa previously considered as widespread have been carefully evaluated using genetic techniques. Among bivalves, for instance, only a few families were thus far genetically investigated, including Ostreidae (Ó Foighil et al. 1999), Limidae (Page and Linse 2002) and Mytilidae (Zbawicka et al. 2019), all of them belonging to the subclass Autobranchia. The results obtained by these studies are not conclusive: the two first confirm the occurrence of widespread species, while Zbawicka et al. (2019) revealed the presence of different (cryptic) species, with allopatric distributions, along the sub-Antarctic area. Considering the great diversification of bivalves in this area, and the broad geographic distributions exhibited by several other bivalve genera, this group of invertebrates appears particularly attractive for providing new insights on the biogeography of the Southern Ocean.

Gaimardia (Gaimardiidae) and Hiatella (Hiatellidae) are two genera members of the subclass Heterodonta which include species currently regarded as circum-sub-Antarctic. According to MolluscaBase (2020), Gaimardia comprises 11 valid species, all of them occurring in the southern hemisphere at high latitudes, including Southern South America, the Scotia Arc islands, Prince Edward Islands, Kerguelen, Tasmania, Macquarie, and New Zealand (Dell 1964; Zelaya 2005). Based on morphological characters, two of these species (Gaimardia trapesina and G. adamsiorum) are considered as widespread in sub-Antarctic waters, although the conspecificity of specimens from distant areas was never confirmed with genetic techniques; the remaining nine species of Gaimardia are considered as having much smaller distributional ranges (i.e., restricted to one or a few archipelagos, or to some particular areas). To date, the distinction of Gaimardia species is not easy. Most of these species remain only known from their original description (usually lacking relevant information for species distinction) and are imperfectly figured or not figured at all. A systematic revision encompassing these species has been never performed. Members of this genus lack a free-living larva and exclusively live on macroalgae (Helmuth et al. 1994; Ituarte 2009; Chaparro et al. 2011; Zelaya et al. 2019). Gaimardia trapesina, for instance, lives on the giant kelp Macrocystis pyrifera, where in fact it is the most abundant epibiotic species on the fronds (Dayton 1985a; Adami and Gordillo 1999; Puccinelli et al. 2018).

The knowledge on Hiatella is not much better than that of Gaimardia. Hiatella is a worldwide distributed genus, but all species have been described based on their shell morphology, a character that has proved to be greatly variable, being dependent on the local / particular conditions, where individuals grow (Lezin and Flyachinskaya 2015). Molecular studies in members of Hiatella are still scarce. Laakkonen et al. (2015) recognized two lineages (molecular species) of Hiatella in the southern hemisphere (which they referred as Hiatella A and Hiatella B), and 11 lineages in the northern hemisphere (Hiatella C-M). Layton et al. (2016) found an additional molecular species in the northern hemisphere (which he named Hiatella N). Out of these molecularly investigated species, only Hiatella A appears in the area under de incidence of the CCA. Members of Hiatella may be found over a great variety of substrates, including the giant kelp (Gordillo 2001; Laakkonen et al. 2015). Species of Hiatella for which some developmental information is available reveal the presence of a planktonic larval stage (e.g., Schejter et al. 2010; Díaz and Campos 2014).

The fact that some species of both Gaimardia and Hiatella live on macroalgae suggests that rafting may be a responsible driver for their current circum-sub-Antarctic pattern of distribution. Several biological and environmental factors have been reported to cause breakage of stipes and detachment of the holdfasts of the giant kelp (Barrales and Lobban 1975; Santelices and Ojeda 1984; Dayton 1985b; Duggins et al. 2001). These factors lead to the generation of kelp rafts, which are dispersed in the Southern Ocean, with the aid of the ACC, consequently dispersing the organisms living on them (Fraser et al. 2010; Nikula et al. 2010; Gillespie et al. 2011; Waters et al. 2018; Avila et al. 2020). In this regard, Gaimardia trapesina appears as an emblematic species in sub-Antarctic waters, because it was the first species in the area in which the dispersion by rafting was documented: Helmuth et al. (1994) provided evidence of specimens of G. trapesina dispersing on Macrocystis rafts from South America to South Georgia (about 1500 km away). Considering that kelp rafts are commonly found in the Southern Ocean (Schiel and Foster 2015) this transport could potentially work for other (more distant) areas too, as in fact it was suggested by some authors (e.g., Dell 1964; Castilla and Guiñez 2000), although this hypothesis was never formally proved.

The aim of this study is to examine if two geographically distant areas of the sub-Antarctic region, the Southern tip of South America (SSA) and the Prince Edwards Islands (PEI), actually share some faunistic elements. For that, members of two genera of bivalves (Gaimardia and Hiatella) with different modes of life and reproduction are selected as particular case studies.

Materials and methods

Studied areas

The SSA encompasses the Atlantic and Pacific coasts of the American continent. The term is here restricted to refer to the waters south of 47°S, including Isla de los Estados and Burdwood Bank. SSA corresponds to an old geological area, originated from the Gondwana break out (Faure and Mensing 2010). The Pacific coast of SSA is under the influence of the West Wind Drift, a subantarctic water flow that contacts the Chilean coast at about 48°S (Camus 2001; Thiel et al. 2007). This gives origin to the Cape Horn Current, which flows towards the south along the Pacific coast, surrounding the southern tip of South America, heading eastwards towards the Atlantic Ocean. The Cape Horn Current gives origin to the Patagonian shelf waters and the Malvinas/Falkland Current, which flow northwards parallel to the Argentine coast. The Malvinas/Falkland Current deflects eastwards off the continental slope at about 40–45°S (Stramma and England 1999).

The PEI is a relatively young volcanic archipelago, comprising Marion Island (originated about 0.45 million years ago) and Prince Edward Island (originated about 0.2 million years ago) (McDougall et al. 2001; Chown et al. 2008). This archipelago is located in the Indian sector of the Southern Ocean (Fig. 1).

Fig. 1
figure 1

a–c Study area (a) and sampling sites of Gaimardia (circles) and Hiatella (triangles), at (b) the Southern tip of South America (SSA) and (c) the Prince Edward Islands (PEI: light blue). ACC Antarctic Circumpolar Current; BB Burdwood Bank (purple); BC Beagle Channel (orange); CF Chilean Fjords (red); IE Isla de los Estados (yellow); PD Puerto Deseado (green); SAF sub-Antarctic Front; SACC Southern ACC Front; TF Atlantic coast of Tierra del Fuego (light orange)

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Sample collection

Studied specimens of Gaimardia were collected by hand from fronds of the giant kelp Macrocystis pyrifera attached to the substrate in the shallow subtidal. In SSA, samples were taken in 2011 and 2012, in the Chilean Fjords (CF) (Pacific coast), in the Beagle Channel (BC), in Isla de los Estados (IE) and Puerto Deseado (PD) (Atlantic coast) and in PEI, in 2016 and 2017 (Fig. 1).

Studied specimens of Hiatella were sorted from sediment dredged from 82 to 415 m depth, in SSA at Tierra del Fuego (TF) and Burdwood Bank (BB) and in PEI, during expeditions of BO Puerto Deseado, GC-189 Prefecto García and RV SA Agulhas II in 2016 and 2017 (Fig. 1). On BO Puerto Deseado and GC-189 Prefecto García a dredge of 4 mm2 mesh-size net of 20 × 60×60 cm, was used, while on RV SA Agulhas II a dredge of 30 × 100×100 cm and a mesh size of 1 cm2 was used.

All samples were preserved in 100% ethanol until further processing. Voucher specimens were deposited at the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Buenos Aires, Argentina.

Due to the impossibility to confirm a-priori the conspecificity of the studied specimens of Gaimardia and Hiatella from SSA and PEI, we refer to these materials as “Gaimardia” and “Hiatella”, in the Results section.

Genetic studies

Total DNA was extracted from adductor muscles using a CTAB/Proteinase K method (Dayrat et al. 2011). For specimens of Gaimardia, fractions of the mitochondrial Cytochrome Oxidase subunit I (COI), mitochondrial ribosomal gene 16S, and the nuclear Internal Transcribed Spacer region (ITS1-5.8S-ITS2, from now on “ITS”) were amplified, using the universal primers LCO/HCO (Folmer et al. 1994), 16Sar/16Sbr (Palumbi 1996) and ITS5/ITS4 (White et al. 1990), respectively. For specimens of Hiatella, a COI fraction was amplified using the HiaHd/HiaHr primers (Laakkonen et al. 2015). Amplifications were performed by routine polymerase chain reaction (PCR).

PCR products were visualized in 2% agarose gels, cleaned-up with enzymes (EXO I-FastAP, Thermo Scientific®) and sent out to Macrogen, Inc. (Korea) for sequencing on both ways. Sequences were trimmed and refined with chromatograms guidance prior to assemblage, and checked manually. MEGA version 10.0.5 (Kumar et al. 2018) was used for aligning sequences using Clustal W with default settings, and to calculate uncorrected p-distances (from now on “p-dist”). DnaSP version 5 (Librado and Rozas 2009) was used to calculate haplotype diversity (H) and nucleotide diversity (π).

Information on voucher specimens, sampled localities and GenBank accession numbers, are provided in (Table 1).

Table 1 Specimens of Gaimardia trapesina and Hiatella O collected as part of the present study for genetic analyses in the Southern tip of South America and the Prince Edward Islands
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Phylogenetic reconstructions of sequences of Gaimardia were performed for each marker individually, and combining the three in a concatenated analysis using a subset of 11 specimens (Table 1). A 16S sequence of Gaimardia trapesina from Tierra del Fuego, Argentina, available from GenBank (KX713220.1) was included in the analysis, as well as COI and 16S sequences of Cyamiomactra laminifera (KC429131, KC429293.1), another Cyamioidea, here used as outgroup. ITS fragments are not currently available in GenBank for any other Cyamioidea, and therefore, the phylogenetic reconstruction performed with this marker could not be rooted.

For the phylogenetic reconstruction of specimens of Hiatella, we included the sequences obtained by Laakkonen et al. (2015); (Hiatella A-M: GeneBank accession numbers KP767805–KP761044). Sequences of Panopea generosa (KJ125418.1) and Panopea globosa (KJ125413.1) were used as outgroups.

The best evolutionary models were selected with AIC criterion in jModeltest version 2.1.10 (Darriba et al. 2012). Maximum Likelihood (ML) reconstructions were performed in MEGA, with node support evaluated through 1000 bootstrap replicates. Bayesian inference (BI) analyses were performed in MrBayes version 3.2 (Ronquist et al. 2012), with four simultaneous runs of 100 generations each and a sample frequency of 100, until average standard deviation of split frequencies reached ≤ 0.001; phylogenetic trees were summarized with a 10% burn-in value. Haplotype networks were built using a median-joining method (Bandelt et al. 1999) in Network version 5.0.1.0 (available at https://www.fluxus-engineering.com/sharenet.htm).

Results

The case Gaimardia

In total, 23 specimens of Gaimardia provided genetic information for analyses: 12 for 16S, 21 for COI, and nine for ITS. Of these, eight specimens successfully amplified for all gene loci, enabling the concatenated analysis.

The alignment of the twelve 16S sequences (Table 1) had a length of 481 bp and resulted in only two variable sites, corresponding to singletons. The phylogenetic reconstructions confirmed the monophyly of specimens of Gaimardia from SSA and PEI altogether, with high support both in the ML and BI analyses. Within the Gaimardia clade, the 16S did not resolve any grouping.

The alignment of the nine sequences of ITS (Table 1) was 1361 bp long, with nine variable sites and six singletons. The phylogenetic reconstructions based on ITS did not resolve any grouping within the Gaimardia clade.

The alignment of the 21 COI sequences (Table 1) had a length of 529 bp with 21 variable sites (4%), with a total of 15 singletons. Low levels of genetic divergence were registered for COI among the considered sequences (p-dist = 0–1.7%) (Table 2). These sequences of Gaimardia showed high levels of differentiation with Cyamiomactra laminifera (p-dist = 16.8%). The phylogenetic reconstructions confirmed the monophyly of specimens of Gaimardia from SSA and PEI altogether, with high support both in the ML and BI analyses. Within the Gaimardia clade, the COI marker consistently recovered a subclade, which encompasses all the specimens from BC, IE and CF, with high Bayesian posterior probability (0.98) but moderate bootstrap support in ML (71%). COI showed relatively low genetic differentiation among the five studied localities (average p-dist = 0.6%).These differences were low among different localities of SSA, and somewhat greater between PEI and each locality at SSA or SSA as a whole (Table 2). The p-dist between PEI and CF, BC, or IE were higher than the respective within-site distances. However, the distances between PEI and PD were lower than the average distance among specimens from PEI (Table 2).

Table 2 Gaimardia COI sequence divergences. Above diagonal: minimum and maximum number of base differences between localities. Diagonal: average uncorrected p distance within locality. Below diagonal: uncorrected p-distance between localities
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The concatenated alignment of 16S, COI and ITS revealed a distinctive grouping within the Gaimardia clade. In this case the BC + IE + CF specimens also formed a subclade, with 80% bootstrap support and 0.97 posterior probability (Fig. 2a). In addition, in the concatenated analysis by BI, a second subclade, comprising the specimens from PD, was recovered with moderately good posterior probability (0.86).

Fig. 2
figure 2

Case Gaimardia. a Tree obtained by Bayesian Inference for COI, ITS and 16S concatenated markers (GTR + G model). Posterior probabilities > 95% are indicated with “*”; maximum likelihood bootstrap support > 80 are indicated with “°”. b, c Median-joining haplotype networks of: (b) COI and (c) ITS. Numbers on branches correspond to number of mutated sites; size of circles is proportional to the number of specimens per haplotype, according to the scale provided. Specimen codes (“G1”– “G23”) correspond to those listed in (Table 1). For colour codes see (Fig. 1)

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Among the 21 COI sequences, 15 different haplotypes were recognized, separated by a maximum of nine mutations. Standard diversity indices revealed high haplotype diversity and moderate nucleotide diversity (H = 0.95; π = 0.007), a fact also evidenced among sequences from BC (H = 0.90; π = 0.003) and PEI (H = 0.97; π = 0.008). Contrastingly, relatively low diversity was found among sequences from PD (H = 0.40; π = 0.0008). The reconstruction of the median-joining network showed a relatively short topology, with low-frequency haplotypes (Fig. 2b). All studied localities showed exclusive haplotypes, with the exception of IE, which shared the same haplotype of BC. The network revealed certain association of the haplotypes with the geographic origin of the specimens. BC, IE and CF were grouped by very similar haplotypes, and connected to the most frequent haplotype of PD by only two substitutional steps. PEI specimens showed the greatest number of haplotypes (eight), and were separated by the greatest number of steps among them (up to eight). These specimens from PEI resulted in two haplotype groups, connected independently to the PD main haplotype. The first group (“PEI I”) showed a star-like topology with its central haplotype separated from the main haplotype in PD by only two steps. The second group (“PEI II”) comprised two additional haplotypes (Fig. 2b). The haplotypes grouped in “PEI I” were similarly differentiated from BC + IE + CF than from “PEI II” (p-dist = 1.1% and 0.9%, respectively).

Among the nine ITS sequences, seven distinct haplotypes were found, most of them represented by a single sequence, resulting in high haplotype diversity and low nucleotide diversity for this marker (H = 0.92; π = 0.002). The ITS haplotype network revealed no geographic structure. In fact, one shared haplotype was found in BC, IE and PEI (Fig. 2c).

When comparing the COI and ITS haplotype networks, no concordant results were found in the position of the specimens. The PD specimens that shared one central haplotype in the COI network were distinct in ITS. Likewise, the PEI specimen that shared a common haplotype with BC and IE in the ITS network, was distant in COI.

The case Hiatella

In total, 17 specimens of Hiatella provided genetic information for analyses. The COI alignment (187 sequences, 796 bp) was highly-conserved among the new sequenced specimens from SSA and PEI, with only three variable sites. However, the inclusion of the sequences of the lineages of Hiatella reported by Laakkonen et al. (2015) resulted in a total of 279 variable sites (35%). High levels of genetic divergence were found between the newly obtained sequences and any other previously known molecular species (p-dist > 19.1%; Table 3), in contrast to the minimum divergence among the material studied herein (p-dist = 0.06%; Table 3), and the divergence between the specimens from SSA and PEI (p-dist = 0.06%). The material here studied was closer to Laakkonen et al.’s (2015) molecular species from the southern hemisphere (i.e., Hiatella A: from the Magellan Strait, and Hiatella B: from New Zealand) than any other lineage from the northern hemisphere (Table 3).

Table 3 Hiatella COI sequence divergences of molecular species: A-M: Laakkonen et al. (2015), N: Layton et al. (2016), O: present study. Diagonal: average uncorrected p-distance within molecular species. Below diagonal: uncorrected p-distance between molecular species. Between brackets: number of specimens per species
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The phylogenetic reconstructions revealed the monophyly of the studied material, with high bootstrap support and bayesian probability (98% and 1.00, respectively) (Fig. 3a). No subclades were recovered among these sequences, which showed a basal position to the previously known Hiatella species, although lacking adequate support (Fig. 3a).

Fig. 3
figure 3

Case Hiatella. Tree obtained by Bayesian Inference for COI (TIM3 + I + G model). Posterior probabilities > 95% are indicated with “*”; maximum likelihood bootstrap support > 80 are indicated with “°”. Terminals “A” to “M” correspond to Laakkonen et al.’s (2015) molecular species (GenBank accession numbers KP767805 to KP761044); terminals grouped in “O” correspond to newly sequenced specimens. b Median-joining haplotype network for COI. Numbers on branches correspond to the number of mutated sites; size of circles is proportional to the number of specimens per haplotype, according to the scale provided. Specimen codes (“H1”–“H17”) correspond to those listed in (Table 1). For colour codes see (Fig. 1)

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Only four haplotypes were recognized in the sequences obtained as part of this study (Fig. 3b): one, the dominant, being present in all sampled localities (TF + IE + BB + PEI), two haplotypes exclusive from BB, and the remaining exclusive from PEI. The last three haplotypes appear separated by only one or two mutational steps from the main haplotype.

Discussion

In recent years, several studies have analyzed the connectivity and origin of the marine fauna from different sub-Antarctic archipelagos (e.g., Waters 2008; Cumming et al. 2014; Moon et al. 2017; González-Wevar et al. 2018; and references therein), although few of these studies have considered the fauna of PEI. González-Wevar et al. (2014) provided genetic evidence of the occurrence at PEI of a gastropod species (Nacella delesserti) that is not present in SSA or any other area in the Southern Ocean (where other species of Nacella are present). This fact would suggest certain degree of faunistic isolation of PEI. On the contrary, the bivalves considered in the present study show extremely low genetic differentiation, together with the presence of shared haplotypes between PEI and SSA, revealing that the same species of Gaimardia and Hiatella are present in these two areas. These results are similar to those reported for the bivalve Limatula pygmaea (Page and Linse 2002), and the isopods Septemserolis septemcarinata and Limnoria stephenseni (Leese et al. 2010; Nikula et al. 2010); suggesting that this wide distributional pattern is not unusual, and that it occurs in species with different modes of life and reproduction. The low genetic differentiation here found between specimens from SSA and PEI does not seem to be originated in the used molecular markers. In fact, two of the fragments here used proved to be successful in distinguishing other cyamioidean and hiatelloidean species: the 16S sequences obtained by Zelaya et al. (2019), resulted in p-dist from 15 to 18.8% among species of Cyamiocardium; similarly, the COI marker shows high divergence of lineages in Hiatella (11.2–20.2%), according to Laakkonen et al. (2015). A taxonomical revision of the species of Gaimardia occurring in the Southern Ocean (Güller and Zelaya, in prep) allows us to confirm that the taxon involved in this study is G. trapesina. The species of Hiatella cannot be determined with confidence, as none of the nominal species of this genus described or reported from the Southern Ocean (and even worldwide) is properly defined. Despite that, the species involved in the present study does not correspond to any of the molecular species, previously recognized by Laakkonen et al. (2015) or Layton et al. (2016), either from the southern or the northern hemisphere, revealing that the molecular diversity of Hiatella is even greater than previously known. For the purpose of the subsequent discussion, the taxon involved in this study is referred as Hiatella O.

The little genetic differentiation found in this study for ITS sequences (fast-evolving nuclear loci), supports the hypothesis of relatively recent events of colonization of PEI by Gaimardia trapesina. In addition, the presence of two different groups of haplotypes of G. trapesina at PEI, which diverge from SSA’s main haplotype (when considering COI marker) is compatible with the occurrence of more than one event of colonization of this archipelago. This is in agreement with the findings by Page and Linse (2002) for Limatula pygmaea (in that case, based on ITS-1 sequences).

How can species occur in geographically distant areas?

The occurrence of a same species in two geographically distant areas is usually explained as a consequence of a common history of these areas (i.e., geological or tectonic connections), anthropogenic transport or natural dispersive events (Castilla and Guiñez 2000). The different geologic histories of SSA and PEI argue against the first of those alternatives to explain the co-occurrence of these taxa in these two areas. Anthropogenic transport does not seem to have major significance for explaining the occurrence of these two taxa at PEI either, as both Gaimardia trapesina and Hiatella O are relatively small-sized species, which do not represent human food-source or are involved in aquaculture activities. On the other hand, the PEI historically have had only occasional ships traffic, due to the distant location of the islands from usual sea routes (Cooper 2008; Leese et al. 2010); and studies on the sea-chests fauna of the few ships visiting this archipelago (Lee and Chown 2007) did not report these species.

On the contrary, some biological and ecological aspects of the two species considered in this study support the hypothesis that natural dispersive events have been the main responsible drivers for their current pattern of distribution. Gaimardia trapesina exclusively live on the giant kelp Macrocystis pyrifera (Dayton 1985a; Adami and Gordillo 1999; Puccinelli et al. 2018), on which all the specimens here studied, either from SSA and PEI, were collected. Specimens of Hiatella O can also be occasionally found on M. pyrifera, where early juveniles are usually present on the fronds and larger specimens on the holdfasts (Zelaya, pers obs). The present study takes Helmuth et al.’s (1994) findings even further, revealing that specimens of G. trapesina may be dispersed in this way, over much greater distances, thus occurring in SSA and PEI (6500 km away). Furthermore, members of the genus Hiatella have been also found living on the bull kelp Durvillaea antarctica (López et al. 2018), which according to Smith (2002) is likely to be more important for dispersal over larger spatial scales than M. pyrifera in the Southern Ocean. Therefore, rafting on macroalgae represents the most probable way in which G. trapesina (and perhaps also Hiatella O) has reached the PEI.

The dispersion of Hiatella O does not merely restrict to the rafting on macroalgae. Members of this genus have been also reported associated to pumice (Velasquez et al. 2018). Considering that pumice is frequently found in several sub-Antarctic localities (Risso et al. 2002; Bryan et al. 2012), and that pumice from South Sandwich Islands have been recovered in quite distant localities, such as New Zealand (Coombs and Landis 1966), this alternative of dispersion cannot be discarded. In fact, rafting on pumice was regarded as the most probable way of long-distance dispersal in Ostrea chilensis (Ó Foighil et al. 1999). In addition to rafting, another way of dispersion for Hiatella O arises from the free-living larval stage present in members of this genus (Gordillo 2001; Greene and Grizzle 2007). In conclusion, the natural dispersion of larvae, juveniles or adults appears as the most probable alternative to explain the presence of Gaimardia trapesina and Hiatella O in PEI, a phenomenon which, in any way, must have been facilitated by the ACC. This current is regarded as the strongest marine current of the world (Olbers et al. 2004), with an average speed of about 40 cm.s−1 and jet surface speeds reaching 0.6 m.s−1 (Whitworth et al. 1982; Meredith et al. 2011). The simulations of particle movements in the Southern Ocean performed by Fraser et al. (2018) provide evidence in favour of regarding the ACC as a potential way allowing the dispersion from SSA to PEI. But even if assuming that the ACC keeps its maximum speed, covering the distance between SSA and PEI (6500 km) would take at least 125 days, which represent more than the estimated floating periods for rafts of Macrocystis pyrifera (100 days, according to Helmuth et al. 1994; Hu and Fraser 2016). Additionally, species of Hiatella have a shorter larval phase (of about 30–60 days, fide López et al. (2018)). However, rare meteorological and extreme oceanographic conditions can reduce the time required for such large-scale dispersive events (Gillespie et al. 2011). An alternative explanation to such successful dispersion arises in the fact that both G. trapesina and Hiatella have been reported in intermediate localities between SSA and PEI, such as the Malvinas/Falkland Islands and South Georgia; and Hiatella also at Gough Island (Dell 1964). In all these archipelagos, M. pyrifera and D. antarctica are also present (Fraser et al. 2009; Macaya and Zuccarello 2010a, b; Caselle et al. 2018). In addition, small-sized specimens of Hiatella have been found in some intermediate localities between SSA and PEI. Thus, the intermediate archipelagos between SSA and PEI could have acted/be acting as stepping-stones for the colonisations of PEI by specimens of Gaimardia trapesina and Hiatella O, which it is compatible with the route of colonization of the different sub-Antarctic islands proposed by Fraser et al. (2012).

Future perspectives

Several issues, to be addressed in the future, arise from this study:

  1. 1.

    Are recurrent events of colonization of PEI coming from a single source or multiple sources? Some of the results obtained for Gaimardia trapesina in this study, as well as previous findings from Page and Linse (2002) on Limatula pygmaea, suggest the presence of different lineages of these species at PEI. Such pattern may be interpreted as a consequence of different events of colonization from a single source or, alternatively, as a consequence of specimens from different localities of the Southern Ocean arriving to PEI. Incorporating specimens from intermediate areas (i.e., Malvinas/Falkland Islands, South Georgia, South Sandwich Islands, Gough Island) could help to evaluate the connectivity of PEI with each of these areas, and the potential multiple origins of the marine invertebrate fauna present today at PEI.

  2. 2.

    How frequent is the circum-sub-Antarctic pattern here reported? The contrasting results between species shared between PEI and SSA (e.g., Gaimardia trapesina, Limatula pygmaea and Hiatella O) vs. endemic species at PEI (Nacella delesserti), reveal the necessity of carefully revising each species previously considered as circum-sub-Antarctic based exclusively on their morphological similarity.

  3. 3.

    What is the proper name for Hiatella O? To solve this issue, a systematic revision of the numerous nominal species of Hiatella reported for the southern hemisphere is crucial. Such revision should encompass topotypic specimens and consider genetic information, particularly taking into account the plasticity exhibited by members of the genus Hiatella in shell morphology (Beu 1971; Lezin and Flyachinskaya 2015).