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Introduction
The state of Tabasco is rich in benthonic fauna (Bueno et al., 2005), among which brackish and marine mollusks are relatively well-studied, while terrestrial micromollusk parasite vectors are in need of attention (Espinosa & Ortea, 2009).
The Streptaxidae Gray, 1860 family includes the Huttonella Pfeiffer, 1856 genus with Hutonella bicolor (Hutton, 1834) as its type species. However, various authors consider it to pertain to the genus Gulella Pfeiffer, 1856 (Vermeulen, 2007) and the subgenus Huttonella (Cowie, 1997; Simone, 2013). It is also known as Rhabdogulella bicolor (Fernández-Garcés, 2008). Huttonella bicolor is a species of India (Christensen & Kahn, 2017). This species has been introduced in various tropical areas, such as islands in the Caribbean region and several regions in the western and southwestern Gulf of Mexico that include the United States of America (USA) and Caribbean countries such as Mexico, Nicaragua, Panama, Venezuela and Brazil. Its distribution in North America comprises: Bermuda, UK (Bieler & Slapcinsky, 2000; Pilsbry, 1900); Florida (Burch, 1962), Mississippi (New Orleans), Louisiana (Baton Rouge), South Carolina and Texas in the USA (Auffenberg & Stange, 2001; Dundee, 1974; Dundee & Baerwald, 1984; Hermann et al., 1965; Schalie, 1948; Turgeon et al., 1988, 1998); and Veracruz and Tabasco in Mexico (Correa-Sandoval, 1999; García et al., 2006). In Central America, it has been found at: Taboga Island, Panama (Pilsbry, 1926); and the old city of Panama (Pilsbry, 1926, 1930); and Nicaragua (López, 1999; Pérez & López, 2003; Pérez et al., 2008). The sites in South America are: Brazil (Pilsbry, 1926; Santos et al., 2008; Simone, 2013); French Guyana (Tillier, 1980) and Venezuela (Ojasti et al., 2001; Venmans, 1963). Presence has been observed in the Antilles-Caribbean at: Jamaica (Rosenberg & Muratov, 2006); Cuba (Aguayo, 1944; Fernández-Garcés, 2008; Maceira et al., 2013; Pilsbry, 1900, 1926; Sarasúa, 1944); Puerto Rico (Schalie, 1948); Trinidad Tobago, Grenada, Saint Thomas (Dundee, 1970, 1974; Pérez & López, 2003; Pilsbry, 1900; Tryon, 1885); Dominica (Robinson et al., 2009) and Guadalupe (Bouchet & Pointier, 2003 in Santos et al., 2008). Distribution includes other regions such as: Africa; the Indo-Pacific (Cole & Herbert, 2009; Herbert & Rowson, 2011; Rowson et al., 2011); Australia (Stanisic, 1981); Borneo, Southeastern Asia (Vermeulen, 2007); Southwestern China (Yen, 1939); the Philippines (Van Benthem Jutting, 1950); India (Ramakrishna & Mitra, 2010; Tryon, 1885; Van Benthem Jutting, 1961); Burma, Cochin China (Southern Vietnam), the Indian Archipelago, China, New Caledonia (Tryon, 1885); Africa, South and East Asia, the West Indies (Van Benthem Jutting, 1950); Seychelles Islands, Indian Ocean (Gerlach & Bruggen, 1999; Tryon, 1885); Japan (Azuma, 1982); Kenya (Clench, 1964); Malaysia (Van Benthem Jutting, 1961); Singapur, Central Asia (Ho, 1995; Van Benthem Jutting, 1961); Sri Lanka, Myanmar (Blanford & Godwin-Austen, 1908); Thailand (Chaijirawowg et al., 2008), Nepal (Budha et al., 2015) and the Society Islands, French Polynesia (Christensen & Kahn, 2017).
Specific studies on H. bicolor published hitherto have been related to its carnivorous type of feeding, status as an exotic species, biological control measures (Davis & Butler, 1964; Mead, 1961), preferred prey (Srivastava, 1968; Srivastava et al., 1975), effectiveness as a predator of Subulina octona (Bruguière, 1798), a Subulinidae (Mead, 1961) and Vertiginidae = pupillids (Dundee & Baerwald, 1984), taxonomic implications (Annandale & Prashad, 1920; Naggs, 1989), and its anatomy, radula and genital system (Berry, 1965; Chaijirawowg et al., 2008; Dundee & Baerwald, 1984; Simone, 2013).
According to reports from other countries, the presence of the predator H. bicolor could pose a threat to Mexico due to its condition as an exotic species. Consequently, the present study contributes a second record of the species in the state of Tabasco. At the same time, it aims to assist governmental and executive authorities to become more aware of the importance of this issue, bringing needed attention to the records from 1999, 2006 and the present work. Such records address the introduction of this exotic species in addition to others that until now had remained hidden, as well as their potential dispersion towards other regions of the country, and in particular, the short and long term impact they may have on biodiversity, agriculture and human health in this and other regions of Mexico.
Material and methods
Huttonella bicolor was collected on August 4th, 2010 at Puente San Manuel, a town in the municipality of Huimanguillo (17° 49´ 01.52” N, 93° 23´ 10.58” W) near the Mezcalapa River, southeast Mexico, state of Tabasco, at a linear distance of 115.71 km and a road distance of 139 km from Coatzacoalcos, Gulf of Mexico. Two empty shells were obtained after lifting a compacted fragment of sandy-mud by hand. The shells were kept dry, and one specimen was placed in the Colección Nacional de Moluscos, Instituto de Biología, Universidad Nacional Autónoma de México, # CNMO 6539. Digital photographs and measurements of the embryonic shell were taken using a Zeiss STEMI 2000 Stereomicroscope.
Results
Huttonella bicolor grows to be 7.5 mm high and 2 mm wide, and has 7 ½ whorls. The shell is smooth and light yellow (Fig. 1A), and the aperture presents four teeth (Fig. 1B). The species was found in sandy mud. No more specimens were located, perhaps because of the environment or because prey is not abundant; nonetheless, a more rigorous effort in the field might demonstrate a true settlement of the species around the site or other areas in the country.
Discussion
Nearly 50 record entries of H. bicolor geographic distribution were found, from its description by Hutton (1834) in India to that by Budha et al. (2015) in Nepal. A patent dispersal trend has been observed from its possible place of origin (India, southern Asia) towards island regions in both the Caribbean (Tryon, 1885) and Asia (since 1908). It has also reached inland tropical regions in the western Atlantic, such as South Carolina, Louisiana and Mississippi in the USA, as well as Mexico, Nicaragua, Panama, Venezuela, French Guyana and Brazil. As for the Pacific region, records are few and far between; López (1999) recorded H. bicolor in Nicaragua (Pacific). Scarce information concerning pathways, environmental factors and food items may be limiting factors.
Huttonella bicolor is distributed in 17 islands, of which about 10 are in the Caribbean region, while the other 7 are in Asia, Africa and Australia.
The Gulf of Mexico possesses rainy areas with wet and muddy substrates; such climatic conditions are perhaps advantageous to the development and survival of H. bicolor. The species, in addition to having been found in sandy mud, lives in forests, beneath rocks, debris and leaf litter, at the base of house foundations, trunks, and garden walls and/or on plant roots (Annandale & Prashad, 1920; Barker & Efford, 2004; Dundee & Baerwald, 1984; Naggs, 1989; Santos et al., 2008; Smith & Stanisic, 1998; Solem, 1988).
Regarding food, H. bicolor is a predator of subulinids such as S. octona (Srivastava, 1968; Srivastava et al., 1975), a land snail that has been widely introduced around the world and is common around disturbed habitats. This prey, a hermaphroditic micro-snail, is a detritus feeder (herbivore), is found within organic debris, and is distributed and frequent in the Caribbean (Deisler & Abbott, 1984) and tropical America (Dundee, 1974; Pilsbry, 1946). It is a gregarious snail, closely associated with human activities such as house gardens (Juřičková, 2006), which gives the species great potential for colonizing new habitats. S. octona, the prey, is distributed in: Florida, USA; Veracruz, Tabasco, Campeche (the cities Campeche and Ciudad del Carmen) and Yucatán (Izamal and Mérida), Mexico; the Caribbean and Central America (Belize, Guatemala, Nicaragua, Costa Rica); South America and the Caribbean Islands.
This close alimentary dependence between H. bicolor and S. octona, both of which presently inhabit Mexico, is of great relevance health wise, since the latter is an intermediate host of the trematode Postharmostomum gallinum (Witenberg, 1923), a parasite of domestic chickens, of Angiostrongylus cantonensis (Chen, 1935) (De Almeida Bessa et al., 2000; De Faria Duarte, 1980) and there is an association with rats (Kliks & Palumbo, 1992). The eventual introduction of the nematode A. cantonensis into Mexico could be of great concern, since it can cause eosinophilic meningoencephalitis (De Almeida Bessa et al., 2000; De Faria Duarte, 1980). S. octona, which is the intermediate host of nematodes and the favorite prey of H. bicolor, is distributed in Mexico in the states of Veracruz, Tabasco, Campeche and Yucatán. It is not known whether H. bicolor started to feed on S. octona after its expansion to different places in the world, or if the presence of S. octona and various pupillid species (upon which H. bicolor also feeds) have been a significant factor in the dispersal of this species.
Introduced molluscan species can be parasite vectors, whether inadvertently introduced or not. H. bicolor is associated to S. octona, a phytophagous snail that also possibly entered tropical countries on ornamental or edible plants; both species can disrupt the trophic dynamics in the environment into which they come. It is thus of vital importance that studies of the impact these vectors may have on public health in Tabasco and other places in Mexico be offered incentives and support.
Malacological studies focused on agriculture are scarce in Mexico, even though exotic species identified as pests live on vegetables. On the other hand, the tropical states of Mexico along the Gulf coast are subject to river floods, clayey-sandy soils with alluvial sediments abound, and additional river water from rain is frequent. Vegetation debris, organic (leaf litter) and non-organic matter and wet decaying wood, together with plant coverage are a refuge and a suitable substrate for colonization by exotic land snails. Human activity, is the main reason that such exotic land snails are able to flourish, in addition to the abundance of food, first of detritivores (such as S. octona) and later of carnivores of the first and second order.
Land mollusks are difficult to control, particularly when they possess an abundant population that will inflict more damage. In scenarios such as this one, several countries are attempting agro-ecological control. Consequently, in-depth knowledge of the taxonomy, biology and ecology of exotic land snails is of vital importance because of the risk they pose to public health, as well as the loss of biodiversity of native species, many of which are unknown to science.
