Life cycles of benthic invertebrates with pelagic larvae may suffer drastic morphological, physiological and behavioural changes at metamorphosis; however, other important changes, although less dramatic, may occur after settlement. Such changes are mostly adaptations to sequential exploitation of different niches during growth and are often accompanied by habitat shifting and, consequently, the spatial structuration of the populations by size (Pardo et al. 2007). The efficient exploitation of ontogenetic niches generally requires different morphologies and colour patterns associated with changing diets (Jensen and Asplen 1998) and predatory risks (Palma and Steneck 2001, Reuschel and Schubart 2007).

A landmark within the post-metamorphic phase is the attainment of sexual maturity, which involves a suite of morphological, physiological and behavioural changes that must occur in harmony for effective reproduction to take place. In crabs sexual maturity is attained progressively through a series of steps. The sequence and synchronicity of these steps are particular to each species (Fernández-Vergaz et al. 2000, Leal et al. 2008) and therefore the size at maturity may vary depending on the particular ontogenic change observed. For most purposes, it is desirable to estimate the sexual maturity based on the direct observation of the size at which individuals are reproducing in nature. However, to do so it is necessary to capture a large number of mating pairs and egg-bearing females, which can be a difficult task in full marine species, given that reproductive individuals tend to hide and/or migrate to areas of difficult access for researchers. Consequently, estimations of the size at maturity of crabs have been traditionally based on a combination of direct observations of the qualitative traits related to reproduction (hereafter “reproductive indicators”) and morphometric analysis (e.g. López-Greco and Rodríguez 1999, Hall et al. 2006, Sal Moyano et al. 2010). Reproductive indicators, such as the morphology of gonads, seminal receptacles and vulvae, have discrete response values (mostly binary as mature/inmature) that are readily observable, so the classification of an individual as mature usually pose no further problems.

In contrast, determining maturity based solely on morphometric traits (hereafter “morphometric indicators”) may be rather problematic. The relative growth that leads to an overall mature shape occurs in a continuum during the ontogeny, thus requiring analyses that are methodologically more complicated. Moreover, the potential existence of different morphotypes of mature males (as evidenced by the growing list of species with alternative mating strategies, see Shuster 2008) complicates the analysis even more. The best method has been a long-debated topic (e.g. Gould 1966, Somerton 1980, Watters and Hobday 1998, Katsanevakis et al. 2007, Packard 2012) but the vision that relative growth follows the allometric function (in the strict sense that the relationship between the two measured quantities fits well to a power law) has prevailed in literature. The most widely used method is that of Somerton (1980), in which the relative growth is assumed to follow a two-phase power function and size at maturity is therefore estimated as the breakpoint in a two-segment regression applied to the log transformed data. This practice has been the object of criticism (Watters and Hobday 1998, Katsanevakis et al. 2007, Packard 2012), mainly because it requires several a priori assumptions that are not necessarily met but are rarely tested.

Platyxanthus crenulatus (Milne-Edwards, 1879) belongs to Platyxanthidae, a family of large edible crabs endemic to South American coasts (Thoma et al. 2012). These are robust crabs, frequently found in crevices of intertidal and subtidal rocky bottoms of the southwestern Atlantic temperate coasts, from 23° to 44°S (Boschi 1964). Their claws have marked heterochely and laterality (i.e. dimorphism between chelae and the tendency for them to appear on a particular side of the body), traits associated with shell-breaking feeding habits (Laitano et al. 2013). Due to their abundance, large size and diet, P. crenulatus presumably has an important ecological role as a predator in benthic communities, similar to that of other stone crabs such as the Mennipidae in the northwestern Atlantic Ocean (Gerhart and Bert 2008).

Here we describe post-settlement changes of male and female P. crenulatus (habitat use and colour patterns; size and sexual differences in relative growth) and estimate the size at sexual maturity on the basis of several reproductive and morphologic indicators. Additionally, we have taken this opportunity to compare the performance of alternative morphometric analyses appearing in literature, testing the consistency of their results within the framework of other ontogenic changes described here and the scarce previous information on the species biology.

Samples were taken on a monthly basis in Mar del Plata (MDP, 38°02’S, 57°31’30”W), from May 2006 to April 2009, and sporadically during November 2007, April 2009 and January 2014 in San Antonio Oeste (SAO, 38°53’S, 62°07’W), Argentina (Fig. 1A). Mean water temperature ranges from 7.5°C in August (austral winter) to 20.6°C in January (austral summer) in MDP and from 4.4°C in August to 25.2°C in January in SAO (Servicio Argentino Hidrografia Naval, http://www.hidro.gov.ar/). With few exceptions, samplings were carried out during daylight. Crabs were collected from 0 (intertidal sampling) to 20 m depth (subtidal sampling) on rocky and sandy bottoms. Subtidal samplings were performed on hard bottoms by SCUBA diving, on soft bottoms with a trawl net towed by a small outboard boat, and on intertidal flats simply by hand, turning small boulders and inspecting caves and crevices. External qualitative traits such as colour pattern and handedness (whether the crusher claw is carried at the right or left side of the body), and those clearly related to reproduction (namely egg-bearing and vulva condition) were observed and registered for each crab. Additionally, the following body dimensions (Fig. 2) were measured with a vernier caliper with an accuracy of 0.1 mm: carapace length (CL) and width (CW); length (L), height (H), and width (W) of the propodus of the crusher (Cr) and cutter (Cu) chelae; gonopod length (GL) of males; and maximum width of the sixth abdominal segment of females (AW). Thus, a total of 11 and 13 measurements/observations of external features were recroded for each male and female, respectively. All statistical analyses (detailed below) were conducted using the R language for statistical computing (R Development Core Team 2011).

In considering our results it is necessary to bear in mind that the data come from two separate populations. This is particularly important for the analysis of relative growth given that exogenous factors (mainly the annual cumulative temperature and nutrition status) control the somatic growth (see Smith and Chang 2007 for a more in depth review) and therefore differing environmental conditions might result in differences in growth at all levels. However, tests for differences between these two populations performed previously to pool the data showed no significant differences. This fact is not surprising given that in this particular case the temperature regime is similar between sites (actually the annual temperature range in SAO encompasses that of MDP) and there are no reasons to think a priori that the nutritional conditions differ significantly between populations. Moreover, in decapods the temperature affects mostly the duration of the intermoult period rather than the size increment per moult (Smith and Chang 2007 and references therein) and therefore differences in temperature will impact the output of age-based models rather than size-based models such as those developed in this work.

There was, however, a considerable difference between MDP and SAO in the number of ovigerous females. The strikingly low number of ovigerous females found in MDP (eight ovigerous females in three years of monthly samples) has two possible explanations. Either the ovigerous females in MDP segregate from the other individuals (something that does not happened in SAO), perhaps because they perform reproductive migrations outside the sampling area or, more simply, because there is a bias in our sampling due to differences in catchability between sites. Subtidal sampling in MDP was performed mostly within the port, in the breakwaters that are built with medium to small, irregularly stacked boulders of orthoquartzite rock, thereby generating an intricate structure with many caves and hollows inaccessible to divers. On the other hand, hard bottoms in SAO are composed of large platforms of sedimentary rocks with countless cracks, hollows and crevices that provide good shelter but are shallow enough for the hidden animals to be within the reach of the human arm. Although we cannot rule out the segregation of ovigerous females outside the sampling area, considering the distinctive behaviour of the ovigerous female P. crenulatus, which tends to hide and remain immotile for long periods (NEF pers. obs.), we believe the most likely explanation is that the highly structured habitat in MDP prevented ovigerous females from being caught.

At a species level, Platyxanthus crenulatus has male-biased sexual size dimorphism, which is likely the rule among free-living brachyurans. The populations are spatially structured by size, suggesting the possibility of different ontogenic niches and a migration of smaller individuals to deeper areas during growth. The species is clearly associated with hard substrates, as shown by the relatively small number of individuals found on sandy bottoms, which were likely roaming among the rocky outcrops scattered in the area. Changes in colour pattern accompany the ontogenic habitat shifting (see Fig. 1B, C), perhaps as an adaptation to the different colour backgrounds, diet and/or predator features that individuals would find while growing and moving to deeper waters.

In relation to the relative growth, the use of a non-pre-defined model revealed morphological changes that would also be involved in size-dependent use of habitat. Although slight in males, the first recognizable change in relative growth rate of chelae (shared by both sexes, see Fig. 6), may be an adaptive response to changes in feeding habits, intraspecific agonisms and/or new predatory risks that might require a different claw shape and relative size. A similar relative growth pattern was described for the chelae of male stone crabs of the genus Menippe. In those crabs the first change in relative growth is also shared by both sexes, occurring at 35 mm, and is considered to be an adaptive response to habitat shifting at that size (Gerhart and Bert 2008). As observed in P. crenulatus during this study, other stone crabs (e.g. Reuschel and Schubart 2007, Manríquez et al. 2008, Krause-Nehring et al. 2010) showed an early habitat shift that was also accompanied by loss or acquisition of cryptic colouration to fit the new environment.

The spline method showed that the relative growth rate of male gonopods decreased gradually while body size increased. Since there is positive correlation between gonopod length and width (NEF, unpublished data), this growth pattern may be explained simply by the trade-off between the total body growth and the limitations imposed by the size of the mature vulvae of females, as stated by Hartnoll (1974). In P. crenulatus the vulvae underwent an abrupt change in size and shape at a body size that coincided with the size at maturity estimated from the abdomen width by the spline method (Fig. 8). Instead of the two linear segments to which the traditional method is restricted, the spline method applied to females’ abdomen showed a rather sigmoid growth pattern (Fig. 5). The sigmoid shape of the growth trajectory is consistent with the expected compromise between the need to expand the abdomen to accommodate as many embryos as possible in order to increase fertility against the restriction imposed by the female sternum width (Hartnoll 1974). Interestingly, in species in which females have determinate growth and terminal moult, a model with two discontinuous phases with different levels of allometry (as defined by the traditional method) seems to fit better the relative growth of female abdomen (e.g. Sainte-Marie and Brêthes 1995, Sampedro et al. 1999). In contrast, in brachyurans with indeterminate growth such as those studied here, an S-shaped model seems to describe the relative growth best (e.g. Luppi et al. 2004, Katsanevakis et al. 2007, this work).

Our results on the attainment of sexual maturity of P. crenulatus revealed the following sequence of maturation in females. First, relative growth of the abdomen reached its maximum (i.e. it will continue growing afterwards but at a lower rate). Later, while the abdomen grew through successive molts, the ovary matured to the point that the vulva changed to its mature form in a single moult, and the abdomen became broad enough to accommodate the egg clutch. When the female had reached sexual maturity, the vulva remained soft, independently of the moult cycle. In turn, physiological maturity precedes morphological maturity in male P. crenulatus and both are lastly followed by behavioural maturity. This pattern is shared by other brachyurans (Watters and Hobday 1998), although in some species such as Chaceon affinis A. Milne-Edwards and Bouvier, 1894 the onset of the different maturity stages is fairly synchronized (Fernández-Vergaz et al. 2000).

This study illustrates the advantages of studying relative growth in a comprehensive manner, using methods that do not rely on previous assumptions. Such an approach may provide further information useful for generating new hypotheses on life history and mating strategies, thus leading future research on the species beyond the mere size at onset of maturity. Whereas conspicuous changes in the morphology of female abdomens and vulvae, and male gonopods, are undoubtedly related to the maturation process, this is not necessarily the case for the chelae. The high predominance of right-handed individuals (those carrying a crusher claw at the right side) in both sexes suggests that natural selection may influence the size and shape of the chelae to fit the durophagous habits (Laitano et al. 2013) as much as sexual selection may enhance the ability to mate. Changes in relative growth are consistent with this hypothesis. Morphometric maturity of males, calculated by the spline method applied to the crusher claw, did not differ significantly from that using the gonopod length. Unexpectedly, spermatophores were present in very small males (physiological maturity 33-37 mm CW) long before their gonopods were morphometrically mature (55-60 mm CW) and also before females developed oocytes for the first time and acquired the open mature form of the vulva that allows gonopod penetration. If small males (i.e. ones that are physiologically mature but morphologically immature) have spermatophores, one might expect them to be able to mate with females larger than themselves, at least in absence of competition with other males, as reported in some other crabs (e.g. Wilber 1989, Sainte-Marie and Lovrich 1994, Gerhart and Bert 2008). This is an interesting idea for future testing under experimental conditions, given the implications regarding mating strategies and the socio-spatial structure of the populations.