Fecundity of Uca uruguayensis and Chasmagnathus granulatus ( Decapoda , Brachyura ) from the “ Refugio de Vida Silvestre

The aim of the present work conducted at the Refugio de Vida Silvestre Bahía Samborombón is to analyse the most relevant aspects of the fecundity of Chasmagnathus granulatus and Uca uruguayensis. Samplings were carried out from March 2001 to February 2003. Ovigerous females of U. uruguayensis (N = 13) and C. granulatus (N = 25) were found during spring and summer, their sizes (CW) varied from 9.1 to 11.7 mm for the former species and from 22.8 to 32.4 mm for the latter. The egg diameter in U. uruguayensis ranged from 245 to 260 μm for embryos in the early stage of development and from 250 to 345 μm for those in mid-developmental stage, while in C. granulatus from 250 to 345 μm and from 260 to 365 μm respectively. Fecundity varied from 1126 to 6745 eggs/brood in U. uruguayensis and 15688-57418 eggs/brood in C. granulatus. For those females with broods in mid-developmental stage, several relationships were made. For U. uruguayensis the best correlation coefficients were obtained for the relationships: female weight vs. egg mass weight and carapace width vs. egg mass weight; for C. granulatus the best association was obtained between female size and the egg number and the egg mass weight.


Introduction
The reproductive aspects and fecundity in brachyurans from the Neotropical region has been treated for different families, among them Portunidae (Mantelatto and Fransozo, 1997), Grapsidae and Varunidae (Spivak et al., 1996;Luppi et al., 1997;Conde et al., 2000;Leme, 2002Leme, , 2004)), and Ocypodidae (Spivak et al., 1991;Costa et al., 2006).These papers refer mostly to Brazil and the Mar Chiquita Lagoon (Argentina).From them it can be appreciated that reproductive aspects are adaptive processes intimately related with environmental conditions, such as latitude, temperature, larval food availability, giving a single species a continuous reproduction in tropical areas of its distribution and a seasonal one in the temperate zones (Spivak et al., 1991;César et al., 2005;Costa et al., 2006).
Chasmagnathus granulatus Dana, 1851 (Varunidae) and Uca uruguayensis (Nobili, 1901) (Ocypodidae) are two endemic species of crabs recorded in the template biogeographic region of the southwestern Atlantic Ocean.The former inhabit estuaries, Spartina sp.salt-marshes, halophyte meadows and mangroves from Río de Janeiro (Brazil) to the Río Negro River (Argentina), while the latter is found as far as Quequén River (Buenos Aires, Argentina).Both species occupy the high levels of the silty or silt-sandy intertidal ecosystem (Spivak et al., 1991;Spivak, 1997aSpivak, , 1997b) ) and are semi-terrestrial species that build burrows and use the habitat in a rather complex way.Very dense populations of C. granulatus and U. uruguayensis inhabit the intertidal ecosystem of Samborombón Bay (César et al., 2005), giving to the landscape a special physiognomy called "cangrejales".The southernmost permanent populations of U. uruguayensis inhabits Samborombón Bay, however some intermittent populations are found in the Mar Chiquita coastal lagoon, 250 km further south (Spivak et al., 1991).The Samborombón Bay includes the socalled "depression of the Salado river" that extends from Punta Piedras (35° 30' S and 56° 45' W) to Cabo San Antonio (36° 22' S and 57° 23' W).This bay is a depocentre of significant magnitude, formed by clayish sediments from the sea and the Uruguay and Paraná rivers generating coastal plains with large marshes (Violante et al., 2001).The coast of the bay is bathed by salty waters of variable salinity corresponding to the estuarine area of the Río de la Plata River (Boschi, 1988).
The aim of the present work conducted at the "Refugio de Vida Silvestre" Bahía Samborombón was to analyse the most relevant aspects of the fecundity of these two crab species.

Material and Methods
Sampling was conducted during each season (three occasions in spring and two in the other seasons) be-tween March 2001 and February 2003 at the mouth of Channel 1 at the Refugio de Vida Silvestre Bahía Samborombón (36° 16' S and 57° 06' W).Ovigerous females were collected by hand using a square of 5 m x 5 m, with 0.5 m x 0.5 m subunits (N = 10) following Elliott (1983).In September 2001, October and December 2002 five ovigerous females of C. granulatus were collected just beside the square.The crabs were kept in plastic bags and frozen.On each sampling occasion physicochemical variables were measured (using the Checkmate Modular Testing system by Ciba Corning Analytical): temperature (°C), pH, and conductivity, then salinity was obtained after multiplying the conductivity by 0.9; due to difficulties with the equipment only the measure of conductivity and pH could be taken in winter.In the laboratory, crabs were thawed at room temperature and fixed in 10% formalin.Total carapace widths (CW), wet and dry weight (60 °C, during 48 hours) were measured.The dry weights of the brooded egg mass and number of eggs per brood (extrapolated from the dry weight of a counted sub-sample of 1000 eggs) were measured in order to determine the brood size (Hines, 1982;Mantelatto and Fransozo, 1997;Luppi et al., 1997;Leme, 2004).Although some authors (Mantelatto and Fransozo, 1997) recommend the use of embryos in developmental stages from late blastula to early gastrula for fecundity analysis, to avoid effects of embryonic diameter swelling during late stages of development, we considered here (following Hines, 1982;Luppi et al., 1997;Leme, 2004) also those embryos in the intermediate stage of development, as determined by formation of eyes.

Results
The registered physical and chemical variations are typical of an estuarine ecosystem and are shown on Table 1.
In both species the ovigerous females captured presented broods with embryos in early and intermedi- Ovigerous females of C. granulatus (N = 25, one of them, with zoeas about to hatch, was not considered in the analysis) were found on the same date reported for the former species (N = 7 on December 2001, N = 8 on December 2002, N = 8 on February 2003) and also on September 2001 (N = 1) and October 2002 (N = 1).Sizes of these individuals ranged from 22.8 to 32.4 mm.The sizes of the broods varied from 15688 to 57418 eggs/ brood (X ± SD = 31371.08± 10217.23).The egg diameter ranged from 250-345 µm (X ± SD = 287.10± 20.33) for embryos in the early stage and 260-365 µm (X ± SD = 314.05± 24.89) for embryos in the intermediate stage of development.The above mentioned relationships between carapace width, female wet weight, brood dry weight and number of eggs were also determined for C. granulatus (Table 2).

Discussion
The "cangrejal" studied is inhabited by large populations of C. granulatus and U. uruguayensis which coexisted in Samborombón Bay, building their burrows in close proximity to one another (César et al., 2005).They were active during the period studied, except in July 2002 when no individuals from either species were recorded in their burrows (at least up to 50 cm deep) or on the surface.This could be due to climate factors, in particular to the frequent frosts registered during that winter.Many brachyurans diminish their activities during the most rigorous seasons of the year and remain apparently inactive, dug into their burrows or hidden inside of crevices of trees.Such behaviour was also reported by Spivak et al. (1996) for C. granulatus in the Mar Chiquita lagoon and for Aratus pisonii (H.Milne Edwards, 1837) in the mangroves of São Paulo State, Brazil (Leme, 2002).
Ovigerous females of U. uruguayensis were found in summer, Spivak et al. (1991) also found ovigerous females of this species in Mar Chiquita lagoon in summer but in higher percentages than ours.Although the population studied in Samborombón Bay has a high density (César et al., 2005), the fact of having collected a low number of ovigerous females could be due to a differential behaviour during this incubation period.Costa et al. (2006) captured the ovigerous females very deep in their burrows and suggested that this species would not feed during this period.On the other hand, U. uruguayensis presented continuous reproduction throughout the year in the north limit of its distribution (Brazil), while in the south (the population here studied in Samborombón Bay and the one studied by Spivak et al., 1991 in Mar Table 2. Regression analysis of fecundity in Uca uruguayensis and Chasmagnathus granulatus in intermediate stage of development (r: correlation coefficient; Fe: number of eggs; CW: carapace width; WBW: wet body weight of ovigerous female; DWB: dry weight of broods); (*, **: regression coefficient significantly different from zero P < 0.05, < 0.01, respectively, ns: statistically not significant P > 0.05).In the case of C. granulatus, the ovigerous females were found in spring and summer, such as reported by López Greco and Rodríguez (1999) at "Faro San Antonio" beach, at the southern point of Samborombón Bay.Also Spivak et al. (1996) reported for this species in Mar Chiquita Lagoon a seasonal reproduction during spring and summer.

Uca uruguayensis
In brachyuran crabs fecundity is related to female size, latitudinal distribution and habitat adaptations.For the organisms that develop in similar environments, female size and the volume of the body cavity appear to limit the brood size, being these features the main factors to determine fecundity and reproductive effort (Hines, 1982;Thurman, 1985;Mantelatto and Fransozo, 1997).
The effective fecundity expressed as the number of eggs per brood would be related as mentioned above, to the different kinds of environments and to the female sizes.Thus, in the Uca species from intertidal brackish environments, such as U. triangularis (Milne-Edwards, 1873), U. annulipes (Milne-Edwards, 1837) and U. rapax (Smith, 1870), fecundity varied among 3390, 6400 and 28500 eggs/brood respectively, being their average sizes 9.6, 11.3 and 15.8 mm of carapace width (Thurman, 1985).On the other hand, the population of U.uruguayensis studied by Costa et al. (2006) in Ubatuba, Brazil, presented a lower fecundity (mean fecundity: 1883 ± 490 eggs) than the population here studied, which is related to the smaller size in the ovigerous females captured (C.W = 6.6 ± 0.8 mm).
In C. granulatus the effective fecundity was similar to that recorded in the Mar Chiquita by Luppi et al. (1997), though a bit lower than that determined by Botto and Irigoyen (1979) for the population at Salado river.Other grapsoids studied showed the association mentioned between fecundity and female size, thus Cyrtograpsus angulatus exhibited similar values to that of C. granulatus, and Aratus pisonii lower ones (Luppi et al., 1997;Conde et al., 2000).In the case of Sesarma rectum Randall, 1840, C. granulatus presented a higher fecundity but the eggs of the former species were almost double in size (Leme, 2004).
In U. uruguayensis the correlation coefficients for the relationship of female weight with egg mass weight and female size and egg mass weight showed the best association between the parameters analyzed.In C. granulatus the female size (CW and female weight) was strongly correlated with the brood (number of eggs and dry weight) as well as reported by Luppi et al. (1997) in Mar Chiquita lagoon.According to Hines (1982), allometric limitations on space available for yolk accumulation in the body cavity appeared to be the main constraint on brood size.

Table 1 .
Physical and chemical variables measured at sites sampled on Channel 1, Bahía Samborombón.The salinity values were obtained after multiplying the conductivity by 0.9.The sizes (CW) of the smallest and biggest individuals were 9.1 and 11.7 mm, respectively, and their brood sizes varied from 1126 to 6745 eggs /brood (X ± SD = 3338.58±1758.86).The egg diameter ranged from 245 to 260 µm (X ± SD = 253.25 ± 4.66) for embryos in the early stage of development and from 250 to 300 µm (X ± SD = 285.97±15.07)for embryos in middevelopmental stage.For those females with broods in mid-developmental stage, the following relationships were analysed: carapace width vs. number of eggs, carapace width vs. brood dry weight, female wet weight vs. brood dry weight, brood dry weight vs. number of eggs and finally, female wet weight vs. number of eggs (see Table2).