Biological aspects of Omalonyx convexus ( Mollusca , Gastropoda , Succineidae ) from the Rio Grande do Sul State , Brazil

Omalonyx convexus (Heynemann, 1868) is widely spread throughout the Rio Grande do Sul State, Brazil. The studied specimens presented in vivo, tegument and mantle coloring in variations between milky-white, orange and beige. The shell presented itself covered by the mantle in different extents; however, it was not completely covered on any of the specimens. The feeding diet is basically constituted by vegetal tissues, although non-vegetal food items were also found. The specimens were found in preserved and in polluted fresh water environments, as well as on natural and artifi cial substrates. The temperature throughout the day infl uences their placement on habitat substrates.


Introduction
Omalonyx d'Orbigny, 1837 includes slugs with a reduced, fl at and fi ngernail-like shell (ARRUDA; THOMÉ, 2008b) and a pattern of yellow coloring with two black longitudinal stripes and blackish stains throughout the entire body, including the mantle (BARKER, 2001).The former covers the visceral mass and the edge of the shell (BAKER, 1926).They are herbivorous, feeding on living plant tissues (BARKER, 2001;FRANCESCHINI, 2003).According to Patterson (1971), they occur in the Caribbean Islands and they are amply distributed throughout South America.
Intending to extend the existent knowledge about this group, this paper focuses on the coloring, shellmantle association, feeding diet, habitat and occurrence of Omalonyx convexus (Heynemann, 1868) in the Rio Grande do Sul State, Brazil.In order to investigate food items constancy in Omalonyx convexus from Rio Grande do Sul State, the crop contents of 49 specimens from Cachoeira do Sul (3), Cachoeirinha (4), Camaquã (5), Estrela (5), Novo Hamburgo (3), Pelotas (8), Porto Alegre (8), Santa Maria (8) and Torres (5) were examined.The animals were dissected under a stereomicroscope and crop contents were fully extracted using a hypodermic needle.The entire content triage was made under an optical microscope, using slides and coverslips.

Results
We found Omalonyx convexus specimens in ten of 19 investigated municipalities (Table 1).
In cloudy days, we found 191 specimens in 163 sampling effort hours, which corresponded to one specimen per 51 minutes.In sunny days, we found 164 specimens in 184 sampling effort hours, which corresponded to one specimen per 67 minutes of effort.
In the early morning (between 7a.m. and 10a.m.) and in the end of the afternoon (between 4p.m. and 6p.m.) of sunny days, the animals were observed on the stem and on the adaxial surface of the macrophyte leaves, on grasses in fl ooded areas and under adjacent vegetation on the banks of dikes and lakes.In this last case, they presented themselves as mimetic to the environment, making visualization very diffi cult.During the hottest hours of the day (between 10a.m. and 4p.m.), they were found on fl owers, roots, abaxial surface of leaves and close to the base of macrophyte fl owers.
The specimens studied presented up to four centimeters in total length and, in vivo, tegument and mantle coloring in variations between milky-white, orange and beige, with irregularly sized and distributed blackish spots, which, depending on their quantity, may give the slug a darkish-gray appearance (Figure 1).Beige was the most commonly observed coloring.Milky-white slugs can present, by translucency, the color of substrates on which they lie.
The shell was translucent, with an amberish tint, and presented itself covered by the mantle in different extents; however, it was not completely covered on any of the specimens.We found Omalonyx convexus on adjacent vegetation in draining ditches of paddy rice irrigations, fl oodplains, dikes and lakes, rivers and streams, and on the trunk (dry portion) of a semi-submerged tree in a temporarily fl ooded location.They were mainly seen on macrophytes Eicchornia azurea Kunth, Salvinia auriculata Aubl., Pistia stratiotis L. and Eryngium sp.; however, they were also found on artifi cial substrates such as canvases, cardboards, plastic bottles, tetra-pak boxes and Styrofoam (Table 2).On macrophytes in draining ditches of paddy rice irrigations X X X On emergent vegetation on dike / lake banks X X On fl oodplain macrophytes and emergent Vegetation X X X X On stream / river macrophytes X X On the trunk (dry portion) of a semi-submersed tree in a temporarily fl ooded location X On fl ooded pasture grass X On artifi cial fl oodplain substrates X beige coloring with a darkish-gray appearance.
The feeding diet of Omalonyx convexus was basically constituted by vegetal tissues.Among 49 examined crop contents, 100% contained vegetal tissues, from which 6% also contained pollen grains and 2% also contained mites (Table 3).
The specimens of O. convexus were found on emergent vegetation in lentic environments, as well as submerged among macrophytes, concurring with d'Orbigny (1837), Martens (1868), Guppy (1872), Doering (1873), Martínez (1993) and Barker (2001) observations and diverging with Parodiz (1963).Hermann and Dundee (1967) reported Omalonyx specimens swimming in the water in Ecuador; however, we did not observe this occurrence.Although Omalonyx species had only been recorded in preserved environments, O. convexus specimens were also found in sites polluted by household sewage.This is the fi rst record of an Omalonyx species living in a polluted area.Given that they avoid desiccation, O. convexus specimens can be found on different substrate portions throughout the day.
The examined crop contents showed bright green and beige fragments of plant tissues.Based on this, we presumed that Omalonyx convexus eats living plant tissues.The only available information regarding crop contents for other Omalonyx species concerns O. brasiliensis.Simroth (1896) reported fi bers and very dark contents, which induced him to conclude that O. brasiliensis fed on dead leaves that had fallen to the ground in the species habitat.Although slug of the genus Omalonyx has been considered herbivorous, non-vegetal food items were found in the O. convexus diet.The low consistency of these items suggests their occasional ingestion.Franceschini (2003) described the damage caused by Omalonyx unguis in macrophytes.According to her, the tissues consumed are the epidermis, palizade parenchyma, aerenchyma, sclerenchyma, phloem, and xylem.Among the ingested tissues within examined O. convexus crop contents, parenchyma was identifi ed.
We have approached original Omalonyx convexus biological aspects which have added knowledge towards the genus Omalonyx and can be used as comparative parameters for future studies about other species of Omalonyx, and other neotropical succineids.

TABLE 1 :
Investigated municipalities of the Rio Grande do Sul State (Brazil) and their collecting data.