© 2005 International Council for the Exploration of the Sea
Does the largest chela of the males of three crab species undergo an allometric change that can be used to determine morphometric maturity?
Murdoch University Centre for Fish and Fisheries Research, School of Biological Sciences and Biotechnology, Division of Science and Engineering, Murdoch University South Street, Murdoch, Western Australia 6150, Australia
*Correspondence to N. G. Hall: tel: +61 8 9360 7215; fax: +61 8 9360 6303. e-mail: normhall{at}murdoch.edu.au.
The allometry of the largest chela of male crabs has often been assumed to undergo a change at the pubertal moult, i.e. the moult to maturity, and could therefore be used as a basis for determining the size of males at morphometric maturity. As initial plots of the logarithms of the length of the propodus of the chela and carapace width (CW) or length (CL) of male Portunus pelagicus, Hypothalassia acerba, and Chaceon bicolor revealed no conspicuous change in allometry, we used an information-theoretic approach and a range of models to explore, in greater depth, whether the chelae of these species did undergo such an allometric change. The candidate models were linear, quadratic, cubic, and broken-stick models, and broken-stick and two-line-segment models with logistic transitions between line segments. There was strong evidence that the largest chela of male P. pelagicus undergoes a subtle change in allometry at 82.0 cm CW, which is only 6.4 mm less than that at which 50% of males attain physiological maturity. Further, as the 95% confidence region of this estimate of size at morphometric maturity overlaps that for physiological maturity, the sizes at which morphometric and physiological maturity are attained by male P. pelagicus are similar. Because the estimate of the carapace length at which allometric change in the chela of male H. acerba was very imprecise, there was far less convincing evidence that the allometry of the chela of this species undergoes a conspicuous change at a certain length. The allometry of the chelae of male C. bicolor changed progressively and continuously with body size, and did not change abruptly at a particular size. In view of the morphometric results for H. acerba and C. bicolor, it would be advisable to base management plans for conservation on the carapace lengths at which 50% of the male crabs of these two species attain physiological maturity, i.e. 68.1 and 94.3 mm, respectively.
Keywords: allometry, chela, Crustacea, male, morphometric maturity, physiological maturity, pubertal moult
Received 29 September 2004; accepted 26 July 2005.
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Introduction |
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The size at which the individuals in exploited stocks become mature is one of the more important parameters used by managers for developing plans that will ensure sustainable exploitation. Indeed, concerns that a reduction in the number of males in those fisheries that exploit only or predominantly males might influence the sustainability of the stocks of such species have highlighted the need for information on the sizes at which the males attain maturity (Conan and Comeau, 1986; Ennis et al., 1988; Gardner and Williams, 2002).
Studies aimed at determining the size at maturity of male crabs have used data on whether individual crabs have become mature from a functional (able to mate), physiological (spermatophores present), and/or morphometric (possession of morphometric characteristics that are distinct from those of immature individuals) perspective (e.g. Comeau and Conan, 1992; Goshima et al., 2000; Conan et al., 2001; Gardner and Williams, 2002; de Lestang et al., 2003a). Evidence that functional maturity has been attained is provided by data proving copulation (Goshima et al., 2000; Gardner and Williams, 2002), whereas males are considered physiologically mature when they produce spermatophores (Warner, 1977; Melville-Smith, 1987; Comeau and Conan, 1992). The attainment of morphometric maturity by a male crab is identified by determining whether the relationship to body size of the dimensions of certain body parts, e.g. chelae, changes conspicuously at the pubertal moult, i.e. the moult to maturity (Somerton, 1981; Conan and Comeau, 1986; Comeau and Conan, 1992; Gardner and Williams, 2002). Such determination has relied on the analysis of whether the allometry of the chelae relative to body size, i.e. the relationship between the relative dimensions of parts of an individual and its overall size (Huxley and Teissier, 1936; Gayon, 2000), changes markedly when the individual crab moults from an immature to a mature state.
A male crab must become both functionally and physiologically mature before it can reproduce, and the ability to mate may depend on the attainment of morphometric maturity. In an aquarium study involving mature female and male Chionoecetes opilio, the latter with carapace widths (CWs) greater than that at physiological maturity, precopulatory pairing behaviour was initiated only by the larger of the males that had become morphometrically mature, i.e. >
95 mm CW (Conan and Comeau, 1986). It was therefore concluded that, although physiological maturity is attained by male C. opilio before they reach a CW of 60 mm, functional maturity is not attained until individuals have moulted to become morphometrically mature and, even then, only by those animals that had attained a CW of 95 mm. However, Paul (1992) queried why physiologically mature males would produce sperm if it was not for immediate use. Subsequently, Sainte-Marie et al. (1995) showed that male C. opilio from the Gulf of St. Lawrence became physiologically mature at a CW of only 38.5 mm, and Sainte-Marie and Lovrich (1994) demonstrated that, in non-competitive aquarium studies, physiologically mature but morphometrically immature males were functionally mature, i.e. mated successfully.
Growth strategies vary considerably among different families of crabs (Hartnoll, 1983), with, for example, the individuals of the Majidae attaining their maximum size after a "terminal" moult to maturity (Conan and Comeau, 1986). The latter authors noted that, as shown by Teissier (1933, 1935) and Hartnoll (1963), plots of the logarithms of chelae dimensions against the logarithms of a measure of the body size of males demonstrate that three distinct segments are present in majid crabs. The first is associated with small, prepubertal, immature individuals, the second is continuous with the first segment, but with a distinct change in slope, and is associated with the pubertal or juvenile stage, whereas the third segment is parallel with the second, but possesses a distinct increase in ordinate. The last of the three line segments is associated with mature animals that are believed to have undergone a final moult to maturity. Conan and Comeau (1986) found that, in many majid species, the length ranges associated with the immature and mature individuals show considerable overlap. In contrast, Hartnoll (1983) observed that individual carcinid and portunid crabs attain maturity before their final moult. For male crabs of species other than majids, plots of the logarithms of chela dimensions against the logarithms of some measure of overall body size may exhibit a less distinct or negligible change in allometry (e.g. Fernández-Vergaz et al., 2000). Such crabs are the subject of this study.
The allometric changes in a body part of a male crab of some non-majid species may not be sufficiently well defined to permit its use with confidence for estimating the size at morphometric maturity (Clayton, 1990; Goshima et al., 2000). For example, in some decapod species, the changes in the "level of allometry" (Hartnoll, 1978), i.e. the allometry coefficient (Huxley, 1924), are gradual and the logarithms of the measurements appear to follow a curvilinear trend, e.g. as demonstrated for Homarus americanus by Conan et al. (2001). Consequently, the straight lines fitted to the presumed juvenile and adult data may be artefactual and, if so, cannot therefore provide a reliable means for distinguishing between pre- and post-pubertal moult animals (Conan et al., 2001). Indeed, Somerton (1980) pointed out that it is essential to test whether a difference between the patterns of growth of that body part in juveniles and adults can be detected.
Somerton (1980) developed a method to fit two disjoint (i.e. overlapping) line segments to the logarithms of the dimensions of various body parts of juvenile and adult crabs, so developing a model that could accommodate data for majid species, such as Chionoecetes bairdi, in which sexual maturity occurs over a range of sizes (Figure 1). His approach involved the identification of three size ranges, representing crabs known to be juveniles, an unknown mixture of juveniles and adults, or adults. Lines were then fitted to the data for the juveniles and adults, respectively, after which the crabs with data points lying within the intermediate size range were classified as juveniles or adults by assigning the points to the closest line. This process of fitting and classification was repeated iteratively until there was no further change in the classification of data points in the intermediate size range.
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Somerton (1980) used an F-test (i.e. essentially a likelihood ratio test) to determine whether the difference between the sums of squares of this and a simple linear model was statistically significant. However, the objective function calculated by Somerton (1980), using his iterative approach, relies on the allocation of intermediate points to one or other of the straight lines, based on the closeness of the value of the dependent variable to the predicted values of the lines at that body size. Therefore, conclusions based on this test are only approximate. Nevertheless, several earlier studies on the maturity of male crabs, including our own on P. pelagicus (de Lestang et al., 2003a), have used models such as that of Somerton (1980) without validating that the model was structurally appropriate.
In Western Australia, the blue swimmer crab Portunus pelagicus (Portunidae) is fished in estuaries and protected coastal waters, whereas the champagne crab Hypothalassia acerba (Eriphiidae) and the crystal crab Chaceon bicolor (Geryonidae) are fished predominantly in water depths of 90–310 m, and 450–1220 m, respectively (de Lestang et al., 2003b; Smith et al, 2004a; Melville-Smith et al., 2005). The main aim of the current study was to determine whether the allometry of the chelae of male P. pelagicus, H. acerba, and C. bicolor undergoes a conspicuous change that reflects a morphological change at a similar size to that at which the males of these species become physiologically mature. For this purpose, the Akaike Information Criterion (Akaike, 1973) was used to explore the extent to which the data supported each of a set of candidate models representing the relationship between the natural logarithms of the length of the dorsal propodus of the largest chela and the body size of the male crabs of each species. These alternative representations were linear (LM), quadratic (QM), cubic (CM), and broken-stick (BSM) models, a broken-stick model with logistic transition (BSMLT), and a two-line-segment model with logistic transition (TLSMLT; Figure 1). An LM assumes that allometry does not change as body size increases, whereas a QM or CM assumes that the pattern of growth changes continuously with increasing body size. In contrast, the BSM, BSMLT, or TLSMLT assumes a distinct change in allometry during growth. If the allometry of the chela in a species was found to change, the estimate of the body size at which this change occurred would be compared with that at which the animal becomes physiologically mature. This required determining the size at which physiological maturity is attained by male H. acerba and C. bicolor, to provide data to complement those already available for P. pelagicus (de Lestang et al., 2003a).
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Collection of crabs
In all, 1287 male P. pelagicus, with CWs ranging from 21 to 154 mm, were collected from Cockburn Sound (32°12'S 115°48'E) monthly between February 1997 and March 2000. The crabs were caught with a beach-seine 21.5 m long, with a bunt made of 3-mm mesh, and a small otter trawl with a codend constructed of 25-mm mesh (see de Lestang et al., 2003a).
A total of 571 male H. acerba, with carapace lengths (CLs) ranging from 16 to 135 mm, was collected bimonthly between July 1999 and April 2002. These were obtained from the trap catches of commercial fishers operating off the lower west and south coasts of Western Australia between 30°10'S 114°30'E (west of Jurien) and 32°30'S 114°60'E (west of Mandurah), and between 34°40'S 115°40'E (south of Augusta) and 34°45'S 119°30'E (south of Bremer Bay), respectively. The crabs were caught using either rectangular wooden slat traps, circular cane traps, or steel-framed traps enclosed with steel or plastic mesh (Smith et al., 2004b). Although such traps do not catch the smallest H. acerba and C. bicolor, they still retain a wide size range of these two deep-sea species, and were the only available methods for their capture.
A total of 392 male C. bicolor, with CLs ranging from 59 to 163 mm, was collected bimonthly between January 2000 and March 2003 from the catches of commercial fishers, who used lines of up to 100 recreational rock-lobster traps. Another 331 males of this species were measured at sea and released. These traps are made of lightweight plastic to prevent their sinking into the soft substratum. They are 675 mm long, 350 mm wide, and 475 mm high, and possess, at their top, an entrance of 170 mm diameter and 200-mm depth.
Further details of the sampling regimes for H. acerba and C. bicolor are given in Smith et al. (2004b).
Measurements and maturity stages
The carapace width (CW) of each male P. pelagicus, i.e. the distance between the tips of the two lateral spines of the carapace, and the carapace length (CL) of each male H. acerba and C. bicolor, i.e. the distance across the gastric region from the midpoint between the bases of the two anterior medial spines and the posterior margin of the carapace, were each measured to the nearest 1 mm. Carapace length rather than carapace width is typically measured in deep-sea crabs (e.g. Levings et al., 1996; Gardner, 1997; Goshima et al., 2000), because it overcomes the problems of using the distance between the two lateral spines of the carapace, which are particularly prone to wear. For randomly-selected subsamples of each species, the length of the dorsal propodus from the proximal edge of the depression below the upper articulation knob to the proximal edge of the depression at the articulation with the moveable finger of the molariform chela of each male (i.e. the "crusher" claw, which is typically the right claw and distinguished by molariform teeth on the inside edge of the chela, rather than incising teeth) was measured to the nearest 0.1 mm with vernier calipers. The decision to use chela propodus length for providing data aimed at ascertaining changes in allometry was based, in part, on the demonstration by Gardner and Williams (2002) that this was the most suitable of several variables for assessing morphometric changes in the giant crab Pseudocarcinus gigas. The latter species belongs to the same family (Eriphiidae) as H. acerba.
The gonads of a randomly selected subset of male H. acerba and C. bicolor, which could be readily distinguished from females by the shape of their abdomen, were examined macroscopically. The crabs were recorded as immature if either their reproductive tract, i.e. the testes and/or vas deferens, was not visible, or if the middle and posterior of their vas deferens were present but thin and either straight or loosely convoluted, and were classified as mature when the middle and posterior regions of their vas deferens were enlarged and highly convoluted (Ryan, 1967; de Lestang et al., 2003a). Note that, when examined microscopically, segments of the anterior vas deferens of large subsamples of each species always contained spermatophores in animals designated as mature, but never with those classified as immature.
Relationship between propodus length and body size
The natural logarithms of the lengths of the dorsal propodus of the males of each species were plotted against the natural logarithms of body size, i.e. CW or CL. In each case, the trend exhibited by the points with increasing body size appeared continuous and without marked disjunction. This suggested that the data for each species might be adequately described by LM, QM, or CM. Therefore, before it could be accepted that there was a marked change in the level of allometry, we needed to demonstrate that the extent to which the data supported such a model exceeded the support for the LM, QM, or CM. Candidate models reflecting a change in allometry included a BSM, i.e. a piece-wise linear regression model that is continuous at the point of intersection (breakpoint; e.g. Shea and Vecchione, 2002), BSMLT and TLSMLT, where the transition between the left- and right-hand line segments is determined by a logistic function of body size, reflecting the probability that a crab has moulted and thus developed chelae and body size consistent with the allometry described by the second line segment (Figure 1).
The LM implies that there is no change in the level of allometry, whereas the QM and CM represent a continuous and smooth change in the level of allometry as body size increases. The BSM, BSMLT, and TLSMLT represent two distinct and constant levels of allometry, reflecting different patterns of growth of morphometrically immature and mature crabs. For the BSM, the change occurs when the animals attain the length corresponding to the point at which the two lines intersect. For the BSMLT and TLSMLT, the expected change is gradual, and its rate is determined by the parameters of the logistic curve that determine the proportion of the mixture of points associated with each line segment. The TLSMLT, which represents the relationship between estimates of the expected values of the logarithms of the morphometric variables for the population, allows for a discontinuity in the line segments representing the allometric relationships for the data for individual crabs, and an overlap in the size range over which the crabs make the transition, through moulting, from one line segment to the other. The LM, QM, and CM may be written as ln DP = a1 + a2 ln S, ln DP = a1 + a2 ln S + a3 (ln S)2, and ln DP = a1 + a2ln S + a3 (ln S)2 + a4 (ln S)3, respectively, and the BSM as
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The BSMLT is
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In these equations, DP is the length of the dorsal propodus, S the body size (where S = CW for P. pelagicus and CL for H. acerba and C. bicolor), aj represents parameter j of each model, and ln is the natural logarithm of the associated variable. The breakpoint of the BSM is at a body size of exp(a4), and the body sizes, S50, at which 50% of the crabs are expected to have made the transition from the first to the second line segments of the BSMLT and TLSMLT, are exp(a4) and exp(a6), respectively (Figure 1).
As the dependent and independent morphometric variables of these allometric relationships are both measured with error, the models were fitted to the logarithmically transformed values for the variables for each species using reduced major axis regression, a form of Model II regression (Sokal and Rohlf, 1995). For this, the horizontal and vertical deviations between each observed point and the corresponding points on the curve with the same value of ordinate (y value) and abscissa (x value), respectively, were calculated. If no analytical expression was available, the abscissa of the point on the curve with the same ordinate as the observed point was determined numerically, using the bisection method (Press et al., 1992). A measure of the deviation of each observed point from the curve was calculated as the geometric mean of the absolute values of these deviations and, to facilitate analysis of the residuals of the fitted models, was assigned the same sign as that of the vertical deviation of the point from the curve. The models were then fitted using the Constrained Nonlinear Regression (CNLR) procedure in SPSSTM to minimize the squared values of these deviations, so minimizing the sum of the areas of the triangles formed by the lines joining the abscissa and ordinate of each point to the curve, and the line joining the points of intersection of those lines on the curve. To improve convergence, initial values of the parameters were refined by employing the "amoeba" procedure described in Press et al. (1992), and the resulting estimates were input as the initial estimates for CNLR.
A simple non-parametric bootstrapping approach was employed to obtain estimates of the 95% confidence limits for the parameter estimates (Efron, 1979; Efron and Tibshirani, 1993). Therefore, data for the individual crabs in the original data sets were randomly resampled, with replacement, to create 1000 different data sets for each species, each containing the same number of observations as in the original data set. Using the values of the parameters obtained when each model was fitted to the original data set as the initial values of the parameters for CLNR, each model was fitted to each of these bootstrapped data sets for each species, and the resulting parameter estimates were recorded. The limits of the approximate 95% confidence intervals of each of the parameters were then calculated as the 2.5 and 97.5 percentiles of the resulting set of 1000 estimates for that parameter.
When fitting the BSMLT and TLSMLT, penalty functions were used to ensure that estimates of the parameters of the logistic functions were such that the values of p calculated for crabs with a body size less than a specified minimum Smin, or greater than a specified maximum Smax, satisfied the constraints that (1 – p) > 0.99 or p > 0.99, respectively, and, when fitting these models and the CM, that the gradient of the curve did not fall below zero. The former constraint emulates the approach used by Somerton (1980), in which crabs with sizes below or above some intermediate size range were assumed to be immature or mature, respectively. For P. pelagicus, the values of these specified minimum and maximum body sizes were set at 60 and 110 mm CW, respectively, as these were the limits used by de Lestang et al. (2003a) for the same purpose. For H. acerba and C. bicolor, the carapace lengths specifying this intermediate size range, i.e. 67–110 mm and 81–134 mm, respectively, were selected from plots of the data for each species, and they encompassed the range presumed to contain the size at which 50% of the crabs would exhibit morphometric variables consistent with the allometry described by the right-hand linear segment of the fitted curve. Three male H. acerba with carapace lengths <42 mm were excluded, because initial analyses demonstrated that had an excessive influence on the fit of the models. In addition, the very few obvious outliers in the data sets for each species were excluded from the analyses.
The extent to which the data for each species supported each of the different models was considered through an information-theoretic approach. For this, values of AICc, a small-sample, bias-corrected form of the Akaike Information Criterion AIC (Akaike, 1973), were calculated as
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AIC differences, 
, were calculated for each candidate model as 
, where 
is the smallest value of AICc determined for the set of candidate models, noting that values for this variable of 0–2 suggest substantial support for the model, while there is considerably less support for the model if the difference lies between 4 and 7, and essentially no support when the difference exceeds 10 (Burnham and Anderson, 2002). The approximate weight of evidence in favour of the i(th) model, within the set of R candidate models, was determined by calculating its Akaike weight:
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Size at physiological maturity
The probability, p, that a male of H. acerba or C. bicolor is physiologically mature is assumed to be related to carapace length by the logistic function
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and ß of this relationship for each species using a non-linear procedure in SPSSTM, and assuming that the maturity status of each crab was the random outcome of a Bernoulli trial, with a probability determined from the above equation and the observed carapace length of that crab. The carapace lengths CL50 and CL95, at which 50% and 95%, respectively, of male H. acerba and C. bicolor are expected to have become physiologically mature, were then calculated as L50 = – /ß and L95 = {loge(19) – }/ß, respectively. |
Results |
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Relationship between chela length and body size
Fitting of the BSM, BSMLT, and TLSMLT to the logarithms of chela length and carapace width of P. pelagicus yielded similar estimates of the carapace width at which 50% of the animals attain morphometric maturity, S50, i.e. 82, 82, and 80 mm CW, respectively (Table 1, Figure 2). After weighting these values by the weight of evidence for each of the different models (Table 2), the average value of S50 was estimated to be 82.0 mm CW (95% confidence limits 77.7–89.3 mm).
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Comparison of the values of the AICc, calculated by fitting each model to the logarithms of DP and CW for male P. pelagicus, revealed overwhelming support (weight of evidence 100%) for the group of models, i.e. BSM, BSMLT, or TSLMLT, that assumed that the allometry of the chelae relative to carapace width of small and large crabs differed (Table 2). The weight of evidence strongly favoured the BSM, with a contribution of 69% to the sum of the Akaike weights, and there was some support for the BSMLT and, to a lesser extent, the TLSMLT, with contributions of 22% and 9%, respectively, to the sum. There was no support for the alternative group of models that assumed that the morphometric data could be described using the LM, QM, or CM.
In contrast to the situation for P. pelagicus, the value of S50 estimated for both H. acerba and C. bicolor by the BSM fell outside the data range, effectively resulting in fitting a linear model and producing a slope for the second line segment that was identical to that of the LM, and with an identical sum of squared deviations (Table 1, Figure 2). The average S50 values for these two species, calculated from the BSMLT and TLSMLT and using weight of evidence as a weighting factor, were estimated to be 83.1 (95% confidence interval from 66.7 to 104.6) and 82.7 (82.4 to 133.2) mm CL, respectively. In the absence of constraints on the values of S50 and S95, the values of S50 estimated for H. acerba using the TLSMLT and for C. bicolor using both the BSMLT and TLSMLT fell below the range of recorded carapace lengths.
Substantial support for the models that assumed a difference in allometry between small and large animals was also evident in the data for H. acerba, i.e. 74% of the sum of the Akaike weights. Although there was some support for both the BSM (3%) and the BSMLT (12%), the weight of the evidence was greatest for the TLSMLT, i.e. 59% (Table 2). However, the data for this species also indicated some support for the LM, i.e. 21% of the total of the Akaike weights. In contrast, the data for C. bicolor provided little support for the BSM, BSMLT, and TLSMLT, i.e. a total of only 7% of the sum of the Akaike weights (Table 2). For this species, the weight of evidence favoured the QM, which contributed 58% to the sum of the Akaike weights, and there was some support for both the LM and CM, i.e. 15% and 20%, respectively.
Size at physiological maturity
Male H. acerba attain physiological maturity at a carapace length of 68.1 (95% C.I. 67.8–68.3) mm, and 95% are physiologically mature when they have reached a CL of 72.0 (95% C.I. 71.2–72.7) mm. For male Chaceon bicolor, the estimates of CL50 and CL95 were 94.3 (95% C.I. 93.7–94.9) and 99.9 (95% C.I. 98.2–101.6) mm CL, respectively (Figure 3).
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Discussion |
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The results show that, for male P. pelagicus, the relationship between the logarithms of the dorsal propodus of the largest chela and carapace width was described better using the BSM than the BSMLT or TLSMLT, and they provided no support for the LM, QM, or CM. The differentiation of the allometry of this chela occurs when animals reach a carapace width of 82.0 mm (95% confidence limits 77.7–89.3 mm). The point estimate of S50 derived previously by de Lestang et al. (2003a) when subjecting the same data to Somerton's (1980) overlapping-lines method is 4.2 mm greater, but it lies within the 95% confidence interval of the BSM estimate.
Although male P. pelagicus become physiologically mature at 88.4 mm CW (95% C.I. 87.8–89.1 mm CW; de Lestang et al., 2003a), 6.4 mm after they have attained morphometric maturity, the 95% confidence limits of the two estimates overlap. Therefore, at least on the basis of chela measurements, the males of this portunid become morphometrically mature at approximately the same size as they become physiologically mature, i.e. when they produce spermatophores. From a management perspective, it would be appropriate to be conservative and base strategies for conserving this portunid species on the maximum of the above two indicators of maturity, i.e. 88.4 mm CW.
Analysis of the data for H. acerba indicated that the weight of evidence for the BSMLT or TLSMLT was high (total for both, 71%), with a weighted average of 83.1 mm CL for S50 for these two models. However, as the 95% confidence interval for S50 ranged widely from 66.7 to 104.6 mm CL and therefore approached the constraints imposed when fitting these two models, the point estimate of the S50 is not very reliable. The analysis demonstrated some support (total 29%) for the LM, CM, and BSM, but the particular values estimated for the parameters of this last model effectively made it a linear model. The imprecision of the estimate of S50 for the two models that assumed a change in allometry, coupled with some support of the data for those models that assumed no such change, suggest that it would be inappropriate to place much reliance on the estimates of the size at morphometric maturity for this species. Therefore, as a basis for developing management plans for this species, it would be better to use the S50 of 68.1 mm CL for male H. acerba at physiological maturity.
In the case of C. bicolor, there was little support for the use of BSM, BSMLT, and TLSMLT (total = 7%), so there was no evidence that the largest chela of this species underwent a conspicuous change in allometry within the wide size range of animals examined. However, the results from the other models, and particularly of the QM, strongly indicated that the allometry of this chela underwent a continuous and progressive change with increasing body size. As with H. acerba, it would be appropriate for managers to use the S50 of 94.3 mm CL when developing management plans for the males of this species.
The results of this study provide strong quantitative evidence that the allometry of the largest chela of male P. pelagicus undergoes a conspicuous change as body size increases. However, our data show that this change is subtle and not as marked as that exhibited by majid crabs (Conan and Comeau, 1986). Although there was also evidence that the largest chela of male H. acerba underwent a change in allometry, the case was far less convincing for this species, and the estimate of the size at which this occurs was very imprecise. Although the allometry of the chelae of male C. bicolor did not change abruptly at a particular body size, it did undergo a progressive and continuous change with body size. From the above it follows that, even in the case of P. pelagicus, there have been no strong selective pressures for one claw of the males of any of our three species to become particularly large at maturity.
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Acknowledgements |
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We thank two anonymous referees for the invaluable advice they offered, Alex Hesp for assisting in analysing data and preparing figures, and the Fisheries Research and Development Corporation and Murdoch University for funding. We also thank the commercial fishers and the skippers and crew of charter vessels for helping us collect samples, and Lobster Australia for providing crabs for our use.
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References |
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