New record of Hygrophila corymbosa ( Blume ) Lindau ( Acanthaceae ) for Mexico , a highly invasive aquatic weed

The presence of Hygrophila corymbosa (Blume) Lindau within the Mexican aquatic flora is recorded for the first time. This species is native to Asia and is commonly used as an ornamental plant in aquariums, so it is possible that its introduction was accidental. Due to its high potential for reproduction and dispersion, H. corymbosa represents a great threat to the native aquatic flora of the wetlands of Mexico. More studies are recommended to explore ecological aspects of the plant to facilitate its control and/or eradication.


Introduction
Aquatic ecosystems are seriously threatened today by natural and anthropogenic causes (Mitsch and Gosselink 2000).The activities that have been most damaging to wetlands and their biota are pollution and land use change (Landgrave and Moreno-Casasola 2011).However, the accidental introduction of exotic aquatic species has also contribute to wetland degradation since, in many cases, these species displace native species in many regions of the world (Mendoza and Koleff 2014).
Although the presence of exotic aquatic plants has been reported in most of Mexico, the border and coastal regions are probably the most susceptible to problems of introduced species.This is perhaps because of the lack of strict regulations by the Mexican government on the introduction of plant specimens, seeds and propagules.This is the case in Tamaulipas which, until now, has been the state with the highest number of exotic strict hydrophytes established in both natural and artificial environments (Mora-Olivo et al. 2013).
In this study, the presence of Hygrophila corymbosa (Blume) Lindau, a plant typically used as ornamental in tropical aquariums, is registered for the first time in Mexico.This aquatic weed is naturally distributed in regions of Asia such as India, Burma, Malaysia and Indonesia (Paffrath 1979;Hutchinson and Williams 2013).Moreover, has been introduced and naturalized in Taiwan water bodies (Wu et al. 2010) and in the United States of America (Wunderlin et al. 2017) where it is considered a potential danger due to its rapid spread and invasive nature (USDA 2015).The first official record for North America was made by Ramamoorthy and Turner (1992) in San Felipe Springs, a Texas town near the border with Coahuila, Mexico.However, some herbarium collections indicate that this plant has been in Florida since 1987 where it has already naturalized (Hutchinson and Williams 2013).With this report, there are now two species of the genus Hygrophila R.Br.(H.corymbosa and H. polysperma) documented in Tamaulipas that are a threat to native aquatic flora due its invasive character.

Study area
The study was conducted within the municipality of El Mante, in the state of Tamaulipas located in NE Mexico (22º44′5″N; 98º58′29″W), at an altitude of 80 m a.s.l.(Figure 1).The climate is sub-humid, with an average temperature of 40 to 46 °C in the months of June and July, and 4 to 26 °C in the months of November and December.The precipitation reaches 1000 mm in this area.The main river is the Mante and there is also an irrigation net (1 to 3 m of depth) that provides water to the agricultural crops of the region (GET 2013).

Registration and identification
The first visual and photographic records of the plant was in March 2007 in an irrigation water channel of the El Mante.Although the species was not identified then, its characteristics indicated that it belonged to the genus Hygrophila.Later, as part of a formal study of the aquatic vascular flora of the El Mante municipality between 2013 and 2017, specimens of the plant species present in natural and artificial aquatic ecosystems were collected.The novel hydrophyte photographically recorded in 2007 was observed to be more common and formed large populations, particularly in the agricultural irrigation water channel habitats.Although most of the observed individuals were submerged and in a vegetative state, some of these were emerging, producing flowers and seeds.Thus fertile material was used for the taxonomic identification of specimens as Hygrophila corymbosa, a member of Acanthaceae family (Wunderlin et al. 2017).

Specimens examined
Next, data of two specimens collected from H. corymbosa that serve as reference for a collection of herbarium are given.MEXICO: Tamaulipas: Municipality El Mante, irrigation water channels within the

Description of the species
The registered specimens of H. corymbosa are submerged or emergent rooted herbs up to 70 cm in length (Figure 2).Almost the entire plant is covered with dense glandular hairs on the aerial parts.The stems are quadrangular and are rooted in the lower nodes.The leaves are opposite, with blades ovatelanceolate to ovate-elliptic, 5 to 12 cm long and 2 to 5 cm wide, dark green in the bundle (sometimes reddish in the submerged leaves) and whitish green on the underside, the margin is whole to crenulated or wavy.The flowers are arranged in axillary corymbs and the corolla is purple with small white spots on the lower lip.The fruit is an elongated capsule of 7.5 to 11 mm in length, with numerous discoid seeds.

Description of populations (meadows)
To date, 25 meadows have been documented, distributed along different irrigation water channels, where they currently occupy an area of approximately 5 km 2 (Supplementary material Table S1).Submerged plants grow profusely in the deepest areas, forming monospecific beds or associated with other submerged hydrophytes such as Hygrophila polysperma, Potamogeton illinoensis Morong and Ludwigia repens J.R. Forst.In the margins of the water channels few emergent specimens are found, which are usually associated with species such as Hymenachne amplexicaulis (Rudge) Nees, Paspalum denticulatum Trin.and Trichocoronis wrightii (Torr.& A. Gray) A. Gray.

Discussion
Most of the species of the Acanthaceae family are terrestrial, although some genera like Justicia Raf. and Hygrophila have aquatic representatives.In Mexico, Acanthaceae are among the 15 most diverse families of vascular plants with 36 genera and 376 native species (Daniel 2013;Villaseñor 2016).Of the 100 known species of the genus Hygrophila (Jiaqui et al. 2011), three of them (H.polysperma, H. costata, and H. corymbosa) are currently present in Mexico (Mora-Olivo et al. 2008).Although only H. costata is native in Mexico, the three species are considered invasive aquatic weeds in different parts of the world (Csurhes 2008;USDA 2015).
As other species of the same genus, H. corymbosa reproduces mainly vegetatively, so a stem fragment may produce new individuals as we observed in this studied area (USDA 2015).Furthermore, during field visits, bees (Apis mellifera L.) were found on flowers and the formation of fruits with possibly viable seeds was observed.It has also been observed that H. corymbosa develops and disseminates fast along the channels, both submerged and emergent growth forms.Vegetative recruitment abilities, successful reproduction efforts, rapid growth, and tolerance to a wide range of environmental characteristics including the disturbance and excess nutrients (Hutchinson and Williams 2013) make H. corymbosa a potential threat to native hydrophytes of Tamaulipas.
Since H. corymbosa is a popularly ornamental plant in tropical aquariums, it is possible that it was accidentally introduced by this route in Mexico where varieties are currently sold online (Martínez 2016).Another possible entry could have been through boats and fishing gear, as the area is frequently visited by tourists and fishermen from other regions.Because the collected plants are fertile, there is also the possibility that the seeds have been dispersed to this place by waterfowl, as has been occurred with other vascular hydrophytes in different parts of the world (Green et al. 2002).
Until now, H. corymbosa has only been registered in the urban area of the municipality of El Mante, although there is a risk of it expanding its distribution to more northern sites as the invaded irrigation channels flow into the Guayalejo river.This would make it possible in the future for this weed to invade the extensive wetlands of the Guayalejo-Tamesí river basin where the states of Tamaulipas, Veracruz and San Luis Potosí converge; an important area because of the high richness of aquatic flora in the country (Rzedowski 1978;Mora-Olivo and Villaseñor 2007).On the other hand, the presence of H. corymbosa in Texas bodies of water that are connected to the Rio Grande could create a supply chainel for this aquatic weed for the Mexican states of Coahuila, Nuevo León or Tamaulipas (Ramamoorthy and Turner 1992).
Although the process of regulating the introduction of invasive species has intensified in Mexico, the results are still insufficient (CANEI 2010;Ortiz-Monasterio 2014).This situation puts the native species that inhabit important wetlands, such as the Guayalejo-Tamesí river lagoon system and Rio Grande delta, at greater risk.For this reason, it is important that Mexican authorities pay special attention to the areas of greatest risk of aquatic invasions, such as border and coastal areas.Particularly in the case of invasive aquatic weeds, further studies on the ecology and dispersal mechanisms of these species are necessary to contribute to their control and/or eradication.

Figure 1 .
Figure 1.Study area and location of registration sites Hygrophila corymbosa in Tamaulipas, Mexico.