1.Introduction

Most of the species of Drepanotrema (Crosse and Fischer, 1880), belonging to the family Planorbidae, are endemic to the Neotropical region. This genus includes nine species, of which six are found in Argentina: Drepanotrema cimex (Moricand, 1839), Drepanotrema anatinum (d'Orbigny, 1835), Drepanotrema depressissimum (Moricand, 1839), Drepanotrema heloicum (d'Orbigny, 1835), Drepanotrema kermatoides (d'Orbigny, 1835), and Drepanotrema lucidum (Pfeiffer, 1839) (^{Paraense, 2005}; Rumi et al., 2006). To date, the ecological and biological aspects of Drepanotrema are poorly known (^{Bonetto et al., 1990}; ^{Rumi, 1991}; Hamman et al., 1993; Rumi et al., 1997 and 2004; ^{Bueno-Silva and Fischer, 2005}); the morphology and taxonomy has been studied by ^{Baker (1945)}, ^{Paraense and Deslandes (1958)}, and ^{Paraense (1975} and 1980). Also, the life cycle of natural systems in Argentina has been little studied, except for some isolated observations (^{Rumi et al., 2004} and 2007).

The aim of the present study was to analyse the individual growth of D. cimex in Arenalcito Pond in the Martín García Island Natural Reserve of Multiple Uses. To date, several investigations have been carried out on the ecology and biodiversity of the Reserve's fauna and flora (^{Lahitte and Hurrell, 1996} and 1997; ^{Rumi et al., 1996}; ^{Martín and Negrete, 2006}; ^{Martín 2008}; ^{César et al., 2008}; ^{Martín et al., 2009}; ^{Martín and Díaz, 2012}).

2.Materials and Methods

Martín García Island is located in the upper Río de la Plata estuary, to the south of the mouth of the Uruguay River (34°11′25″ S and 58°15′38″ W) (Figure 1). The island constitutes an outcropping of the Brazilian massif of Precambrian crystalline basement rocks, overlaid by sediments from the Holocene and Pleistocene (Quaternary). In the central-eastern sector of the island, there is a plateau-like area gradually sloping downwards towards the sea and more abruptly towards the north of the island, where the terrain continues as a plain that ends in the river (^{Ravizza, 1984}). The study site was Arenalcito pond located near Day Península (on the eastern sector of the island). This pond is an inland eutrophic body of water fed exclusively by rainfall and thus undergoing occasional desiccation periods. The vegetation present consists in aquatic free-floating herbs, belonging to the Hydrocaritaceae family (^{Lahitte and Hurrell, 1996}).

A total of 14 samples were collected from February 2006 to June 2007 and five environmental variables measured with a digital Water Quality Meter (Sper Scientific Ltd): water temperature, dissolved oxygen (mg L^–1), electrical conductivity (µS cm^–1), total dissolved solids (TDS), and pH.

Snails were collected with sieves of 20 cm diameter and 0.15 cm mesh size and then fixed with Henry-Raillet and Carnoy solution. Snail relative abundance was calculated as captures per unit effort as specimens (CPUE), that is, specimens/60 min/person. The snails were identified under a stereoscopic microscope according to the morphological guidelines of ^{Paraense (1975)}. The maximum shell diameter of each specimen was measured with a caliper to a precision of 0.01 mm. The shell length was selected as the parameter for determining the monthlysize frequency histograms and identifying the modal lengths (Lm) for subsequent graphical representation of these parameters as a function of time. Individual growth was analysed by means of the von Bertalanffy growth equation (Bertalanffy, 1960): Lt = Lmax(1 - e - k(t - to)), where: Lt = the length at a specific time t; Lmax= the asymptotic maximum body length; to= the hypothetical time in which the length of the individuals is “zero”; t = time; k = the growth rate.

To make the intervals on the time axis among the samples proportional to the time spent in each sampling period, the sampling dates were transformed into parts of the year (between 0 and 1), according to the following expression: Parts of the year = [(m - 1)*30 + d]/365, where m = number of the month considered, and d = number of days spent in the month cited. The parameter L∞ (maximum body length that an individual of a cohort can reach) was calculated by the Ford-Walford method, where estimations of Lt + 1 were graphically represented as a function of L for each time t. The parameters k and towere determined by linear regression between ln[(1 - Lt)Lmax^–1], the dependent variable, and t, the independent variable; where kTo= the ordinate value at the origin and becomes the (a) parameter of the Ford-Walford Method with -k, the slope, becoming the (b) parameter and Lt being the length at time t. Finally, the curve obtained was compared with the one plotted from the von Bertalanffy model through the use of the X² test for goodness of fit.

3.Results

The population of D. cimex was characterised by a complex and dynamic size structure throughout the annual cycle (Figure 2). The analyses of the curves revealed both unimodal (2006) and polymodal (April to June 2007) distributions, allowing the detection of cohorts within the population studied.

The cohort detected in 2006 was used to analyse the individual growth. The breeding period of this cohort took place during the summer and continued to develop until late summer 2007. During the sampling periods conducted in 2007 juveniles (2.5 to 3.1 mm), adults (3.2 to 5.8 mm) and seniles (5.9 to 7 mm) were observed within the population, thus indicating an average longevity of a year to a year and a half (Figure 3).

The maximum length reached by individuals according to the Ford-Walford Method was 5.6 mm (Figure 4), the growth rate k = 2.0592 and to = 0.293, the time during which they are “zero-length” or in the breeding period (Figure 5). The curve obtained did not show significant statistical differences according to the von Bertalanffy model (X² = 0.639, df = 9, p > 0.05) (Figure 6), indicating that this model is a useful tool to predict the growth of populations belonging to this species (Rumi, 2007). The relationship between population structure and size revealed the presence of dominant classes, corresponding to (4 to 5.2 mm) at the beginning of the reproductive period in February and March 2006 and from November 2006 to March 2007.

In February and March 2006 and from November 2006 to March 2007 the highest values of water temperature were recorded (23 to 27.5 °C). The percentage of dissolved oxygen was low throughout the year; while the pH was stable and alkaline during the sampling period, ranging between 5.5 and 6.58. The electrical conductivity was low (6 to 62 µS cm²) attributable to a poor contribution to the pond from rainfall. Of all the environmental parameters, the water temperature was the most preponderant condition affecting population growth (Figure 7).

4.Discussion

The population of D. cimex showed unimodal and a polymodal distribution, this finding enabled a differentiation among the cohorts that indicated no coexistence of different generations throughout the annual cycle. The peak of abundance of the D. cimex population in that period agrees with the data recorded by Michelson and Mota (1982) and Pointier and David (2004); indicating that species of Planorbidae such as D. depressissimum, D. lucidum, and some species of Biomphalaria, are favored by rainy seasons. These results agreed with those of ^{Bueno-Silva and Fischer (2005)} for D. cimex in the Barigüí Park, Curitiba (Brazil). In the Cantera Grande Lake, located at the southern end of the Martín García Island, this species was also abundant during the summer and fall of 1995 (^{Rumi et al., 2004}).

^{Ituarte (1989)} observed the unimodal characteristic of the growth curves of populations of Biomphalaria straminea (Dunker, 1848). We conclude that unimodality and only a single reproductive period of a given species are distinctive characteristics of the ecology of the Planorbidae family. ^{Rumi et al. (2007)} observed that the growth rate of D. kermatoides in the Paiva Pond of Corrientes, Argentina was much higher than that observed for D. cimex in the Arenalcito pond mainly because the temperatures recorded on the Island are lower than those registered in Corrientes.

Among the most significant environmental parameters affecting growth of populations of D. cimex, water temperature is the most influential. According to ^{Rumi (1991)}, this characteristic is a limiting condition for the development of the populations of all the species belonging to the Planorbidae family.