Craspedacusta sowerbii Lankester, 1880 (Fig. 1) is a hydrozoan species belonging to the family Olindiidae and characterized for inhabiting inland water environments (Jankowski et al., 2008). The life cycle of this species include both a benthic polyp and a free-swimming medusa stages. The polyps are inconspicuous, minute and without tentacles. Medusa has an umbrella of 10–20 mm in wide, broad and well-developed velum with a large manubrium extending beyond the umbrella margin. The statocysts are within elongated vesicles enclosed in velum (Bouillon et al., 2004). Craspedacusta sowerbii has a cosmopolitan distribution, colonizing all continents apart from Antarctica; actually, it is the most widely distributed freshwater medusa (reviewed in Dumont, 1994 and Jankowski et al., 2008).

Within the genus Craspedacusta Lankester, 1880, more than ten species have been originally described (Jankowski, 2001). However, the lack of a systematic revision at world level has not yet resolved the main question about the number of valid species in Craspedacusta and further molecular data are needed before to may conclude this issue (Fritz et al., 2009; Zhang et al., 2009; Karaouzas et al., 2015). Eastern Asia seems to be the center of diversity for Craspedacusta, showing the highest concentration of species (Jankowski, 2001; Zhang et al., 2009). Moreover, from the classical works of Sowerby (1941) and Kramp (1950), the idea of a putative center of origin in China for Craspedacusta continues to be a suggestive study hypothesis on the basis of biological and biogeographical indicia (Dumont, 1994), in replacement of its early neotropical ascription (Lankester, 1880). Particularly, C. sowerbii is widely considered native to the Yangtze valley (e.g.: Fritz et al., 2007, 2009; Gasith et al., 2011; Karaouzas et al. 2015), until a conclusive phylogeographical study emerges.

The first description about the existence of C. sowerbii occurred in 1880, at the Royal Botanic Garden of London (Lankester, 1880). This finding already revealed the potential invasiveness of this species and the importance of human activities as vectors of its long-distance dispersal. In mainland Europe, C. sowerbii was initially reported in 1901 (Sowerby, 1941). Since then, it was recorded from a rapidly increasing number of localities through the first half of the 20^th century, both in artificial and natural environments (Kramp, 1950). During the last years, published citations on the presence of C. sowerbii in different European countries have increased again (e.g.: Arbaciauskas & Lesutiene, 2005; Lundberg et al., 2005; Pérez-Bote et al., 2006; Fritz et al., 2007; Jacovček-Todorović et al., 2010; Stefani et al., 2010; Gomes-Pereira & Dionísio, 2013; Morpurgo & Alber, 2015; Minchin et al., 2016), with the occurrence of more than one independent colonization event (Karaouzas et al., 2015). It is likely that this scenario will not only be due to a plausible biological reality (i.e.: progressive colonization of new localities), but also to a greater effort of sampling, observation, monitoring and control of water bodies in relation to exotic species (Cardoso & Free, 2008), together with the phenomenon of universalization of information made possible by new communication technologies (Silva & Roche, 2007).

In the Iberian Peninsula, the first known identification dates back to 1968, when it was recorded in the Sau reservoir (Barcelona) (Margalef, 1977: 246). After fifty years, C. sowerbii is well distributed throughout the Iberian territory, with records in numerous Spanish provinces and several Portuguese districts (Table 1, Fig. 2). Unfortunately, most information about the presence of this invasive species in the Iberian Peninsula is provided by regional academic publications, technical reports of limited access, sparse naturalists’ observations or press news on environmental issues. This circumstance makes it difficult for the scientific community and environmental management institutions to stay informed of the updated distribution of C. sowerbii in this territory. Therefore, the goals of the present work were: (i) to present a synthesis of known Iberian localities for this species, and (ii) to report the presence of an established population of C. sowerbii from the low Guadalquivir River (western Andalusia, southwestern Spain). In addition, environmental data associated with its occurrence are provided.

Iberian Distribution. To our knowledge, C. sowerbii has been detected at least in 50 localities from 26 Spanish provinces and 6 Portuguese districts (Table 1, Fig. 2). All annotated records occurred in the bell-shaped jellyfish stage, which must be taken into account as a possible factor of underestimation about its real distribution in Spain. Indeed, the presence of C. sowerbii is usually ignored when the organism is only present as a polyp (Duggan & Eastwood, 2012).

Lentic waters define the characteristic biotope of C. sowerbii (Jankowski, 2001), which explains that reservoirs and lakes were the dominant habitat in the Iberian Peninsula (74%). Medusae were also frequent in temporary rivers (22%), where inhabit ponds and backwaters from middle to low courses (Table 1). In these environments, the absence of strong currents and the remarkable increase of the water temperature during the summer favour polyp fixation and medusa budding respectively (De Vries, 1992).

Warm temperature, low salinity and high dissolved organic matter have been interpreted as some of the main environmental factors favouring the development of C. sowerbii medusae (Zhang et al., 2016; Caputo et al., 2018). Unfortunately, the autoecology of this species is still little explored in the Iberian Peninsula and limnological data associated to its records are uncommon. Nevertheless, these scarce data suggest that C. sowerbii prefers mesotrophic lentic water-bodies and it is usually found in calm, freshwater reservoirs, lakes or pounds connected to river systems, with a maximum depth of 10 m and summer temperature above 15°C. The occurrence of medusae is between July-November, but they could be absent during several consecutive years (Ferreira, 1985; Ruiz et al., 1992; Pérez-Bote et al., 2006; Fraile et al., 2008; Gomes-Pereira & Dionísio, 2013; Montserrat, 2017).

Low Guadalquivir River. On August 19^th 2006, a population of C. sowerbii was found at Sietearroyos River, in Villaverde del Río, Seville, southwestern Spain (37°37’23’’ N -5°53’13’’ W). Specimens were identified according to macromorphological diagnostic characters reviewed by Jankowski (2001) (Fig. 1). This locality has been monitored since then, having detected medusa blooms during most summers of the last 12 years. It is the first documented record of C. sowerbii from low Guadalquivir and also the first published for western Andalusia.

Sietearroyos is a seasonal Mediterranean stream, direct tributary of the Guadalquivir River and one of the few courses of the province that maintain water throughout the year. During summer, including the driest years, the lower and middle course have permanent waters with a marked influence of the Guadalquivir in their confluence, while the upper course only retains points with isolated stagnant water-bodies or weakly connected to each other with very low flows. As a whole, the lower course is integrated in an alluvial-plain geomorphological unit with deep and fertile soils, accompanied by a deciduous riparian forest (Nerio oleandri-Populetum albae) within a cropland landscape matrix. On the other hand, the upper course is enclosed in a narrow valley excavated on micaschistous bedrock, embedded within an agroforestry mosaic with open sclerophyll vegetation (Flueggeion tinctoriae and Oleo-Ceratonion). Particularly, C. sowerbii population inhabits the middle-upper course of the stream, in a system of shallow ponds that form a single water-body of meandering topology, with a total length of 200 m, maximum width of 15 m and depth up to 3 m (Fig. 3). This zone is located at altitude of 33 m above sea level, separated 8.6 km from the confluence with the Guadalquivir River and freely exposed to direct sunlight throughout the day. Its main physicochemical water parameters associated with the occurrence of C. sowerbii are listed in the Table 2.

Water temperature is a key factor in jellyfish blooms (Purcell, 2005; Prieto et al., 2010). In particular, the polyps of C. sowerbii release medusae at a threshold of 21°C (Folino-Rorem et al., 2016), which is consistent with summer temperature in the study area (Table 2). Alkaline levels of pH recorded in Sietearroyos were similar to others reported for the Iberian Peninsula (Ruiz et al., 1992; Ferreira, 1985; Pérez-Bote et al., 2006) or zones with comparable latitude in Europe (Karaouzas et al., 2015). Nevertheless, records in pH from 6.5 (Litton, 1984; Fraile et al., 2008) to more than 8.5 (Raposeiro et al., 2011; Kozuharov et al., 2017) indicate the wide range of pH tolerance for this species. Analogously, values of electrical conductivity in Sietearroyos are also included within the wide range of values reported in the literature for this species: from 60–70 µS/cm² (Fraile et al., 2008) to 1260–1340 µS/cm² (Ruiz et al., 1992), in the Iberian Peninsula. However, as a surrogate of salinity, values of electrical conductivity would be expected low in optimal habitats for C. sowerbii medusae (Zhang et al., 2016). Moreover, measurements of water quality reveal high values of salinity from July to October (28.5–61.2 mg/l Na⁺) downstream of the sample point (41503 DMA Station, Hydrographic Confederation of the Guadalquivir Basin), in spite of freshwater nature of this species.

Craspedacusta sowerbii is considered an invasive species in freshwater systems worldwide, which has established populations in natural, as well as artificial and human-modified habitats (Dumont, 1994; Jankowski et al., 2008; Duggan & Eastwood, 2012). Its wide range of sexual and asexual reproductive strategies makes it an efficient invader. Furthermore, the life cycle of this species includes an encysted phase and it can remain dormant for many years (Bouillon et al., 2004). This characteristic makes this species a perfect invader, allowing withstand by long periods of extreme environmental conditions and facilitating the anthropogenic transport (Jankowski, 2001). Several dispersal vectors have been suggested to explain the cosmopolitan pattern of C. sowerbii, all them associated with the larval (frustule) and resting stages (cysts): water transfer, ship drift, use of alluvial sands for the construction of hydraulic infrastructures, malpractice in aquarofilia, introduction of fish and aquatic invertebrates for sports and commercial purposes, natural movements of wetland birds, importation of aquatic plants, etc. (Turquin, 2010; Fraire-Pacheco et al., 2017). Indeed, several invasive fish species (Alburnus alburnus Linnaeus, 1758; Ameliurus melas Rafinesque, 1820; Lepomis gibbosus Linnaeus, 1758) coexist at the study pools, which are frequented by fishers. Nevertheless, it is also necessary to consider the hypothesis of the spontaneous entry of C. sowerbii from the Guadalquivir River, since this is a natural route of introduction of exotic species in the area (Encina et al., 2006). Moreover, in dry years in which the disruption between water-bodies from upper course and the Guadalquivir are more pronounced, medusa blooms were not usually observed (e.g.: 2016, 2017).

Craspedacusta sowerbii is classified by the European Commission as an alien species of high impact in the European freshwaters (EASIN, 2017). The predatory impact of C. sowerbii on zooplankton community has been described within natural environments and in the laboratory (Boothroyd et al., 2002; Jankowski et al., 2005; Smith & Alexander 2008). In fact, different authors point out that blooms of C. sowerbii can increase its negative effect on native zooplankton community, alter the aquatic food webs and reduce the dissolved oxygen in water (Jankowski, 2000; Smith & Alexander, 2008; Gasith et al., 2011; Folino-Rorem et al., 2016). Dodson & Cooper (1983) conclude that the size range of prey is about 0.2–2.0 mm, though it can kill, but not eat, preys up to 8.8 mm in length. Thus, the food web effect of C. sowerbii is by reducing the density of other invertebrate predators and increasing the abundance of herbivorous zooplankton.

The loss of biodiversity by the introduction of alien and invasive species could not simply be considered in terms of displacements of some native species in favour of their equivalent newcomers (González-Duarte et al., 2016). All associated fauna and relationships can be finally altered (Byers et al., 2002). The possible ecological, socio-economical and evolutionary implications of the introduction and dispersion of C. sowerbii around the world are far from correctly understood. The impact of this invasive species on Iberian ecosystems is still preliminary, but this study contributes to know its geographic distribution along with some important physicochemical parameters.

Only further molecular analysis will be able to provide additional information about the true origin of this species. In addition, long-term studies of the populations will provide more data about the relationship between the environmental factors and the fluctuations of the population, and the influence of C. soberwii in the trophic web.