The loss of environments suitable for the development of autochthonous species and invasion by exotic species are the basis of global biodiversity damage [17, 18].
Hypersaline aquatic ecosystems, like coastal and epicontinental lagoons, salt lakes, even exploited solar saltworks are dramatically suffering from these impacts [19–21]. Many human activities threaten or have already damaged salt marshes and hypersaline ecosystems through inflow diversions, pollution, biological disturbances, and diverse anthropogenically-induced damage like abandonment in favour of other economical activities. Small solar saltworks managed according to old artisanal techniques are amongst the most threatened of these ecosystems. The data shown in Table 1 confirm that of about 60 saltworks reported from both archipelagos (52 in Canary Islands and 8 in Cape Verde), only 8 of these exploitations can be considered to be still working. Most are small, were established during the 18th and 20th centuries, occupy areas ranging between 1 and 3 ha, and are still managed in an artisanal way, producing between 110 and 500 metric tonnes of sea salt per year. The only exceptions are the new Fuencaliente saltworks in La Palma, and Janubio saltworks in Lanzarote. Janubio covers an area of about 44 ha and produces about 13,000 tonnes per year . Most of these saltworks still persist because they lie in natural areas protected by government environmental agencies. This is the case of Fuencaliente and Janubio in the Canary Islands and Pedra Lume in Cape Verde, which still exist because of their natural landscape, biodiversity protection and tourist interest. Briefly, it can be asserted that biotope loss in terms of abandoned saltworks is about 85% in both archipelagos.
It is not possible to assert the origin of the A. franciscana population found in El Médano sea shore pool (Tenerife, Canary Islands) and in the Pedra Lume, Santa Maria and Sal Rei salterns (Cape Verde), all far from the native distribution range of this American species. Most probably A. franciscana cysts reached these biotopes for deliberate commercial reasons (salt production) or through erroneous or inadvertent intrusive inoculations (pet trade). But, beyond question, these inoculations occurred at different moments in the past. As stated above, the presence of native diploid parthenogenetic Artemia populations in the Canary Islands has been known since the 1980’s, but no Artemia populations have been described in the western Canary Islands, i.e. in Tenerife, since then.
The presence of native Artemia in the Canary Islands can be closely related to the natural dispersion of Artemia cysts from Europe (Spain, Portugal) [11, 13, 14, 22] or from the African continent, especially Morocco, where this strain was found in Salines Marocaines (32º53’50”N-9º50’20”W) and in the saltworks at Larache and Asilah (35º11’50”N-6º07’08”W) . In the western Sahara, not more than 100 km away from Lanzarote and Fuerteventura islands, there are several hypersaline biotopes and saltworks, e.g., Dait Um Saad El Aaiun (27º09’52”N-13º11’44”W) and Sebhet Taazgha (27º32’59”N-13º00’31”W), which could be considered a hypothetical source of dispersion towards the Canary Islands, although there is no information about the presence or biodiversity of Artemia populations there.
The Cape Verde A. franciscana populations could have reached this archipelago earlier because they show an important level of genetic divergence compared with other American populations that invaded the western Mediterranean biotopes in Spain, Portugal, France, Morocco and Italy . There is no information about the possible existence of native Old World Artemia populations in this archipelago before the arrival of A. franciscana as an exotic species.
Even considering the previous presence of native species as a result of a natural dispersion from the African continent, the nearest hypersaline biotope that could be the source of this dispersion is located in Senegal, in the Kaolack saltworks (14º06’54”N-16º05’37”W) about 750 km away from the easternmost Cape Verde saltworks (15º59’29”N-22º47’10”W) in Curral Velho (Boa Vista island). Nevertheless, there is no information available on the biodiversity of Artemia in Senegal, although some publications on the global geographical distribution of Artemia species mention the presence of at least three localities in this country where Artemia could be present: Dakar, Lake Kayar and Lake Retba. In these publications Kaolack saltworks are not cited. No additional information is available on the taxonomy of the species present in these locations or on their way of reproduction [12, 23, 24].
The presence of A. franciscana in El Médano could be the origin of the dispersion of this exotic species towards other hypersaline biotopes existing in the eastern Gran Canaria, Lanzarote and Fuerteventura Islands, as well as towards the west (La Palma). In addition to possible anthropic intrusive inoculation, the role of migratory and resident aquatic birds in spreading brine shrimp cysts, attached or stuck to feathers and feet, has been demonstrated . They can also survive passage through the gut after capture as food by birds, and cysts may be excreted during displacements in search of food or during seasonal migrations . This possibility has been broadly tested after verifying the presence of large numbers of viable cysts of invasive A. franciscana and native A. parthenogenetica in the faeces and pellets of common redshank (Tringa totanus), blacktailed godwit (Limosa limosa) and dunlin (Calidris alpina) collected in saltworks in the south-west of the Iberian Peninsula, at Castro Marim (Portugal) and Cadiz Bay (Spain) [25, 26]. Several species of these shore bird groups have been reported as regular winter visitors in the south of Tenerife (El Médano) and in other saltmarshes and saltworks in the eastern Canary Islands [27, 28]. These sites probably play a role as stopover and feeding places for these birds as they migrate from the European continent to African stating areas along the East Atlantic flyway [25, 26]. If this is so, the dispersion of brine shrimp cysts and their inoculation in new biotopes is almost guaranteed.
The probable arrival of A. franciscana cysts from the El Médano population to the other islands housing active saltworks and native diploid parthenogenetic Artemia populations would have triggered a competitive coexistence between both forms and, as is well known, parthenogenetic strain usually comes out as loser .
Knowledge of the mechanisms leading to the success of invasive species relies on our understanding of any interactions triggered between the exotic and the native species, particularly as regards the attributes of biological fitness characterizing both invaded and invasive species. Assessment of this biological fitness led to the study of life table parameters for species under laboratory experimental conditions, which provided the knowledge necessary to explain the results of competition between native species and congeneric invasive exotic species [29–31].
In the case of Artemia, much information is available on competitive interactions between different species and populations, and between sexual species and parthenogenetic strains [20, 32–35]. Competitive superiority begs the classical questions referring to invasive species: (i) why do invasive exotic species colonize and displace native species that should be better adapted to local environments [36, 37] and (ii) why do invasive species flourish despite reduced genetic diversity in the recipient region ?
A. franciscana in the Old World should display less genetic diversity than in its American native range. Founder events and population bottlenecks in the early stages of introductions are considered responsible for the loss of biodiversity of many invasive species. Genetic diversity for nuclear markers must be lower as a consequence of the founder effect during introduction. However, in the case of A. franciscana, and as regards its introduction into the Macaronesian islands (Tenerife in the Canary Islands), the probability of an alien species thriving, when it has a combination of fitness traits superior to any native species, is obviously much higher on small isolated islands and in hypersaline isolated biotopes, which necessarily display much less variation, than in large continents with large populations and high biodiversity .
Also, in the case of A. franciscana, in its role as an introduced species, it may well show a similar or higher accumulation of diversity than native American populations. This may result from a combination of multiple local introductions of several origins (anthropic, birds) and numerous translocations from these sites of introduction, or have other origins like local mutations that have no negative effects on individual fitness or as a result of a small population size allowing the accumulation of deleterious mutations (Muller’s Ratchet effect) .