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All issues Volume 58 (2022) Int. J. Lim., 58 (2022) 5 Full HTML
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Issue
Int. J. Lim.
Volume 58, 2022
Article Number 5
Number of page(s) 10
DOI https://doi.org/10.1051/limn/2022007
Published online 01 June 2022

© EDP Sciences, 2022

1 Introduction

Brazil has the third-highest species richness of freshwater crabs in the Neotropical region. Most of this fauna comprises representatives of the family Trichodactylidae (H. Milne-Edwards, 1853) (10 genera and 31 species) (Cumberlidge et al., 2014). The vast majority of Trichodactylidae occur in river floodplains below 100 m altitudes, therefore, some species can be distributed over extremely broad areas (Yeo et al., 2008). One of the species of this family, Sylviocarcinus pictus (Milne-Edwards, 1853) occurs in an area of over 20,000 km2 (Magalhães, 2016) and is distributed in 7 countries of South America, namely Colombia, Brazil, Bolivia, Peru, Argentina, Guyana, and French Guiana (Cumberlidge et al., 2014). In this extensive region, S. pictus is commonly found in rivers, streams, and lakes at different types of habitat, such as cracks in rocks or between submerged branches, roots, trunks of hollow trees, associated with aquatic macrophytes, underneath stones, in the litter or between roots and leaves of aquatic vegetation (Magalhães, 2003; Silva, 2010; Silva et al., 2012).

Besides its ecological role, crabs of this genus are commercially exploited as live baits for sport fishing in the Pantanal region (Mansur and Hebling, 2002) and are important food sources for Yanomami Indigenous in the Brazilian Amazon (Magalhães et al., 2006). Although S. pictus occurs in several Brazilian watersheds, little is known about its life history and the reasons for its wider distribution if compared to most other Trichodactylidae (Magalhães, 2016). Also, in a changing world, where the species are constantly under stressful conditions and continuous degradation, it becomes important to understand how an annual drastic reduction in the precipitation and a consequent change in a river regime (from lotic to lentic) affect a population and how they react to the environment (Strachan et al., 2015; Shields, 2019).

Recently, this crab was reported for the first time in an intermittent river (Pralon et al., 2019) from the semi-arid northeastern region of Brazil. This is a type of environment often neglected in limnological research and environmental monitoring. Despite the importance of freshwater decapods and the vulnerability of this kind of river, only a few researchers have examined its decapod populations (Yarra and Magoulick, 2018).

Here, the term “intermittent river” is used according to Datry et al. (2014), to characterize all temporary, ephemeral, seasonal, and episodic rivers in defined channels. This ephemerality offers several physicochemical and biological challenges to the river's biota, such as continuously decreasing habitat size in the dry season and increasing water temperature, a heatstroke that reduces the amount of dissolved oxygen and increases the concentration of ions, thus altering the pH of the water body (Williams, 1996). Alongside the extremes abiotic changes, in this type of environment, macroinvertebrates lose refuges for predation and feeding sites (Boulton, 2003). As a response, animal populations commonly show a certain synchronization between their fluctuations in density, sex ratio, and recruitment and the various directly influential environmental factors (Ruetz et al., 2005; Bernardo et al., 2019).

Therefore, the present study evaluated populational and reproductive parameters such as size at onset of maturity, sex ratio, reproductive period, and youth recruitment to verify the influence of seasonal environmental changes in a population of S. pictus of the Guaribas River, an intermittent river of the semi-arid northeastern region of Brazil. The hypothesis raised here was that the population structure of the freshwater crab S. pictus is modulated by the seasonality of dry and rainy seasons. Under this hypothesis, three predictions were tested: (1) the abundance of individuals must be greater in the rainy season; (2) individuals with mature gonads must be registered throughout the rainy season; (3) recruits should be observed at the end of the rainy season and the beginning of the dry season. Such predictions are supported by several empirical studies that found that the availability of nutrients in freshwater systems tends to increase during the rainy season and this reflects in reproductive peaks (Etim and Sankare, 1998; Litulo, 2005; Novak et al., 2015).

2 Material and methods

2.1 Study area and sampling

Sampling was carried out in the Guaribas River (0648'20.4” S/4118'29.4” W) ( Fig. 1), which belongs to the hydrographic basin of the Parnaíba River, and is one of the main water bodies of the state of Piauí, northeastern Brazil. This area of the Guaribas River is within the semi-arid macroregion and features caatinga vegetation on its banks. According to the Köeppen classification, this region has a dry, megathermal climate (average annual temperature 27.3 C). The southeast of Piauí, where the sub-basin of the river in question is located, is one of the most critical regions of the state in terms of water availability. It is characterized by an irregular rainfall regime for almost 3 months a year (rainy season – February to April) (Fernandes et al., 2000). Despite its characteristic intermittency, the water body at the collection site was flooded throughout the study period, albeit at varying depths, providing conditions for the occurrence of crabs.

The collections occurred monthly from October 2013 to September 2014. The specimens of S. pictus were captured manually according to a standardized sampling effort of one hour, by two collectors, in a stretch of approximately 100 m of the riverbank, and always at the start of the evening (between 06:30 p.m. and 08:30 p.m.). At the same collection site in all sampling months, a multiparameter meter (Hanna® – HI 9146) was used to measure the abiotic factors: water temperature (C) and pH. Total rainfall for the sampling months was obtained from the nearest meteorological station (Picos, Piauí – OMM: 82780), in the INMET (National Institute of Meteorology) database.

The captured crabs were packed in plastic bags, anesthetized on ice, and taken to the laboratory. The brooding females (ovigerous or with juveniles attached to its body), a typical behavior in freshwater crabs with direct development (Micheli et al., 1990; Cumberlidge and Sachs, 1991; Liu and Li, 2000; Vogt and Tolley, 2004) were kept individually in a plastic bag to prevent the loss of offspring until laboratory procedures and measurements were performed.

thumbnail Fig. 1.

Stretch from the Guaribas river in the municipality of São Luís do Piauí - PI from where specimens of Sylviocarcinus pictus (H. Milne-Edwards 1853) were collected.

2.2 Population structure

In the laboratory, the crabs were identified at the level of species according to Melo (2003). Each individual was identified to determine sex according to the morphology of the abdomen (narrow for males and wide for females) and the number of pleopods (2 pairs for males and 4 pairs for females) (Silva and Oshiro, 2002). Subsequently, the greatest carapace width (CW) was measured with a digital caliper (0.05 mm). Finally, the coloration and volume of the reproductive system were examined macroscopically concerning the thoracic cavity and the perigastric organ. Thus, the stage of development of the gonad was classified as immature (IM), developing (ED), developed (DEV), and spawned (S) ( Tab. 1) (adapted from Silva et al., 2012).

With the results obtained from the analyzes of maturity and body measurements, the size at which 50% of the individuals in the population showed morphologically mature gonads (CW50) was calculated. This method analyzes the distribution of individuals according to size classes based on carapace width (CW). A graph based on the logistic curve equation y = 1/(1+ er(CW-CW50)) was plotted to relate the size of the animal (independent variable), where “r” is the slope of the curve, for 50% of the population to reach the adult phase (CW50) to the relative frequency of mature individuals (ED, DEV, and S), the dependent variable (adapted from Herrera et al., 2013). The size at gonad maturity is defined by the interpolation point corresponding to a relative frequency of 50% on the graph. The equation of the logistic curve was fitted to the data by the Least-Squares method (Vazzoler, 1996).

The crabs of the total sample (males and females) were grouped in size classes (of CW) according to Sturges (1926) (K = 1 + log2n; K = number of classes; n = number of individuals). Subsequently, sexes were distributed separately among the size classes. That way, the sex ratio was estimated by the quotient between the number of males and the total number of individuals both for each month and between CW size classes (Pescinelli et al., 2020). Thus, values of sex ratio greater or lower than 0.5 indicated deviations in the population in favor of males or females, respectively. Deviations in sex ratio were analyzed using the binomial test (α = 0.05) (Wilson and Hardy, 2002). Normality was checked using Shapiro-Wilk’s test and homoscedasticity a Levene’s test.

Table 1

Macroscopic characteristics of each male and female gonadal states in Sylviocarcinus pictus (H. Milne-Edwards 1853) (Adapted from Silva et al., 2012).

2.3 Seasonal changes at population

The annual cycle was divided into two periods, each represented by five sampling months with equal sampling effort. The first period was the rainy season (from October 2013 to February 2014), when the increased monthly averages of accumulated precipitation, with a minimum of 1.3 and a maximum of 116 mm per month (mean ± SD = 54.30 ± 54.31), made the river overspill and transition back to lotic condition, up to the flooding peak of Guaribas River, which, in this sample year occurred in March and April 2014 (National Institute of Meteorology, INMET). The second period was the dry season, from May to September 2014, after the large flood. In this period, the monthly averages of accumulated precipitation decreased radically, with a minimum of zero and a maximum of 23.8 mm per month (mean ± SD = 7.04 ± 10.54) (National Institute of Meteorology, INMET), making the river increasingly lentic with progressively less flooding. Rainy and dry periods were compared in terms of the measured environmental parameters (precipitation, temperature, and pH of water) by the Mann-Whitney (nonparametric) test. The level of significance was set at 5% for two-sided p-values (Zar, 2010).

Multivariate analysis and autocorrelation were carried out considering two main seasons (rainy vs. dry). This separation enables the testing of the hypothesis that the population structure of S. pictus is modified seasonally. Seasonal population changes were also analyzed to investigate whether the structure of the population in terms of gonad development varied over the two seasons. For this analysis, non-metric multidimensional scaling (nMDS) was conducted, based on Bray-Curtis similarity matrices (Clarke, 1993). Later, an One-way crossed analysis of similarity (ANOSIM R) tested for significant differences in the population structure between seasons (p < 0.05) (Clarke, 1993).

Furthermore, correspondence analysis (CA) was used to evaluate the relationship between the sampling month and the number of crabs by size classes (Lepš and Šmilauer, 2003). In this analysis, absolute abundance values were used, considered the number of crabs in each month and size class.

3 Results

3.1 Population structure of S. pictus

A total of 178 crabs were recorded, 111 (62.36%) males and 67 (37.64%) females. Of the 111 males, 31 (27.93%) were collected in the rainy and 80 (72.07%) in the dry season. And of the 67 females, 24 (35.82%) were in the rainy and 43 (64.18%) in the dry season. Despite the effort, no crabs were collected in March and April 2014. No ovigerous female was collected during this study, and only two female brooding juvenile crabs were recorded, one in November 2013 (rainy season) and the other in July 2014 (dry season). The body size of the smallest and largest crab (both were male) observed during the sampling period was 14.42 and 45.00 mm CW, respectively. The mean size recorded for the sampled population was 33.32 ± 6.43 mm CW. For gonad sexual maturity, the adjustment of the data based on the logistic curve (CW50) resulted in a CW at gonad maturity of 36.75 mm for males and 34.22 mm for females ( Fig. 2). The size-frequency distribution analysis indicated a bimodal and non-normal distribution for the population (W = 0.939, p < 0.001) ( Fig. 3A).

The overall sex ratio deviated significantly from 1:1 and was skewed toward males (males: females = 1.66:1; p < 0.001). Skewed sex ratios were verified for males in the initial and final size classes (Fig. 3B). The body size of males (34.92 ± 6.35; range 14.42 to 45.00 mm CW; n = 111) was significantly larger than females (30.65 ± 5.66; range 18.23 to 40.37 mm CW; n = 67; Mann–Whitney U test, U = 1989.50, p < 0.001) (Fig. 3B). Juvenile crabs (IM + ED) were distributed in the size range from 14.00 to 38.00 mm CW. Adults were recorded mainly from 26.00 to 47.00 mm CW ( Fig. 4).

thumbnail Fig. 2.

The logistic curve indicating the point at which 50% of males and females of Sylviocarcinus pictus (H. Milne-Edwards 1853) are mature, estimating the size (CW) in which males and females reach gonadal maturity.
thumbnail Fig. 3.

Population distribution and sex ratio of the crab Sylviocarcinus pictus (H. Milne-Edwards 1853) at Guaribas river, semi-arid region of Brazil. A, observed population distribution of all crabs collected in this study; B, Sex ratio (estimate ± SE) during the months sampled and size-frequency distribution of body size (CW) in males and females. The black square indicates a deviation from a 1:1 sex ratio.
thumbnail Fig. 4.

Frequency of Sylviocarcinus pictus (H. Milne-Edwards 1853) by size class at Guaribas river, semi-arid region of Brazil. JM, juvenile male; AM, adult male; JF, juvenile female; AF, adult female.

3.2

Seasonal changes in the population

Concerning the measured environmental factors, rainfall was the only one that differed significantly between the periods (rainy and dry) (Mann–Whitney, p < 0.05). It ranged from 0 to 116 mm throughout the sampling period, with an average of 54.30 ± 54.31 mm per month in the rainy season and an average of 7.04 ± 10.53 mm per month in the dry season. Moreover, rainfall similarly varied in the sampling months as the number of crabs sampled according to the gonadal development category ( Fig. 5). The water temperature, in its turn, ranged from 28.50 C to 31.50 C, with an average of 29.90 C ± 1.10 C in the rainy and 29.40 C ± 0.60 C in the dry season, and the pH of the water ranged from 6.60 to 7.00, with an average of 6.70 ± 0.10 in the rainy and an average of 6.80 ± 0.10 in the dry season.

Of the 111 males analyzed for gonadal maturation, 38 were immature (juveniles) (34.20%), 31 were developing (27.90%), 32 were developed (28.80%) and 10 were spawned (9.10%). The developed males were more abundant in November 2013 and June 2014. Of the 67 females analyzed, 40 were immature (59.70%), 10 were developing (14.93%), 9 were developed (13.43%), and 8 were spawned (11.94%). All the females were immature in February 2014. Mature females (DEV + S) were present in November and December 2013 (rainy season) and through all the dry season. The nMDS ordination, derived from the population structure in terms of the individuals' gonadal condition, recorded two groups, as seen in Figure 6. An Analysis of Similarity (ANOSIM) indicated a significant difference in the composition of the population in terms of the individuals' gonadal condition between the two analyzed seasons (rainy vs. dry) (ANOSIM; R = 0.386; p <0.05; Fig. 6).

Recruitment of individuals in the population also varied across months and seasons (rainy and dry). Juveniles were more frequent in the months of the end of the rainy season (February) and the beginning of the dry season (May), and this variation was observed in CA ( Fig. 7).

thumbnail Fig. 5.

Correlation between the number of individuals of each gonadal state of Sylviocarcinus pictus (H. Milne-Edwards 1853) and rainfall at Guaribas river region, semi-arid region of Brazil. A, Immature crabs (IM); B, Developing crabs (ED); C, developed crabs (DE); D, spawned crabs (S).
thumbnail Fig. 6.

Non-metric multi-dimensional scaling (nMDS) plots evaluating the seasonal changes in the structure of the population of the crab Sylviocarcinus pictus (H. Milne-Edwards 1853) at Guaribas river, semi-arid region of Brazil.
thumbnail Fig. 7.

Correspondence analysis (CA) evaluating the seasonal changes in the structure of body size in the population of the crab Sylviocarcinus pictus (H. Milne-Edwards 1853) at Guaribas river, semi-arid region of Brazil.

4 Discussion

The results here provide valuable information on the population and reproductive biology of S. pictus. The hypothesis that the populational structure of S. pictus is modulated by the seasonality of dry and rainy seasons was corroborated. The three predictions raised here were also confirmed: (1) the abundance of individuals was greater in the rainy season; (2) individuals with mature gonads were registered throughout the rainy season; (3) recruits were observed at the end of the rainy and the beginning of the dry season. Individuals with developed gonads were frequent in all months (except March and April 2014, with no crabs collected) in the studied stretch of the Guaribas River.

Based on the results, there was little evidence of a seasonal pattern of reproductive activity. Although in the region there is a remarkable seasonality in terms of precipitation, the population seems to be well adapted to the extreme drought period when there is practically no rainfall because there were no indications of fully interrupted reproduction. Even in an unfavorable situation, the population of S. pictus manages to follow the typical pattern for freshwater crabs from temperate or subtropical regions of Asia (Okano et al., 2000; Chen et al., 1994), Europe (Micheli et al., 1990), and South America (Taddei et al., 2015). The results found corroborate the same continuous breeding pattern throughout the year reported by Dobson et al. (2007) in the study conducted in tropical Africa with the genus Potamonautes (“Chinga” freshwater crab). Possible continuous reproduction was also observed for Trichodactylus fluviatilis (Latreille, 1828) in a study realized by Silva et al. (2014) in northeastern Brazil, in a remnant patch of Atlantic Forest, where juveniles were found throughout the year. However, in the population of S. pictus studied here, the reproductive peak (indicated by the number of sexually mature individuals) and the recruitment peak occur in the rainy season, as it was found for other crustaceans (Etim and Sankare, 1998; Litulo, 2005; Novak et al., 2015).

In March and April 2014, the depth of the Guaribas River increased and the water became extremely turbid. Because the collection method used here (manual capture) depends on visually identifying individuals, specimens in these months were probably missing due to adverse conditions. Another possible explanation for the absence of individuals during these two months is that crabs may have emigrated to places downstream of the collection site since the largest amount of water in the river created a mild current at the site, which was not observed in the other collection months. Therefore, it can be considered that the capture of these organisms depends on sampling efficiency and habitat characteristics, namely, vegetation, physicochemical non-suitability, food shortages, and presence of prey, as found for other crustaceans (Collins et al., 2006; Lercari and Defeo, 1999).

Males of S. pictus in the Guaribas River are significantly larger than the females, which is common in natural populations of Brachyura (Hartnoll, 1969; Abelló et al., 1990; Parvizi et al., 2017). Some hypotheses may explain patterns of sexual dimorphism, such as Sexual Selection and Niche Divergence (Shine, 1989). Males being larger than females have been explained by the intersexual differences in the trade-off between growth and reproduction (Marochi et al., 2018). Sexual Selection theory predicts that this pattern is expected in gonochoric species with strong male competition for receptive females (Correa and Thiel, 2003). In these cases, the reproductive fitness of an individual is positively correlated to its size, since investments in somatic growth increase their resource holding potential and, thus, their opportunities to access and defend receptive females from other male competitors (Baeza and Asorey, 2012). Previously, Pralon et al. (2019), studying this same population in the Guaribas River suggested the presence of agonistic behavior in this same population of S. pictus. Unlike our findings, Silva (2010) did not observe any differences between the average sizes of males and females for a population of S. pictus in the Amazon basin. Therefore, it is not a general rule for populations from different environments to have the same patterns in terms of average sizes for males and females. Evolutionary history in the environment probably determines the specific pattern for the average size of each sex in the population.

Although males are larger than females when they reach gonadal maturity, both sexes reached sexual maturity at the same CW size class range (34–38 mm). According to Herrera et al. (2013), this is the consequence of their reproductive behavior. Trichodactylid crabs don’t exhibit cohort and the mating behavior in this family seems to involve forced copulation (Liu and Li, 2000; Almeida et al., 2019). In this type of mating, the male intercepts the female by force, without first checking her state of receptivity (Senkman et al., 2015). For a crab to be able to reproduce, it must reach both its physiological and morphological maturity (Hartnoll, 1982). Our results indicate that when S. pictus reaches gonadal maturity, it is already able to mate and achieve reproductive success. Comparing with the results of Pralon et al. (2019) of morphological sexual maturity for both sexes (Males: 30.82 mm; Females: 28.63 mm) was achieved in sizes smaller than the estimated gonadal maturity (Males: 36.80 mm; Females: 34.20 mm) for this population of S. pictus in this study.

The frequency bimodality of the studied population indicates that mature and juvenile adults coexist (Collins et al., 2006). Since the highest frequencies of juveniles occurred in February and May, the recruitment in these months may have caused the recorded bimodality. Moreover, the only female with juveniles in the abdomen was obtained in July. Thus, the population of the Guaribas River seems to reproduce throughout the year, with discrete recruitment pulses in some months. The environment studied here seems to offer conditions for reproduction throughout the year with little variation. The proximity of the studied site with the Equator line may explain this pattern because the small oscillation in water temperature during the study period possibly means abundant food resources throughout the year (Dobson et al., 2007). Besides that, in most Brachyura, spermatheca has evolved into sperm storage and, in some species, sperm can survive for more than a year (Liu and Li, 2000). Sperm retention for at least one molting event has been reported in some crabs, including the freshwater species (Liu and Li, 2000; Teixeira et al., 2017). This storage increases the probability of reproductive success of females, especially when males are absent at certain times of the year, and may explain the temporal pattern of reproduction in the population of S. pictus (Farias et al., 2017).

This continuous reproductive pattern would reveal an exception in the trichodactylid populations previously studied in South America. In populations of Dilocarcinus pagei Stimpson, 1861 (Taddei and Herrera, 2010; Davanso et al., 2013) and Sylviocarcinus australis (Magalhães and Türkay, 1996; Mansur et al., 2005), for example, reproduction is clearly seasonal. In continental aquatic environments, the recruitment season usually coincides with the time of increase in precipitation rate, as occurs in the studied population, with juveniles more frequent at the end of the rainy season in February/14 passing through March and April/14 (months of higher flood) up to the early months of the dry season. This reproductive strategy ensures that when juveniles are released from female care, they will find favorable conditions for survival and development. It is possible to find, for example, available shelters and a greater amount of materials from allochthonous sources that enrich the water with nutrients and remove organic matter from sediment (Davanso et al., 2013).

The general sex ratio of the population is skewed to males. However, the proportion was significantly higher in males than in females only in a few months. The sex ratio may differ from the Mendelian theoretical pattern of 1:1 due to sex reversal, differences in longevity, migrations, and differential mortality rates (Díaz and Conde, 1989; Colpo et al., 2005). Reproductive behavior is another possible explanation for the higher number of males in the population of S. pictus. According to Mansur and Hebling (2002) and Senkman et al. (2015), the relative vulnerability during the egg incubation period forces the females to search for hiding places and almost cease foraging. Consequently, it was particularly difficult to find ovigerous females (no record) or females with juveniles in the environment of the studied population of S. pictus. Similarly, studies conducted with freshwater crabs in different environments, in such as S. australis Brazil (Mansur and Hebling, 2002), Travancoriana schirnerae Bott, 1969 in India (Devi and Smija, 2013), and Sodhiana iranica Sharifian et al., 2014 (Sharifian et al., 2017) in Iran, also reported difficulties in obtaining ovigerous females or females with juveniles.

The higher number of males in the present population, especially in larger size classes, may be linked to differential behavior between the sexes, which causes greater exposure of males during sampling. Therefore, a certain bias may have occurred in this study regarding the sampling method since the females should remain hidden during the incubation period (Colpo et al., 2005). When males are proportionally more numerous than females in the larger size classes, we can assume they never mate with females larger than themselves but can mate with small females (Chen et al., 1994). Since most mature males were larger than most mature females, individuals of S. pictus of both sexes are highly likely to find a receptive partner.

Systems with annual flood pulses, such as the intermittent rivers of the Brazilian Caatinga, exhibit a permanently imbalanced behavior (Leigh et al., 2016). Temporal variability is a complex function that depends on the input and output of energy and materials in different places of the watershed and on the opportunities of populations to adjust their distribution and abundance (Datry et al., 2014). In the population studied here, and despite the significant differences in rainfall between the rainy and dry seasons, other abiotic parameters such as pH and water temperature did not oscillate or differ significantly between the seasons. This stability may explain the seasonal pattern of reproduction reported here.

The information obtained in the present study provides important insight into the life cycle of the species S. pictus in the Caatinga biome due to the intermittency of the ecosystem of occurrence of this species. This information can also support the conservation and maintenance of natural stocks of this species in the region. The study was conducted in an area that does not have fully conserved riparian vegetation since the marginal area was occupied by plantations, which is a common anthropogenic pressure in the region. The extent to which these environmental degradations may have contributed to the results of this research is unknown and may have collaborated to the relatively low number of sampled individuals, since the riparian vegetation, in addition to playing an important role in the health of the water body (Rivaes et al., 2017), is also a preferred habitat of the species (Silva, 2010).

Disclosure statement

No potential conflict of interest was reported by the authors.

Acknowledgements

The authors are thankful to laboratory co-workers Antonio Adriano Silva Sousa and João Gabriel Cavalcante Farias who helped with field samplings.

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Cite this article as: D.P. Rios, V.E.S Damasceno, D.F.R. Alves, W.M.S. Souto, B.G.N. Pralon. 2022. Seasonal variation in population structure and sexual maturity of Sylviocarcinus pictus (Decapoda: Trichodactylidae) in a Neotropical intermittent river. Int. J. Lim. 58: 5

All Tables

Table 1

Macroscopic characteristics of each male and female gonadal states in Sylviocarcinus pictus (H. Milne-Edwards 1853) (Adapted from Silva et al., 2012).

In the text

All Figures

thumbnail Fig. 1.

Stretch from the Guaribas river in the municipality of São Luís do Piauí - PI from where specimens of Sylviocarcinus pictus (H. Milne-Edwards 1853) were collected.
In the text
thumbnail Fig. 2.

The logistic curve indicating the point at which 50% of males and females of Sylviocarcinus pictus (H. Milne-Edwards 1853) are mature, estimating the size (CW) in which males and females reach gonadal maturity.
In the text
thumbnail Fig. 3.

Population distribution and sex ratio of the crab Sylviocarcinus pictus (H. Milne-Edwards 1853) at Guaribas river, semi-arid region of Brazil. A, observed population distribution of all crabs collected in this study; B, Sex ratio (estimate ± SE) during the months sampled and size-frequency distribution of body size (CW) in males and females. The black square indicates a deviation from a 1:1 sex ratio.
In the text
thumbnail Fig. 4.

Frequency of Sylviocarcinus pictus (H. Milne-Edwards 1853) by size class at Guaribas river, semi-arid region of Brazil. JM, juvenile male; AM, adult male; JF, juvenile female; AF, adult female.
In the text
thumbnail Fig. 5.

Correlation between the number of individuals of each gonadal state of Sylviocarcinus pictus (H. Milne-Edwards 1853) and rainfall at Guaribas river region, semi-arid region of Brazil. A, Immature crabs (IM); B, Developing crabs (ED); C, developed crabs (DE); D, spawned crabs (S).
In the text
thumbnail Fig. 6.

Non-metric multi-dimensional scaling (nMDS) plots evaluating the seasonal changes in the structure of the population of the crab Sylviocarcinus pictus (H. Milne-Edwards 1853) at Guaribas river, semi-arid region of Brazil.
In the text
thumbnail Fig. 7.

Correspondence analysis (CA) evaluating the seasonal changes in the structure of body size in the population of the crab Sylviocarcinus pictus (H. Milne-Edwards 1853) at Guaribas river, semi-arid region of Brazil.
In the text

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