On the presence , distribution and habitat of the alien freshwater snail Ferrissia fragilis ( Tryon , 1863 ) ( Gastropoda : Planorbidae ) in the oceanic islands of the Azores

The present work reports on the presence and distribution of an alien and invasive gastropod, Ferrissia fragilis (Tryon, 1863), in freshwater systems of the oceanic islands of the Azores archipelago. First registered in 2007, F. fragilis was found in S. Miguel, Santa Maria, and Graciosa. This study increases to 6 the number of freshwater species of mollusks occurring in the Azores. Remarks are added on the possible impact of invasive species in island freshwater systems.


Introduction
The geographic position of oceanic islands strongly limits dispersal and colonization of freshwater systems, resulting in a "disharmonic" fauna, characterized by low levels of diversity and high abundance of insect orders when compared to continental systems.Oceanic islands are potentially highly vulnerable to invasive species due to the low levels of diversity (and therefore competitors) and the resulting availability of ecological niches (Covich 2009).
Ferrissia fragilis has a near-cosmopolitan distribution in temperate and tropical freshwater pond ecosystems, owing to its small body size, hermaphroditism and ability to live in stagnant water (Walther et al. 2006).First records in Europe were from artificial habitats -aquaria and botanical gardens in Germany (Boettger 1949) and England (Brown 1977).
The early studies of the malacofauna of the Azores clearly stated the absence of freshwater mollusks (Morelet 1860).The first fresh-water species, Physa teneriffae Mousson [=Physella acuta (Draparnaud)], was recorded by the Azorean malacologist F. Arruda Furtado (1881) and soon after Jules de Guerne (1887) described Pisidium dabney [=Pisidium casertanum (Poli)] from Faial.E. Fisher-Piette (1946) added Ancylus striatus Q. and G., but its presence in the Azores was never confirmed.Backhuys (1975), in his revision of the land and freshwater malacofauna of the Azores, added Helisoma trivolvis (Say), Galba truncatula (Müller) and Radix peregra (Müller).F fragilis was recorded for the first time from S. Miguel, Santa Maria, and Graciosa by Raposeiro et al. (2007); the present contribution analyses its distribution and habitat in Azorean waters, and discusses the impact of the alien species in these freshwater ecosystems.

Study area and sampling procedure
Located between 36º-39º North, and 24º-31º West, the Azores is an atlantic oceanic archipelago comprising nine islands (Figure 1).Being 1,300 km from the nearest continental coast (Cabo da Roca, Portugal), and 1,900 km from the American Continent, it is the most remote Macaronesian archipelago.
Collections were made during the biological freshwater monitoring program of surface waters during 2005-2007(Gonçalves et al. 2007, 2008)), during Graciosa (Azevedo et al. 2005) and Pico Expedition (Gonçalves et al. 2006) and other campaigns in all islands.The sampling covered several types of freshwater habitats in the Azorean archipelago.Samples were collected with a hand net (mesh size: 500 µm) along the surface waters of ponds, lakes, streams, reservoirs and animal drinking troughs.All collected material was fixed in 70% ethanol and deposited in the mollusks collection of the Department of Biology of the University of the Azores (DB_FW_AZ_0004).

Results
In the Azores, Ferrissia fragilis was recorded in S. Miguel, Santa Maria, and Graciosa.First observed in São Miguel in Sete Cidades Lake (25º46'41''N/37º52'10''W in January, 2004) and was later found in Furnas Lake (25º19'58''N/ 37º45'59''W in March, 2004).F. fragilis was also collected in the same year in a rivulet in Santa Maria (25º07'09''N/36º57'43''W in March, 2004) and in a wet zone in Graciosa (39º02'35''N/ 28º01'38''W in August, 2004).In the lakes, it was found most commonly on submerged leaves of macrophytes (e.g. the introduced invasive Egeria densa in Sete Cidades) in the onshore shallow zone (depth 0.2-0.3m); it was also seen on submerged stones covered by periphyton or on plant detritus deposited on the bottom.In streams, F. fragilis was found attached to submerged stones, more frequently in pools or in zones where there is a predominant low flow.Description: shell thin, light amber to greenish-brown, with elliptical, laterally flattened dorso-ventral outline, covered with marked growth rings; apex on the posterior third, slightly dislocated to the right side (Figure 2A); height from base to apex about twice its width.Length and width measurements were made in the case of ten specimens, using a square grid calibrated at 0.1mm.Specimens were on average 1.6 mm length (max: 1.9 mm; min: 1.0 mm) and 1.0 mm width (max: 1.1 mm; min: 0.7 mm).Animal completely covered by the shell; headfoot whitish to light-brown, tentacles moderately long and thin, eyes black, conspicuous, sessile, located at the posterior base of the tentacles; visceral mass brown, irregularly mottled with darker spots (Figure 2B).

Discussion
The record of Ferrissia fragilis in the Azores corresponds to the only register of the species in the oceanic islands of Macaronesian region.As matter of fact, this species is not reported in the last updated checklist published for the region (Izquierdo et al. 2001;Bank et al. 2002;Arechavaleta et al. 2005).The record of F. fragilis increases to 6 the number of species occurring in the archipelago, distributed across 2 classes and 4 families (Table I).Apparently, all the gastropods found in Azorean freshwater systems are introduced (Backhuys 1975; Frias Martins 1991).
Molecular work by Walther (2008) has revealed that all the Azorean specimens, as well as the European F. clessiniana, are in fact a single North American species, F. fragilis.This finding highlights the importance of the Azores as a possible stepping-stone for colonization and dispersal of species across the Atlantic, possibly mediated by nearctic birds or insects.Birds like American Black Duck (Anas rubripes), Common Snipe (Gallinago gallinago), Mallard (Anas platyrhynchos), Moorhen (Gallinula chloropus) and, Teal (Anas crecca) are possible vectors in bringing F. fragilis to the Azores archipelago because they live in freshwater systems, breed locally, and migrate from America passing through the Atlantic islands on their way to Europe.Once arriving, the key for successful establishment of populations might be the capacity for self-fertilization of this hermaphroditic gastropod.In Sete Cidades and Furnas lakes, F. fragilis occurs simultaneously with two other alien invasive species: the crayfish -Procambarus clarkii (Girard, 1852) (Correia and Costa 1994), and the plant -Egeria densa Planch.Both species are present in Lagoa das Sete Cidades, but only the crayfish is covered by Portuguese law (Dec.Lei 565/99).Egeria densa, also known as Brazilian elodea, in contrast to the crayfish, is not mentioned as introduced, potentially harmful or an invasive species under national law.However, the conspicuous presence and growth of this plant has provided evidence for public awareness of the eutrophic state of the lake.As a consequence, the local government has shown environmental concern by taking measures such as mechanical harvesting, in order to mitigate its impact on the eutrophication of the lake (Costa 2005).F. fragilis is likely to become an element of crayfish diet providing another source of animal protein for the crayfish, and both will benefit from the habitat provided by E. densa.The extent of trophic relations among these three non-native species in Sete Cidades lake remains to be established but their co-existence underlines the degraded state of this lake.Another possible pathway for arrival of F. fragilis to the Azores is through the aquarium industry, as reported by Frias Martins (1991), in Terceira island for Helisoma trivolvis.In Graciosa, F. fragilis was present in a small pond, 35 by 15 m, very turbid, with a strong dominance of Euglenophyta, namely Phacus longicauda and Trachelomonas bacillifera (Azevedo et al. 2005), indicating strong water pollution by nutrients.Also, in Santa Maria it was found in a small rivulet enriched with nutrients (Inova 2006).The presence of non-native and invasive species in organic polluted and degradated aquatic systems has long been observed and it has been recently proved that such polluted habitats can favour invasive species (Crooks et al. 2010).
Oceanic islands have a reduced, but unique diversity due to the different levels of separation caused by various factors for example, distance to mainland, island age and origin, all of which strongly limit the dispersal, colonization and evolution of the organisms therein.Insular systems are potentially highly vulnerable to invasive species due to the low levels of diversity (and therefore competitors) and the resulting availability of ecological niches.The establishment and consequences of introduced species has been object of discussion in a lot of studies (e.g.Ricciardi 2003; Gilbert and Parker 2010), but we are still not able to predict either the outcome of the introduction of any particular species, for example F. fragilaris, or the impact of invasions in general to historically highly vulnerable insular systems such as the Azores.

Figure 1 .
Figure 1.Location of Azores archipelago and islands (in dark) where Ferrissia fragilis is present.