PRESENCE OF THE NEARCTIC WATER BOATMAN TRICHOCORIXA VERTICALIS VERTICALIS ( FIEBER , 1851 ) ( HETEROPTERA , CORIXIDAE ) IN THE ALGARVE REGION ( S PORTUGAL )

This paper describes the presence of the nearctic water boatman Trichocorixa verticalis verticalis in southern Portugal. This species has been cited recently for the first time in Europe from individuals captured in southern Spain. This species, native to Atlantic coast of America, has also been cited from New Caledonia and South Africa, and has been found in the open sea. Two kinds of introduction are reported for this species: involuntary introduction with exotic fish, and passive dispersion through marine currents and severe storms. The possibility of this kind of introduction in Europe is discussed.


Introduction
Trichocorixa Kirkaldy, 1908 is a strictly nearctic and neotropical genus distributed from Canada to Argentina.Trichocorixa verticalis (Fieber, 1851) is a corixid divided into several subspecies throughout North America and the Caribbean islands, inhabiting preferably brackish or saline waters (Sailer, 1948).This species is considered, with the dragonfly Erythrodiplax berenice (Drury, 1770) and several insect larvae, as an extremely euryhaline insect (Hutchinson, 1993).The nomi-nal subspecies, T. v. verticalis (Fieber, 1851), is distributed throughout the Atlantic coast from Labrador to the north of Mexico, and the Caribbean islands (Sailer, 1948;Jansson, 2002).The presence of T. v. verticalis outside the American continent is reported from New Caledonia (Jansson, 1982), several sites in the KwaZulu-Natal region in South Africa (Nzimane River, Umhlatuze River and Charter's Creek; Jansson & Reavell, 1999) and in the province of Cádiz, Spain (Sanlúcar de Barrameda; Günther, 2004).Other species of the genus Trichocorixa are also found outside their distribution area, such as T. reticulata (Guérin-Ménéville, 1857) in Hawaii (Sailer, 1948) or T. kanza Sailer, 1948in Mali (Jansson & Reavell, 1999).Although the presence of T. reticulata from Shanghai is also reported (Hutchinson, 1931;Sailer, 1948), Jansson (1982) doubts the validity of the citation and considers that they are mislabeled specimens from the Hawaiian Islands.
In 2002, adults of the corixid Trichocorixa verticalis verticalis were observed in a puddle on a path on the campus of the Universidade do Algarve, in Gambelas (S Portugal).Later on, during a sampling campaign on large branchiopods, the same species was detected in four temporary ponds, one in Gambelas and three in Castro Marim.Finally, checking macroinvertebrate samples from another study on large branchiopods carried out in the Algarve between 1997 and 1998 (Machado et al., 1999;Machado, pers. comm., 2003), individuals of the same species were identified from a temporary pond near Sagres.

Material and Methods
All the individuals of adult corixids in qualitative macroinvertebrate samples of 16 temporary ponds of the Algarve region (Fig. 1) were sorted and identified under a stereomicroscope.Individuals were captured with a dip net of 30.5 cm diameter and a mesh size of 1 mm.Sailer (1948), Jansson (1986) and Nieser et al. (1994) were used to identify the corixids.Other types of aquatic ecosystems were not sampled, because all samples corresponded to temporary pond community studies.The names of the ponds of Vila do Bispo council follow Alcazar (1998) and those of Castro Marim council follow Cardoso et al. (2000).

Discussion
The presence of Trichocorixa verticalis verticalis in the south of Portugal represents the second citation of this genus in Europe.This species has been cited recently in the province of Cádiz, Spain (Günther, 2004).Thus, the citations in Portugal verify the establishment of this exotic species in the south of the Iberian Peninsula.T. v. verticalis differs from the rest of the Palaearctic species of Corixidae in that it is less than 5.5 mm long, and has sinistral asymmetry, triangular pala, and an apically produced protibia in males (only 2 Palaearctic genera present sinistral asymmetry: Corixa Geoffroy, 1762 and Heliocorisa Lundblad, 1928.Corixa spp.have body lengths superior to 7 mm, and Heliocorisa spp.do not have the pala and the tibia as above).
The discovery of this species in the Algarve region from at least 1997 shows that the presence of T. v. verticalis in the Iberian Peninsula cannot be considered occasional, although the sporadic prospections performed for this and other studies (Günther, 2004) do not allow us either to define with clarity the actual distribution of this species nor to know if the different populations have settled in the area.
The genus Trichocorixa has been cited several times outside its distribution area, being present in Africa, the Pacific region and now Europe.The presence of T. v. verticalis and T. kanza in New Caledonia, South Africa and Mali has been related to the introduction of Gambusia affinis (Baird & Girard, 1853) (Jansson, 1982;Jansson & Reavell, 1999).Two species of Gambusia (G.affinis and G. holbrooki Girard, 1859) have been widely introduced worldwide as a biological control of mosquito populations.These two species are sympatric with T. v. verticalis and T. kanza: G. holbrooki is distributed along the east coast of USA from New Jersey to Florida, while G. affinis replaces it further westwards along the coast of the Gulf of Mexico (Rauchenberger, 1989).On the Iberian Peninsula, G. holbrooki was introduced in 1921 (Nájera, 1944) and arrived in Portugal during the 1930s (Boto, 1932).On the Iberian Peninsula, another exotic fish is also present, Fundulus heteroclitus (Linnaeus, 1766), which is also sympatric with T. v. verticalis in North America, being distributed, in its native area, from Canada to Florida along the Atlantic coast.Fundulus heteroclitus was introduced supposedly in the province of Huelva in 1973, and is now distributed in the Spanish provinces of Cádiz and Huelva, and in the Portuguese region of the Algarve (Gutiérrez-Estrada et al., 1998).Fundulus heteroclitus and G. holbrooki are euryhaline species, although F. heteroclitus inhabits waters with higher salinity.On the Iberian Peninsula, we must allow for the possibility of the accidental introduction of T. v. verticalis with F. heteroclitus, and not only the hypothesis of introduction with G. holbrooki.In fact, the coincidence of the distributions of T. v. verticalis and F. heteroclitus in the Iberian Peninsula, and the absence of citations of T. v. verticalis in the recent revisions of the distribution of Corixidae on the Iberian Peninsula (Nieser & Montes, 1984;Baena & Vázquez, 1986;Nieser et al., 1994) or in Europe (Jansson, 1986;Polhemus et al., 1995) makes it more plausible to consider that, if introduced with an exotic fish, it could be with F. heteroclitus.Although the citations of several species of Trichocorixa outside the American continent seem to be related to anthropic activity, Hutchinson (1931) and Sailer (1948) do not reject the hypothesis that the presence of T. reticulata in the Hawaiian islands could be caused by a dispersion due to the marine current between the Pacific coast of the continent and the Hawaiian islands.Nevertheless, these authors also admit the possibility that its arrival in the Hawaiian islands could be due to an anthropic factor, and Jansson (1982) considers rather improbable the hypothesis of marine dispersion.On the other hand, Sailer (1948) describes the colonization of the island of Bermuda by T. v. verticalis transported by a severe storm in 1927.
Trichocorixa verticalis verticalis has the capacity to inhabit saline environments (Wurtsbaugh, 1992;Aiken & Malatestinic, 1995) or even to survive in the open sea (Hutchinson, 1931;Gunter & Christmas, 1959).In fact, there are twelve genera of Corixidae which can inhabit saline waters, but only the genus Trichocorixa has been found in the open sea, and T. verticalis has a high osmoregulatory ability (Scudder, 1976).This ability to survive in the open sea and the possibility of being transported by severe storms implies that natural transportation of T. v. verticalis cannot be completely ruled out (Hutchinson, 1993) and it could be the way of introduction of T. v. verticalis into Europe.The existence of the Gulf Current between the American and European Atlantic coasts, and the predominantly easterly winds in the Atlantic support this hypothesis.According to that, the occasional presence of the migratory butterfly Danaus plexippus (Linnaeus, 1758) on the Western European coast is related to these predominant winds (Vanholder, 1996;Fernández Vidal, 2002).The wind has been considered an important way for the intercontinental dispersion of several insect species, as in the dispersion of the orthopteran Schistocerca gregaria (Forskål, 1775) between Africa and the Caribbean (Rosenberg & Burt, 1999), or the dispersion of the lepidopteran D. plexippus between New Caledonia and Australia (Clarke & Zalucki, 2004).

Fig. 1 .
Fig. 1.-Map of the sampled temporary ponds in the Algarve region.Black dots show sites with presence of Trichocorixa verticalis verticalis, and empty dots show sites without its presence.

Table 1 .
-Presence of corixid species in several temporary ponds in the Algarve region.Tabla 1.-Presencia de especies de coríxidos en diversas lagunas temporales en la región del Algarve.