Marine exotic isopods from the Iberian Peninsula and nearby waters

Discussion

At present, 12 marine exotic isopod species are known to be present in European waters. Ten of them are free-living species, most of them considered to be established, and two are parasites and considered to be casual (Streftaris, Zenetos & Papathanassiou, 2005; Zenetos et al., 2010; Galil, 2011; Noël, 2011; Lavesque et al., 2013; Chainho et al., 2015; Lorenti et al., 2016; Marchini, Ferrario & Occhipinti-Ambrogi, 2016a; Ulman et al., 2017) (see Table S1). The Iberian Peninsula alone hosts 50% of these ten free-living species, proving to be an important monitoring point for spread as well as future arrivals of exotics. Moreover, 50% of the marinas sampled in 2017 had increased their number of exotic isopods within the timeframe of only six years (Table 1). The case of the marinas in Cádiz Bay (Strait of Gibraltar) is to be noticed. Only Paracerceis sculpta was found in 2011, but they hosted P. sculpta, Paradella dianae, Sphaeroma walkeri and Paranthura japonica in 2017 (see the case of St. 12, 13 and 14.1 in Table 1). It is to be noticed that, despite more habitat-forming species were analyzed in 2017 in comparison with 2011, the increase in NIS was verified for the same species. In fact, a previous study conducted by Ros et al. (2013) demonstrates that about 50% of the dominant sessile species present throughout the year in Puerto América marina (St. 14.1) are introduced. Several factors may be favouring the introduction and establishment of exotic species in this area. Some of these factors may be due to particular environmental conditions of each marina; but others are most likely human-related, like the proximity of these marinas to a major international port in southern Spain (Cádiz Port), together with the high maritime traffic occurring across the Strait of Gibraltar.

History of introduction, pathways, vectors and potential spread of each species are discussed below.

Histories of introduction and worldwide distribution

Ianiropsis serricaudis is native to the western Pacific, from the Sea of Okhotsk to the Sea of Japan, including Russia, Japan and Korea (Kussakin, 1962; Jang & Kwon, 1990; Shimomura, Kato & Kajihara, 2001; Yokoyama & Ishihi, 2007) (Fig. 3A). It was reported as NIS in San Francisco Bay, California (Carlton, 1979) in association with the introduced ascidians Ciona intestinalis Linnaeus, 1767 and Styela clava Herdman, 1881, possibly transported in shipping associated with the Vietnam War (Carlton, 1979). In the following years, reports of unknown Ianiropsis or erroneously identified specimens started to appear in the East and West coast of the United States and in 2004 it was already present in Europe, associated with the introduced ascidian Syela clava in Southampton (England) (see Hobbs et al., 2015 and references herein). In the Netherlands it was first observed in 2000 (Faasse, 2007) in an estuary used for shellfish aquaculture, and near the port of Rotterdam, among other locations. In 2010 and 2011, Hobbs and collaborators realized that all the mentioned reports involved the same species, potentially globally distributed by ships. From 2010 to 2013 it was widely reported from Maine to New Jersey (United States, western Atlantic), in association with both native and introduced algae, bryozoans and ascidians from fouling communities on floating dock sites and pilings (Pederson et al., 2005; McIntyre et al., 2013; Janiak & Whitlatch, 2012; Johnson, Winston & Woolacott, 2012; Wells et al., 2014; Hobbs et al., 2015). Also in California and Washington (United States, eastern Pacific), in mudflats near reefs of the introduced Australian serpulid polychaete Ficopomatus enigmaticus (Fauvel, 1923) (Heiman & Micheli, 2010) or in association to the non-native tunicate D. vexillum colonizing mussel aquaculture facilities (Cordell, Levy & Toft, 2013).

The first evidence of its occurrence in the Mediterranean Sea took place in 2012, when it was found to be abundant in the Lagoon of Venice (Adriatic Sea, Italy) (Marchini, Ferrario & Occhipinti-Ambrogi, 2016a; Marchini, Ferrario & Occhipinti-Ambrogi, 2016b). The Lagoon of Venice is a big center for recreational and commercial harbour as well as flourishing mariculture that hosts a high number of introduced species (Occhipinti-Ambrogi, 2000; Marchini et al., 2015). A couple of years later it was present in Olbia (Ferrario et al., 2017), again a major site for mussel farming which, in fact, imports stocks from Adriatic lagoons (Marchini, Ferrario & Occhipinti-Ambrogi, 2016b); and subsequently in French marinas (Ulman et al., 2017). Our results fill a gap in its distribution, providing the first record for the Iberian Peninsula and the Lusitanian province. We now have evidence that it was already present in 2011 in the North of Spain, in La Graña marina (Ferrol, Galicia). Ferrol city has been a major naval shipbuilding centre for most of its history, and today, aquaculture and fishing stand as its primary industries.

Interestingly, the specimens found in Ferrol bear four marginal denticles on pleotelson (Fig. 1D). There are some minor discrepancies regarding this character; Gurjanova (1936) described it as possessing four or five, Kussakin (1962) established a range of four to seven, Jang & Kwon (1990) showed four on the material from Korea, Doti & Wilson (2010) established “five denticles or more” but not “up to four denticles” and Marchini, Ferrario & Occhipinti-Ambrogi (2016a); Marchini, Ferrario & Occhipinti-Ambrogi (2016b) reported three or four from the specimens collected from the Mediterranean Sea. In any case, Hobbs et al. (2015) considered this to be a variable character and they relied on additional characteristics instead. They suggested a founder effect from the narrower range of denticle counts in introduced populations (three to four) versus the reported from native regions (up to seven). Moreover, our specimens were considerably large (males up to 5 mm and ovigerous females up to 3 mm) in comparison to those reported from Russia (2.9 mm for males and 2.7 for females) (Kussakin, 1962, Kussakin, 1988) from the East coast of the United States (largest male being 3.2 mm and female 2.4 mm) (Hobbs et al., 2015) and from the Mediterranean Sea (around 3 mm) (Marchini, Ferrario & Occhipinti-Ambrogi, 2016a; Marchini, Ferrario & Occhipinti-Ambrogi, 2016b). Whether these morphological changes imply changes in the ecological performance of the species in the new range and whether these are the result of changes at the genetic or only phenotypic level are uncertain. The biological, social and economic impact I. serricaudis may have in the introduced areas cannot be estimated until further ecological studies are carried out, since there is a severe lack of information for this species, even in its native range (Hobbs et al., 2015).

In the Iberian Peninsula, the arrival of I. serricaudis is probably linked to accidental introduction with shellfish transfers. This is a likely associated vector (see Marchini, Ferrario & Occhipinti-Ambrogi, 2016a; Marchini, Ferrario & Occhipinti-Ambrogi, 2016b), judging by the occurrence of the species in European mussel aquaculture facilities and hotspots for mariculture and shellfish trade. In fact, Galicia, together with Cataluña, bear the highest oysters, clams and mussel production of Spain, including production of non-native species such as the Pacific oyster (Crassostrea gigas) and the Japanese clam (Ruditapes philippinarum), and export to other countries of Europe (Instituto Galego de Estatistica, 2017; Ministerio de Agricultura y pesca, 2017). This vector has been attributed to several species with similar routes of introduction (see Gruet, Héral & Robert, 1976), including the isopod Paranthura japonica (see Figs. 3A, 3E) (Lavesque et al., 2013). Nevertheless, shipping transport is an associated vector of this species as well, given its presence in ports and its nature as fouling species of hard substrates such as docks, as well as its adaptability to different substrates (Hobbs et al., 2015). Our finding in a marina adds recreational boating as a vector, at least, for secondary transport. This means I. serricaudis has the potential to spread to further Mediterranean marinas as well as along the Iberian Peninsula coast. This would be not surprising since this species bears broad temperature tolerance and is expected to spread through Europe as was well as temperate waters of the southern hemisphere (see Hobbs et al., 2015). As a small-size organism, it is likely to be overlooked though; therefore, we call for prevention for the detection of this species in the mentioned areas.

Paracerceis sculpta is the most widespread species within the genus and a successful species colonizing new areas. Its type locality is San Clemente Island, California (USA) (Menzies, 1962) and its putative native range includes the northeastern Pacific region, including California (Richardson, 1905; Wallerstein, 1980; Austin, 1985; Reed & Hovel, 2006), San Quintin Bay, southern Baja California (Menzies, 1962); Puerto Peñasco, Sonora (Ohmart, 1964) and northern and central Gulf of California (Mexico) (Brusca, 1980). It has been present in Hawaii at least since 1943, probably introduced by naval shipping from southern California (Miller, 1968; McCain, 1975); and at least since 1978 in Brazil (Pires, 1980a; Pires, 1981; Loyola e Silva, Masunari & Dubiaski-Silva, 1999). It was only detected from the Gulf of Mexico in 2009 (Munguia & Shuster, 2013). From the 1990s onwards, it was reported from distant locations; from China (Yu & Li, 2001), Hong Kong (Bruce, 1990; Yu & Li, 2003), Taiwan (Yu & Li, 2003) and Japan (Ariyama & Otani, 2004), to Australia (Hass & Knott, 2000; Hewitt & Campbell, 2001) and northwest Indian Ocean, in Pakistan (Yasmeen & Yousuf, 2006). It is also considered introduced in South Africa, being ship fouling and/or ballast water its associated vector (Barnard, 1940; Griffiths, Robinson & Mead, 2009; Mead et al., 2011). In the Mediterranean Sea, it is known from the central region since the 1970s (Rezig, 1978; Forniz & Sconfietti, 1983; Forniz & Maggiore, 1985; Lombardo, 1985; Savini et al., 2006; Cosentino, Giacobbe & Potoschi, 2009; Vincenzi et al., 2013), and decades after it was reported from the eastern (Katsanevakis et al., 2014) and western Mediterranean as well (Marchini, Ferrario & Minchin, 2015). In the eastern Atlantic Ocean it was found for the first time in 1988–1989 in Cádiz bay (Spain) (Rodríguez, Drake & Arias, 1992). In the Macaronesia biogeographical region (northwestern Africa) it was detected only in 2014 (Marchini et al., 2017), collected from Ponta Delgada marina (Azores archipelago) and in 2015 (Ramalhosa et al., 2017), collected from Funchal marina (Madeira archipelago).

There is evidence for attributing shipping, including recreational boating, as vector to Paracerceis sculpta (Hewitt et al., 2004; Katsanevakis et al., 2014; Mead et al., 2011; Marchini et al., 2017). It is commonly found in locations of intense vessel traffic; in marinas, bays or coastal lagoons near major harbor facilities (Rezig, 1978; Forniz & Sconfietti, 1983; Rodríguez, Drake & Arias, 1992; Castelló & Carballo, 2001; Espinosa-Pérez & Hendrickx, 2002; Marchini et al., 2017). In the 1990s it was already present in the Mediterranean Sea and the Strait of Gibraltar. From there, it has been subsequently found in additional marinas along the southern and eastern sides of the Iberian Peninsula coast from 2011 to 2017 (Table 1); and it currently occurs from southern Portugal to eastern Spain. We report it for the first time for Alboran sea ecoregion, where all the males found belonged to the alpha morph sensu Shuster (1992). This supports the idea that only the alpha morph has made it into the introduced populations, consistent with the lack of beta and gamma male records in other non-native locations (Pires, 1981; Forniz & Maggiore, 1985; Rodríguez, Drake & Arias, 1992; Loyola e Silva, Masunari & Dubiaski-Silva, 1999; Hewitt & Campbell, 2001; Yu & Li, 2001; Ariyama & Otani, 2004; Munguia & Shuster, 2013; Marchini et al., 2017). In fact, Shuster & Wade (1991) hypothesized that the shorter lifespan of beta and gamma males is a handicap for surviving long trips and colonizing new regions.

In the Iberian Peninsula, Paracerceis sculpta is mainly associated to the introduced/cryptogenic bryozoan Bugula neritina and the introduced A. verticillata, which may have facilitated the transport and establishment of this exotic isopod (Marchini, Ferrario & Minchin, 2015; Marchini et al., 2017; Gavira-O Neill, Guerra-García & Moreira, 2016). Additionally, we have observed a non-overlapping presence of P. sculpta and the native isopod Dynamene edwardsii in most of the stations. A further study investigating the interspecific interaction of these two species is scheduled, in order to determine the potential biological impact of Paracerceis sculpta.

Similar to Paracerceis sculpta, Paradella dianae was first reported from Bahia de San Quintin, Baja California and Mexico (Menzies, 1962). Its native range is supposed to be Northeast Pacific, from Ventura County (California, USA) to Michoacán (Mexico), including the Gulf of California (Iverson, 1974; García-Guerrero & Hendrickx, 2004) (Fig. 3C). Before the 1980s it was reported in the western Atlantic in Puerto Rico (Menzies & Glynn, 1968) and Brazil (Pires, 1980b). First record outside of its native range was in Marshall Islands in 1967 (Glynn, 1970). From the 1980s onwards, it was found in distant areas of the world. In western Pacific, in Hong Kong in 1986 (Bruce, 1990); in Australia (Harrison & Holdich, 1982; Furlani, 1996; Hass & Knott, 2000), collected from small boats jetties; and at the other side of Indian Ocean in Pakistan (Arabian Sea) in 1984 (Javed & Ahmed, 1987). At the same time, Paradella dianae arrived to the southeastern coast of USA (western Atlantic) (Clark & Robertson, 1982; Van Dolah, Knott & Calder, 1984; Fox & Ruppert, 1985; Kensley & Schotte, 1989; Nelson & Demetriades, 1992), being ship fouling the most likely vector (Knott & De Victor 2010).

It is unknown whether P. dianae arrived to the Iberian Peninsula and the Mediterranean Sea from the Indian Ocean, from the Atlantic Ocean, or from both through multiple introductions. It was reported from the Italian coast in 1980 (Forniz & Maggiore, 1985) and the coast of Alexandria (Egipt) (Atta, 1991); but at the same time reported across the Strait of Gibraltar, in Cádiz Bay (Atlantic side of the Strait) in 1988–1989 (Rodríguez, Drake & Arias, 1992) and Algeciras Bay (Mediterranean side of the Strait) in 1992 (Castelló & Carballo, 2001). From 2000 onwards it was collected and reported from additional locations in Central Mediterranean Sea (Bey et al., 2001; Ferrario et al., 2017; Ulman et al., 2017); and Eastern Mediterranean Sea (Zgozi, Haddoud & Rough, 2002, Kirkim et al., 2010; Çinar et al., 2008, Doğan, Bakir & Katağan, 2015, Kirkim, Özcan & Katagan, 2015; Ulman et al., 2017).

As well as P. sculpta, it was probably introduced to new locations by hitchhiking on the hulls or other surfaces of ships (Rodríguez, Drake & Arias, 1992; Galil, 2011). Hass & Knott (2000) also point to recreational boating as a likely vector, at least for its introduction to Australia. Our study supports this hypothesis, since it was found again in marinas located in Cádiz Bay (Strait of Gibraltar’s vicinity) plus others along the Alboran Sea coast. Marinas of southern Iberian Peninsula coasts are well connected by frequent local traffic; 90% of visiting boats in the sampled marinas are Spanish, plus a percentage of foreign boats usually coming from Europe (UK, France, Holland) and other parts of the world (America, Australia, Arabic countries) (marina staff, personal communication). In fact, our data shows an ongoing expansion of Paradella dianae into additional marinas, potentially colonizing the eastern side of the Iberian Peninsula into the western Mediterranean Sea. Even having the same native range and potentially bearing a similar pattern of introduction than P. sculpta, P. dianae does not seem to be as successful, bearing lower densities than P. sculpta and a smaller introduction range (Figs. 3B, 3C).

Sphaeroma walkeri is the most widespread of these species, reaching numerous ports worldwide (see Carlton & Iverson, 1981). Stebbing (1905) first described it from in Ceylon (now Sri Lanka, Indian Ocean), with the northern Indian Ocean being its native range, including India, Arabian Sea and Bay of Bengal (Carlton & Iverson, 1981). It was known from the Persian Gulf some years later and the introduction status in this locality is doubtful, thus considered cryptogenic (Fofonoff et al., 2017) (Fig. 3D). Carlton & Iverson (1981) propose an episodic dispersal for this species. An initial local transport (pre-1870 period) would have occurred around the Indian Ocean plus South Africa (Stebbing, 1917), where it was found in fouling on pilings, Mozambique (Barnard, 1955) and Australia (Baker, 1928; McNeill, 1932; Iredale, Johnson & McNeill, 1932). A second period would be related to the opening of the Suez Canal in 1869. The record of this species in Port of Suez already in 1904–1905 (Stebbing, 1910) is doubtful; therefore, we agree with Fofonoff et al. (2017) and consider S. walkeri cryptogenic from this locality as well (Fig. 3D). From there, it would have travelled through the Suez Canal into the Mediterranean Sea (Omer-Cooper, 1927; Larwood, 1940). A post 1940 period would have been coincident with World War II. Sphaeroma walkeri would have been transported to the American continent associated to the intense shipping traffic since that time. It was found in Brazil (Loyola e Silva, 1960), Puerto Rico (Menzies & Glynn, 1968), Florida (Miller, 1968; Camp, Whitino & Martin, 1977; Nelson & Demetriades, 1992) and Hawaii (Miller, 1968). From those areas, it continued to increase its distribution to different parts of the world. To the western Pacific in Hong Kong in 1972 (Vrijmoed, 1975; Morton, 1987), Hainan (southern China) from pier fouling samples (Kussakin & Malyutina, 1993) and other locations in Australia (National Museum of Natural History (Smithsonian Institution) collections (NMNH), 1967; Montelli & Lewis, 2008). To the eastern Pacific in San Diego Bay (California), it was first detected in 1973 in fouling on pilings, floats and small boats at yacht harbours (Carlton & Iverson, 1981). Along the western Atlantic coast it was found in other locations of the Gulf of Mexico (Clark & Robertson, 1982; Cházaro-Olvera et al., 2002), Cuba in 1994 (USNM 280039, US National Museum of Natural History 2007) and Isla Margarita (Venezuela) in 2004 (Gutiérrez, 2012). Along the Northwest coast of Africa, it was also associated with harbours (Jacobs, 1987). On the Indian Ocean it was reported from Malaysia only in the 1990s (Rai-Singh & Sasekumar, 1996) and from Iran in 2006–2010 (Khalaji-Pirbalouty & Wägele, 2010). Across the Mediterranean Sea, it continued spreading to further eastern locations until the present year (Glynn, 1972; Kocataş, 1978; Galil, 2008; Ulman et al., 2017). It was recorded in the Italian Peninsula (Lodola, 2013) and found to be completely established with successful populations in Tunisia harbours and lagoons (Ben Souissi et al., 2004; Ben Amor, Ben Slaem & Ben Souissi, 2010). In was also reported in the western Mediterranean (Zibrowius, 1992), being reported from Spain for the first time in 1981 (Jacobs, 1987). In 2017, we report Sphaeroma walkeri from the southern Iberian Peninsula, in Cádiz Bay.

The route of introduction to southern Spain and the Strait of Gibraltar is unknown and several are possible. Initially, specimens may have arrived to the Mediterranean Sea from faraway ports in Indian Ocean or Australia; or from the long-established population in Suez Canal, and subsequently spread towards the western Mediterranean Sea, arriving to France and eastern Spain. It may also have arrived from western Atlantic populations from America or northwestern Africa and entered through the Strait of Gibraltar (Spanier & Galil, 1991; Galil, 2008); or from both Indian and Atlantic populations through multiple introduction events. In any case, its presence in Puerto América marina also indicates a transport via shipping, including recreational boating as vector. This supports the findings of Ulman et al. (2017), who collected individuals of S. walkeri directly from hull fouling of recreational vessels in Mediterranean marinas. Interestingly, S. walkeri was first reported from the Macaronesia biogeographical region only two years ago; at Funchal marina, presumably introduced by means of recreational boating from populations in the Canary Islands (Spain) or the Madeira island system itself (see Ramalhosa et al., 2017). Considering that S. walkeri was already present in Marocco and Mauritania (northwestern Africa) since the early 1980s (Jacobs, 1987), it could have introduced to marinas across Madeira, Canary Islands and the Strait of Gibraltar years ago, even though it was detected only now. An interspecific competition pressure among S. walkeri and its congener S. serratum has been suggested for the Lagoon of Tunis (Ben Amor, Rifi & Ben Soussi, 2015), but further studies are necessary to evaluate its biological impact in the Iberian Peninsula.

Finally, Richardson (1909) first described Paranthura japonica from material collected from Muroran (North Japan). Its native range only includes localities from Japanaese coasts (Nunomura, 1977; Yamada et al., 2007), eastern Russia (Sea of Japan) (Nunomura, 1975; Moshchenko & Zvyagintsev, 2004), Kurile Islands (Kussakin, 1975) and eastern China (Che & Morton, 1991; Li, 2003; Zhang et al., 2009; Wang, Ren & Xu, 2010) (Fig. 3E). It was reported as alien for San Francisco Bay in 1993 (Cohen & Carlton, 1995) and found to be widespread in southern California harbours in 2000 (Cohen et al., 2005). Between 2007 and 2010 it was first found in European waters; in Arcachon Bay (Bay of Biscay, France), probably introduced with oyster transfers. This Bay is one of the major French oyster farming sites (Verlaque et al., 2008), and during the 1970s, the exotic Pacific cupped oyster Crassostrea gigas (Thunberg 1793) from the Senday Bay (Japan) was massively introduced (Mineur et al., 2014), in order to sustain the local industry after a viral disease of Crassostrea angulata (Lamarck 1819). Paranthura japonica probably remained unnoticed or misidentified since then (see Lavesque et al., 2013). It was found in the Mediterranean for the first time in the Lagoon of Venice, probably in 2000 (Marchini et al., 2014). It is thought to have arrived as shellfish import directly from Arcachon Bay, associated with the clam Ruditapes philippinarum (Adams and Reeve 1850) during the 1970s; and secondary spread to further Mediterranean marinas (see Marchini et al., 2014; Marchini, Ferrario & Minchin, 2015; Lorenti et al., 2016; Ferrario et al., 2016b; Dailianis et al., 2016; Tempesti et al., 2016; Ferrario et al., 2017; Ulman et al., 2017).

It was reported only recently from the Iberian Peninsula, from samples collected from fouling assemblages in marinas of the eastern coast in 2016 (Ulman et al., 2017). Nevertheless, our study proves that P. japonica has been present in Barcelona and Valencia (eastern Iberian Peninsula) at least since 2011. Ulman et al. (2017) suggest this species to be ‘polyvectic’ (meaning it has been transported by multiple mechanisms, according to Cohen (1977), Carlton & Ruiz (2005)), and points at recreational boating as vector for its secondary spread across the Mediterranean Sea. Our data supports this hypothesis, since P. japonica was found in Barcelona, Benicarló and Mallorca (Balearic Islands), which are popular destinations for vessels cruising the western Mediterranean in between Barcelona to the West and northwestern Italy to the East (Ulman, personal communication). In 2014, two individuals of P. japonica were found within the Strait of Gibraltar’s vicinity, in Chipiona rocky shores (Cádiz) (Cabezas, pers.comm); and three years later, it was abundant in marinas located in Cádiz Bay. Cádiz is a great hotspot for both international commercial shipping and pleasure craft, as well as a center for aquaculture production, including the Japanese clam Ruditapes philippinarum (Junta de Andalucía, 2014). Just as in Italy, this clam was intentionally introduced for commercial use in Spain in the 1970s. Despite having conducted several samplings in Cádiz marinas before 2014, this species was never found to be present before that date. On one hand, it is possible that P. japonica has arrived to Cádiz bay due to shellfish transfers since the 1970s, but have remained unnoticed and located only in aquaculture facilities instead of spreading to nearby marinas, thus undetected during sampling campaigns. On the other hand, it seems more likely that it spread via recreational boating from the Italian Peninsula to the eastern Iberian Peninsula (present in 2011), and later on to Cádiz marinas (present in 2017). It is to be noticed that P. japonica was not present in the bryozoan B. neritina in Puerto América marina in 2011; but it was found associated to the same host in 2017. This fact supports this record as a new arrival of NIS into a particular region, and thus represents a Marine Strategy Framework Directive indicator to establish Cádiz Bay as a hotspot for marine introductions, following Olenin et al. (2016).

Conclusions

We have reported a distribution range extension for all exotic isopod species present in the studied areas, some of them proving to be polyvectic and well established in marinas. The next step is to evaluate their potential biological, social and economical impact, however, there are gaps of knowledge that hamper this task. Baseline studies delving into the ecology of all these species (i.e. role as prey-predator in the trophic chain, habitat selection, role in their ecosystem functioning) are of great need in here (see Table 1 Blackburn et al., 2014). Although none of the NIS found in the present study were found in the extensive survey of natural coastal habitats by Guerra-García et al. (2012), future surveys including natural areas would be necessary to detect a potential secondary spread into these habitats.

There is a critical problem that keeps recurring and needs to be reduced: the lags in detection of a new arrival. In many occasions, much time lapse between the initial introduction and the report of it, with a bias for noticing invaders only after they become an abundant nuisance, due to inadequate monitoring or lack of taxonomic expertise (see Crooks, 2005). This happens often in the case of small-sized and scarcely studied organisms, which often remain overlooked until they reach high densities and the spreading process is advanced. But small does not mean “unimportant” (Carlton, 2011) and, since biological invasion processes are “irritatingly idiosyncratic” (Richardson et al., 2000), exotics can exist in relatively low numbers before exploding. This means we risk underestimating the potential impact of taxa like the Order Isopoda that, as shown in the present study, can subsequently spread across additional marinas within a short timeframe.

In order to be ready for decision making and implementation of invasion control, as well as assessment of future arrivals, prevention is the key; and all this starts with building comprehensive data on the presence and distribution range of exotic species, especially on new arrivals (see Bishop & Hutchings, 2011; Groom et al., 2015; Olenin et al., 2016). We consider this account serves as documentation and update about the marine exotic isopods dwelling in the Iberian Peninsula, a hotspot for exotics arrival; as well as drawing attention to these overlooked organisms and the risk of recreational boating as vector for introduction and secondary spread.

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