Research Article |
Corresponding author: Verónica Williner ( vwilliner@inali.unl.edu.ar ) Academic editor: Ingo S. Wehrtmann
© 2014 Verónica Williner, María Victoria Torres, Debora Azevedo Carvalho, Natalia König.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Williner V, Torres MV, Carvalho DA, König N (2014) Relative growth and morphological sexual maturity size of the freshwater crab Trichodactylus borellianus (Crustacea, Decapoda, Trichodactylidae) in the Middle Paraná River, Argentina. In: Wehrtmann IS, Bauer RT (Eds) Proceedings of the Summer Meeting of the Crustacean Society and the Latin American Association of Carcinology, Costa Rica, July 2013. ZooKeys 457: 159-170. https://doi.org/10.3897/zookeys.457.6821
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The relative growth of a number of morphological dimensions of the South American freshwater crab Trichodactylus borellianus (Trichodactylidae) were compared and related to sexual dimorphism. Crabs were collected from ponds in the Middle Paraná River in Argentina. A regression model with segmented relationship was used to test for relative growth between these measurements where breakpoints infer the body size at which crabs reach sexual maturity. In both sexes the carapace width and the length, height, and thickness of the right and left chelae were measured, as well as the male pleopod length and the female abdomen width. All of these measurements were found to show positive allometry with the exception of the male pleopod length and the left chelae, which did not show a breakpoint. In females the breakpoint for the abdomen width inferred a morphological sexual maturity at carapace width 6.9 mm. In males the break point for the pleopod length was at carapace width 6.6 mm, with that for the chelae measurements was between carapace widths 6.4 and 6.9 mm. The relative growth pattern in T. borellianus was found to be similar to that recorded for other species of the family Trichodactylidae.
Chelipeds, cephalothorax width, pleopods, reproduction, growth, regression model
The onset of maturity in crustaceans is signaled by a series of morphological, physiological and behavioral transformations through which immature individuals become able to produce reproductive cells and copulate (
The Trichodactylidae is a neotropical family of freshwater crabs found throughout the river basins of tropical and subtropical South America, with the exception of the Pacific slope rivers (
There are only a few recent studies of the allometric changes associated with growth within the genus Trichodactylus (
Monthly collections were carried out during the day for fourteen months from August 2001 to October 2002. The samples were taken from three sites of the Paraná alluvial valley: Las Sandias Stream (S31°41'15.3"W 60°31'31.6"), Aliviador Stream (S31°40'17.9", W60°34'45.9") and Santa Fe River (S 31°38'35", W60°40'05.5"). The latter river is the biggest channel of La Plata Basin, containing 85% of the total freshwater in Argentina. The Paraná River system consists of a main channel with an alluvial valley ranging from approximately 13 to 56 km in width with a slope of 0.036 m km-1. The main channel is located on the eastern margin and is between 2 and 4.2 km wide, and is 2.3 km wide at the study site. The western area of the river has several secondary rivers, streams, ponds and islands, extending approximately 10 km in width to the main channel. The flow along the main channel varies between 10,600 m-3s-1 and 31,000 m-3s-1 causing a primary runoff in spring and summer (November to March), which originates annual floods. Autumn and winter are the low water seasons (
In the laboratory, a total of 337 crabs were analyzed of which 155 were males and 182 females. The following variables were recorded: carapace width (CW), right cheliped length (RChL), right cheliped height (RChH) and right cheliped width (RChW), left cheliped length (LChL), left cheliped height (LChH) and left cheliped width (LChW), abdomen width (AW) in females, and left first pleopod length (PL) in males (Fig.
Characterization of how the morphometric variables were obtained. CW: carapace width, ChL: cheliped length, ChH: cheliped height, ChW: cheliped width, AW: abdomen width, PL: pleopod length.
Body part (morphometric variable) | Measuring mode |
---|---|
CW | Distance between the first postorbital spines of the carapace |
ChL | Ventral distance between distal end of the propodus and the carpus articulation |
ChH | Maximum propodus thickness |
ChW | Maximum distance between lateral margins of the propodus |
AW | Maximum distance of the third segment of female abdomen |
PL | Maximum length of the left pleopod |
The CW was used as the independent variable to test relative growth in the other measurements (the dependent variables). Linear regressions, comparisons of slopes, and calculations of the onset of morphological sexual maturity were made with the software R version 2.13.2 (
The Davies’ test was used to test for significant differences of slopes between juveniles and adults and to test for a non-constant regression parameter in the linear predictor (
Male crabs had a CW of 6.34 ± 1.48 mm (ranging from 2.9 to 10.4 mm CW); female crabs had a CW of 6.12 ± 1.92 mm (ranging from 2.7 to 12.4 mm). There were no statistically significant differences between the CW of male and female crabs (t= -1.21; p = 0.227). The regression using CW as the independent variable indicated that measures of the right cheliped of male crabs were best adjusted to two straight lines rather than to one because these presented statistically significant differences between the slopes of both lines (Fig.
Results of Regression Model with Segmented Relationship with breakpoint studied for males and females of T. borellianus with Davies' test for change in the slope, This includes the estimate slopes for J: juveniles and A: adults.
Sexes | Relationship | Estimated break point CW (mm) | R-squared | Intercept | Davies' test for change in the slope p-value | Stage | Slopes | Allometry |
---|---|---|---|---|---|---|---|---|
Males | RChL vs. CW | 6.6 | 0.89 | -0.02 | <0.0001 | J | 0.55 | - |
A | 1.19 | + | ||||||
RChH vs. CW | 6.5 | 0.85 | -0.01 | <0.0001 | J | 0.21 | - | |
A | 0.66 | - | ||||||
RChW vs.CW | 6.9 | 0.82 | -0.02 | <0.0001 | J | 0.14 | - | |
A | 0.54 | - | ||||||
PL vs. CW | 6.6 | 0.60 | -0.24 | 0.2836 | J | 0.39 | - | |
A | 0.25 | - | ||||||
LChL vs. CW | (*) | 0.81 | 0.89 | without slope change | - | - | ||
LChH vs. CW | (*) | 0.73 | 0.25 | without slope change | ||||
LChW vs. CW | (*) | 0.53 | 0.19 | without slope change | ||||
Females | RChL vs. CW | 6.0 | 0.90 | 0.66 | <0.0001 | J | 0.37 | - |
A | 0.62 | - | ||||||
RChH vs. CW | (*) | 0.85 | without slope change | |||||
RChW vs.CW | (*) | 0.70 | without slope change | |||||
AW vs. CW | 6.9 | 0.91 | -0.79 | <0.0001 | J | 0.73 | - | |
A | 1.30 | + | ||||||
LChL vs. CW | 5.7 | 0.91 | 0.54 | <0.0001 | J | 0.37 | - | |
A | 0.68 | - | ||||||
LChH vs. CW | (*) | 0.82 | without slope change | |||||
LChW vs. CW | (*) | 0.66 | without slope change |
In females, there were three characters that best adjusted to two straight lines rather than to one: the lengths of both chelae and the abdomen width (Table
Results of allometric relationship study of each stage of Trichodactylus borellianus. J: juveniles; A: adults.
Sexes | Relationship | Stage | Linear equation y= a + bx | R-squared | Allometry |
---|---|---|---|---|---|
Males | RChL vs. CW | J | RChL = -0.0179 + 0.5478 CW | 0.66 | – |
A | RChL = -4.2291 + 1.1899 CW | 0.80 | + | ||
RChH vs. CW | J | RChH= -0.0109 + 0.2117 CW | 0.50 | – | |
A | RChH = -3.003 + 0.6643 CW | 0.76 | – | ||
RChW vs. CW | J | RChW = -0.0217 + 0.1378 CW | 0.40 | – | |
A | RChW = -2.7944 + 0.537 CW | 0.68 | – | ||
PL vs. CW | J | PL = -0.3109 + 0.3938 CW | 0.46 | – | |
A | PL = 0.8324 + 0.2228 CW | 0.28 | – | ||
Females | RChL vs. CW | J | RChL = 0.6661 + 0.3747 CW | 0.59 | – |
A | RChL = -0.7479 + 0.6187 CW | 0.84 | – | ||
AW vs. CW | J | AW = -0.7905 + 0.7309 CW | 0.79 | – | |
A | AW = -4.7357 + 1.3027 CW | 0.73 | + | ||
LChL vs. CW | J | LChL = 0.5427 + 0.3747 CW | 0.56 | – | |
A | LChL = -0.9007 + 0.6268 CW | 0.87 | – |
The knowledge of the life history of a species involves understanding such aspects as the development of sexual maturity, changes in allometric growth, and the age at which each of these occur. In crabs, morphological maturity is often observed together with allometric changes in growth (
The results obtained in this study are in line with the points proposed by
For males, the three measurements around the right cheliped showed a good fit to the regression model, as well as differences in slopes of the regressions. These three measurements revealed a similar size from which growth patterns morphometrically change. According to
Similar relative growth patterns were registered in other trichodactylids such as Dilocarcinus pagei, Sylviocarcinus australis and Trichodactylus fluviatilis (
Against this background, the changes in growth shown for the equations provide evidence of the reproductive function of the abdomen width in females and the chela in males. As in the crab T. fluviatilis, the relationship between RChL vs. CW also showed a positive allometry in both sexes (
For both sexes, the differences in the size of the chelae could involve some of different issues concerning reproduction. One of them could be related to the modification of feeding options. Variations in size of the chelae may also involve the possibilities of the range expansion of trophic items. This crab, found in the area of middle Paraná River, changes natural diet with body size (
We consider that the application of the Regression Model with Segmented Relationship with breakpoint with the package ‘segmented’ is an appropriate way to determine patterns of relative growth and sexual maturity. The present study also establishes the size range corresponding to the pubertal molt in this species. In this study, one might assume that, with the exception of traits that showed differential growth rates between adults and juveniles, for the remainder of the measured characters, that a shift in allometry between juveniles and subadults occurs in a continuous, gradual manner.
Considering the present contribution to the knowledge of reproductive traits, it is now necessary to assess sexual maturity in its other dimensions, mainly from the histological perspective, with a description of ducts and sexual cells. These results show that it is necessary to inquire about the sexual and mating system of this species in order to evaluate differences in measurements and trends found in this study (
The current study also provides a starting point for addressing additional aspects relevant to allometry of T. borellianus; however variations in allometry in other parts of the range of this species and among other populations still need to be evaluated.
This work was supported by the grants of National Agency for Science and Technology, through the Fund for Scientific and Technological Research (FONCYT), PICT Bicentenario 2159, National Council of Scientific and Technical Research (CONICET) PIP 2011-2013 052, and Universidad Nacional del Litoral CAI+D 2011 PI 119.