The evolution of sexual dimorphism in Chondrichthyes: drivers, uncertainties, and future directions

Sexual dimorphisms are abundant in natural systems; however, their ecological and evolutionary significance have largely been neglected with respect to Chondrichthyes. A number of dimorphisms have been reported in this ancient clade, yet there remains considerable uncertainty regarding the disparity and variation in dimorphisms present in extant taxa, and the evolutionary processes that have resulted in their manifestation. In this review, I summarise our current understanding of sexual dimorphisms in chondrichthyans and consider the extent to which existing studies favour the two predominant theories regarding their evolution. Throughout, I consider the major limitations and open questions in the field, arguing ultimately that additional studies are required (both with regard to the phenomenon of sexual dimorphism itself, and several related fields including evolutionary genetics) if we wish to fully understand the evolutionary and ecological significance of sexual dimorphism in Chondrichthyes.

Sexual dimorphism occurs where male and female conspecifics (in dioecious taxa) differ with respect to one or more morphological traits (Hedrick and Temeles 1989). Such traits commonly include body size and shape (Frayer and Wolpoff 2003), as well as sex-dependent differences in internal anatomy beyond those related directly to the reproductive organs (McFall-Ngai and Dunlap 1984). The phenomenon of sexual dimorphism has received attention from ecologists and evolutionary biologists for decades (Mori et al. 2017), primarily attempting to identify the ecological implications and evolutionary drivers of specific morphological dimorphisms. Despite such questions being studied since the times of Darwin (Kottler 1980), there remains much uncertainty in the field, with both the drivers and consequences of sexual dimorphism thought to be subject to contextual and taxonomic nuance (Fryxell et al. 2019).
Whilst the concept of sexual dimorphism has existed in the literature for a relatively long time (Mori et al. 2017), it is only comparatively recently that quantitative studies of sex-based trait differences have been reported in chondrichthyans. This ancient and morphologically diverse vertebrate subclade has persisted for over 400 million years (Stein et al. 2018;White et al. 2022) and remains an important component of global marine and freshwater ecosystems (Heithaus et al. 2022). Performing evolutionary studies on extant chondrichthyan taxa has often proven difficult, both due to intrinsic life-history traits such as slow generation time (Compagno 1990;Martin et al. 1992), and logistical constraints associated with their ecology and husbandry in laboratory conditions (Smith et al. 2004). Nevertheless, studies are increasingly revealing cases of sexual dimorphism in sharks and rays, particularly those that concern external morphological features that can be readily studied in natural populations, such as teeth and skin (Kajiura and Tricas 1996). These dimorphisms have been recovered using a variety of methodologies, and in a number of taxa, however, questions remain regarding their universal applicability to Chondrichthyes as a clade. There are several cases in which dimorphisms found in one species are absent in another even where both share similar evolutionary histories and ecologies.
In this review, I synthesise current understanding of sexual dimorphism in chondrichthyans, including the range of traits and taxa in which it has been reported, and the techniques that have been used in respective studies. I also suggest to what extent two predominant theorised drivers of sexual dimorphism may apply to chondrichthyans on the basis of current evidence, as well as major uncertainties regarding the evolution and ecological consequences of dimorphism for these taxa. Finally, I discuss future directions for research in this area, and the evolutionary/ ecological insight that could be gained.

Sexual dimorphism in chondrichthyans: existing studies
A number of sexual dimorphisms have been reported in various chondrichthyan taxa, encompassing both the evolution of unique morphological structures in one sex, and significant differences between sexes in the morphometry or shape of shared morphological structures (Table 1). Whilst several 'categories' of sexual dimorphisms have been detected in multiple taxa, there exists much variation in the nature and timing of their expression, as well as the observed magnitude or intensity of sex-based differences. Gyandric heterodonty (sex-based differences in dentition) represents perhaps one of the most documented cases of sexual dimorphism in chondrichthyan taxa (Feduccia and Slaughter 1974;Berio et al. 2020). However in some species it appears only seasonally (Ellis and Shackley 1995;Kajiura and Tricas 1996) whereas in others, dimorphism develops over the course of ontogeny (Gutteridge and Bennett 2014;Straube and Pollerspöck 2020), and in others, it is absent entirely (Cullen and Marshall 2019). In fact, almost all reported cases of sexual dimorphism in chondrichthyans demonstrate clear ontogenetic trajectories (Table 1), at least qualitatively suggesting that their presence is in some way related to the onset of sexual maturity. Of course in the case of sexual size dimorphism (e.g. Colonello et al. 2020), which is known to occur in a large number of taxa (Blanckenhorn 2005), the link between dimorphism and ontogeny is intrinsic. The same need not be true in dimorphism of traits such as dentition on dermal armoury, the ontogenetic manifestation of which warrants further study. Importantly, many studies have failed in recovering evidence of sexual dimorphism in chondrichthyan taxa (Braccini and Chiaramonte 2002;Coelho and Erzini 2008;Weeton 2014), prompting speculation regarding the potential drivers of sexual dimorphism evolution.
The genetic basis of these sexual dimorphisms in Chondrichthyes is not yet well understood, although circumstantial evidence based on observation of hermaphroditic individuals suggests that at least in some cases, dimorphism and sex determination may be linked genetically (Scenna et al. 2007), a relationship that is thought to be common to many taxa (Williams and Carroll 2009). Teleost fish are known to exhibit both environmentally and genetically induced sex determination (Uno et al. 2020), and whilst empirical studies are taxonomically restricted, it is thought that chondrichthyans use a genetic sex determination system (Uno et al. 2020). A true understanding of the evolution and expression of sexual dimorphism requires knowledge of the gene regulatory networks underlying sex determination (Williams and Carroll 2009), and as such, this should form a major focus of future work.
Perhaps the single greatest limitation affecting our knowledge and understanding of sexual dimorphism in chondrichthyans is the taxonomic and morphological restriction of existing studies. Chondrichthyes consists of around 1300 species (Fricke et al. 2023), collectively exhibiting significant ecological and morphological disparity (Kuraku 2021;Heithaus et al. 2022). Despite this, sexual dimorphism has only been investigated in a fraction of these taxa, with several large subclades (e.g. Pristiophoriformes) entirely lacking in representation. This presents an issue for several reasons: perhaps most notably, it restricts the potential for the application of comparative phylogenetic methods, which might help unravel the evolutionary history of specific sexual dimorphisms. Such an approach, utilising analyses such as the phylogenetic independent contrasts (Garland Jr. et al. 1992) could identify 'hidden' biological correlates of dimorphism, enabling inference of the selective regimes underlying its evolution. The utility of these methods depends greatly on the taxonomic range of available data, however, which is at present extremely limited. Crucially, a number of taxon-specific morphological structures (such as the extended rostrum of Pristiophoriform taxa) have not been assessed for the presence of sexual dimorphisms, and given that such structures are taxon-specific, the presence or absence of these dimorphisms cannot be inferred by the usage of comparative phylogenetic methods, irrespective of sample size and taxonomic coverage. For this reason, additional studies focussing on morphologically and ecologically diverse taxa are warranted, with the eventual goal of conducting  Raikow and Swierczewski (1975) comparative phylogenetic analyses to unravel the evolutionary significance of individual dimorphisms in chondrichthyan taxa. Even amongst chondrichthyans for which studies of sexual dimorphism exist, the range of methodologies utilised presents challenges for robust interpretations of such dimorphisms, their general applicability, and the factors influencing their evolution. The simplest method by which sexual dimorphism can be established is through simple morphological observations. Such an approach was used by Tsai et al. (2015), presenting evidence of dimorphism in pectoral dermal armoury, posited to play a role in copulation. Whilst useful as a baseline for further studies, this approach is merely qualitative, and hence only valuable in cases where dimorphism is expressed as the presence or absence of some morphological character. The majority of studies use linear morphological measurements, typically of deceased specimens, using microscopes where relevant (Pratt 1979;Crooks et al. 2013;Gutteridge and Bennett 2014). This approach is quantifiable and replicable, such that evidence for continuous dimorphisms can be evaluated through the use of multivariate statistics. Moreover, the use of measurement rather than qualitative observation facilitates the extraction of ontogenetic trajectories, and direct quantitative comparisons between studies and species. Such an approach is nonetheless undesirable, as it relies typically upon a relatively small number of linear measurements, and as such may fail to capture the majority of morphological variation observed in a given structure (Bookstein et al. 1985). Indeed, the field of comparative anatomy underwent a revolution following the development of geometric morphometric techniques that take into account variations in overall shape rather than specific linear measurements (Sidlauskas et al. 2011). Whilst empirical comparisons of these methodologies are absent in most systems (Sidlauskas et al. 2011), it is generally accepted that geometric morphometrics provides a more robust system for analyses of morphological variation, both due to greater statistical power (Rohlf and Marcus 1993), and the retention of geometric information that is lost in traditional morphometric analyses (Slice 2007). Despite these obvious benefits, relatively few studies have utilised geometric morphometrics to study sexual dimorphism in chondrichthyans (see Kajiura et al. 2005 for an example). The price of scanning procedures required for 3D geometric morphometrics is falling rapidly (Lawing et al. 2010), and combined with the advent of 2D alternatives (Cardini 2014), I suggest that a shift towards geometric morphometrics is required if we wish to fully comprehend the scale and variety of sexual dimorphism in chondrichthyans.

Sex, conflict, and the evolution of dimorphism
Explanations for the evolution of sexual dimorphisms typically fall into two broad categories, yet both rely on the presence of differences in the nature of the selective regime (the sum total of all selective pressures acting upon an individual at a given time) to which males and females of a given species are subjected respectively (Connallon et al. 2010). Such differences could imply the presence of selective pressures that are wholly unique to a given sex (Mank et al. 2010), or merely differences in the magnitude or direction of pressures that act upon both sexes (Singh and Punzalan 2018). More subtly, patterns of spatial and temporal variations in the strength or direction of selective pressures may vary between sexes. This background of sex-dependent selection provides the framework for the evolution of sexual dimorphism. However, there are multiple potential routes by which such differences in selective regime can arise. The first and most prevalent route in the literature is through evolutionary phenomena associated with sexual reproduction, including sexual selection and sexual conflict. In the absence of clonal reproduction, genetic differences are present between all individuals in a population, and thus, there will be some intrinsic level of conflict between the genetic interests of these individuals (Arnqvist and Rowe 2005). In sexually reproducing species, conflict between the genetic interests of sexes (sexual conflict) is elevated due to differences in the theoretical lifetime reproductive success of each sex (Chapman et al. 2003). Males are thought to have an unlimited theoretical lifetime reproductive success, and thus maximise their evolutionary 'fitness' by engaging in as many mating events as possible, regardless of mate quality. Contrasting this, females have a finite number of oocytes and are thought to maximise evolutionary 'fitness' by mating selectively with only the highest-quality males (Chapman et al. 2003). Logic dictates that such an imbalance will result in sexually antagonistic coevolution, by which 'ardent' males drive the evolution of female traits maximising both pre-copulatory and post-copulatory control over paternity, which in turn drive the evolution of traits facilitating coercive and forceful matings in males (Rowe and Day 2006). Coevolution does not need to be explicitly antagonistic in nature, and sexual dimorphisms can also evolve through the expression of male quality signals (Shine 1979;Selander 2017), or traits associated with male-male competition that are absent in females (Lawler 2009). However, even in these cases, signals typically evolve to exploit pre-existing sensory bias or evolved preferences in females (Ryan and Rand 1993), and even where signals appear honest, there should be some stable frequency at which cheaters (utilising dishonest signalling) persist within the population.
There is some evidence for sexual conflict in Chondrichthyes in the literature. Whilst reproductive behaviour is rarely observed (Whitney et al. 2004), observations of dermal lacerations that appear to have been inflicted during copulation have been reported in multiple taxa (Kajiura et al. 2000;Ritter and Amin 2019;Rangel et al. 2022;Whitehead et al. 2022). Given the severity of these wounds (Whitehead et al. 2022), some have suggested that they represent a behavioural manifestation of sexual genomic conflict, with males attempting to increase their lifetime reproductive success by forcing females to mate with them. Mating wounds also provide logical explanations for the presence of sexual dimorphisms in dentition and dermal composition. Particularly convincing are case studies in which females possess skin that is significantly thicker than that of males (Pratt 1979) and where heterodonty is restricted to the mating season of the taxon in question (Kajiura and Tricas 1996). Indeed, it has been suggested that thicker skin in female chondrichthyans represents an adaptation to withstand coercive mating attempts (Ritter and Amin 2019), and that heterodonty enables males to better 'grip' or 'restrain' females during copulation (Kajiura et al. 2000;Berio et al. 2020). The same could also be true of the pectoral spines observed in male Squatina guggenheim individuals (Tsai et al. 2015) or the prepelvic spines of chimaeriform males (Kemper et al. 2010), although this has yet to be observed. The alar thorns of rajiform taxa are also known to be used to restrain and position the female for mating (Luer and Gilbert 1985). Of course, it is important to note that negative fitness consequences for females resulting from this putative coercive mating behaviour have yet to be shown empirically. The literature is currently devoid of functional studies evaluating how shifts in tooth morphology would better facilitate copulation events. It also remains to be seen whether seasonal variation such as that observed in the dentition of several species is also observed in corresponding female traits such as skin thickness. Another phenomenon that supports the concept of sexual conflict in Chondrichthyes is that of genetic polyandry, where multiple males are responsible for paternity of a given litter (Taylor et al. 2014). A number of studies have found evidence for multiple paternity in this clade Portnoy et al. 2007;Barker et al. 2019;Pirog et al. 2019), and unlike other taxa in which polyandry is thought to be driven by females attempting to gain indirect genetic fitness benefits (Hosken and Stockley 2003;Slatyer et al. 2012), it is thought that polyandry in chondrichthyans is symptomatic of coercive mating and thus sexual conflict (Portnoy et al. 2007;DiBattista et al. 2008). In extreme cases, polyandry has resulted in the evolution of intrauterine cannibalism (Gilmore et al. 2011;Chapman et al. 2013). The proposed relationship between polyandry and sexual conflict (Portnoy et al. 2007) raises the intriguing prospect of the sexual dimorphism abundance and intensity varying with levels of genetic conflict, although this has yet to be tested quantitatively. Moreover, there are several reports of genetic monogamy in chondrichthyans, even in taxa for which sexual dimorphisms have been detected (Chapman et al. 2004;Kajiura et al. 2005), and thus the true relationship between polyandry, sexual conflict, and dimorphism remains poorly constrained. Evidence for dimorphisms in sexual signals is far less abundant; however, one putative example is presented by Claes and Mallefet (2010), who detect an unambiguous sexual dimorphism in the luminescence patterns of Etmopterus spinax. The photophores responsible for bioluminescence in this taxon are located in the same region of the body as the reproductive organs (Claes and Mallefet 2008) and the ontogenetic trajectory of bioluminescence is consistent with a role in sexual signalling (Claes and Mallefet 2009). It appears that sexual selection associated with signalling could be responsible for some dimorphisms in chondrichthyans; however, the paucity of current studies has broadly prevented further consideration of this concept.
There is a third path through which sexual reproduction could theoretically result in the evolution of sexual dimorphism, unrelated to any direct interaction between the sexes or their genomes. Sexual reproduction imparts a number of functional constraints on morphology and anatomy in taxa which utilise internal fertilisation (Wake 2003). In all chondrichthyan taxa, it is the female that provisions embryos Awruch 2015). Consequently, female chondrichthyans are likely subject to different selective pressures than their male conspecifics as a product of the physiological and anatomical consequences of provisioning offspring. In a number of taxa, female fecundity has been shown to depend on body size (Briegel 1990;Honěk 1993). Whilst such a relationship has not yet been established in chondrichthyans, it is clear that this provides one potential explanation for sexual size dimorphisms observed in many constituent members of this clade. This hypothesis is loosely supported by a general trend relating to reproductive mode (Colonello et al. 2020) in which females mature at larger body sizes in viviparous but not oviparous taxa (Cortés 2000; Ebert et al. 2006). It is important to note, however, that the importance of female fecundity in driving sexual size dimorphism has been challenged, with male-male competition posited as a potential alternative (Shine 1988). Such an explanation is only posited where males attain greater body size at maturity however (Parker 1992), and is thus insufficient to explain sexual dimorphisms in viviparous chondrichthyans.

Ecology, multivariate selection, and the evolution of dimorphism
The presumed ubiquity of sexual selection, male-male competition, and genetic conflict between male and female genomes (Arnqvist and Rowe 2005), combined with the clear ontogenetic trajectories of many dimorphisms (Gutteridge and Bennett 2014; Straube and Pollerspöck 2020) might lead us to assume that all dimorphisms in Chondrichthyes are intrinsically associated with sexual reproduction. However, it has also long been argued that ecology provides a viable alternative mechanism by which dimorphisms can evolve (Slatkin 1984;Shine 1989). The influence of ecology on selection is multifaceted, encompassing foraging, locomotion, predator evasion, and other ecological interactions. It is also well known that ecological differences between individuals can be of great evolutionary importance, contributing to the maintenance of genetic polymorphisms (Hedrick 2007), the accumulation of reproductive isolation (Funk et al. 2006), and in some cases, speciation (Rundle and Nosil 2005). Slatkin (1984) describes three distinct mechanisms by which ecology could directly result in the evolution of sexual dimorphism, even in complete absence of sexual selection or conflict. The first of these is the dimorphic niche model, where intrinsic differences between the sexes result in different optima for certain traits (Slatkin 1984). The previously described functional constraints hypothesis falls under the umbrella of dimorphic niches; however, this model is not restricted to sex-based differences in morphology or physiology but incorporates factors such as differences in social roles. This model has received support in the literature, with several putative examples of dimorphism driven by intrinsic differences between the sexes, predominantly related to reproductive roles (Hedrick and Temeles 1989;Bulté et al. 2008;Cassini 2020). Alternatively, under the bimodal niche model, the selective regimes of each sex are similar (both consisting of two optima for the dimorphic trait), with each sex simply evolving to a separate optimum trait value (Slatkin 1984). Whilst theoretically viable, this model is lacking in substantial empirical evidence and warrants further study. The final and most convincing ecological driver of sexual dimorphism is competitive displacement or resource partitioning (Slatkin 1984;Shine 1989). This model suggests that where males and females compete for access to some limited resource (such as a particular prey species), selection will favour differential trait evolution in each sex such that competition is minimised and niche overlap is reduced (Slatkin 1984). Evidence for this model is more robust than for either the bimodal or dimorphic niche models (Pearson et al. 2002;Shine et al. 2002), although all three are relatively difficult to test, and in the majority of cases, sexual explanations for dimorphism cannot be ruled out (Shine 1989). Nonetheless, it is clear that in some taxa, and under certain biological conditions, ecological factors could drive the evolution of sexual dimorphism.
Whilst rigorous quantitative studies evaluating each of these models in sexually dimorphic chondrichthyan taxa are yet to be conducted, there is a fairly large quantity of evidence to suggest that such hypotheses are at least biologically feasible. Sexual segregation is well-studied in a number of chondrichthyan taxa (Mucientes et al. 2009;Simpson et al. 2021), and ontogenetic shifts in both trophic niche (Matich et al. 2019) and habitat usage (Grubbs 2010) appear to be abundant. The link between ontogenetic shifts in ecology and morphology has been quantified in several taxa (Gayford et al. 2023), although the resulting allometric niche shift hypothesis has yet to be considered from the perspective of sexual dimorphism. Sexual segregation or trophic niche differences provide an intriguing alternative explanation to sexual conflict for the evolution of gyandric heterodonty, particularly in cases where such dimorphism is not known to be restricted to the mating season of the taxon in question (e.g. Berio et al. 2020). Other cases of sexual dimorphism, however, such as pectoral spines (Colonello et al. 2020), are more difficult to explain in terms of ecological differences between sexes. One study has explicitly considered the contribution of ecological factors to the evolution of sexual dimorphism in chondrichthyans, finding qualitative evidence for the resource partitioning model (Feduccia and Slaughter 1974). There are, however, a lack of empirical studies addressing this question, and none that explicitly consider the dimorphic or bimodal niche models. Even where robust evidence of sexual segregation or trophic niche differences exists, such studies typically consider a single population or geographical location, and given what is known about the connectivity of chondrichthyan populations (Hirschfeld et al. 2021) cannot be taken as taxon-wide evidence of such sexual differences in ecology.
As some degree of sexual conflict is likely to be present in all sexually reproducing taxa, reconciling the interactions between ecology, sexual selection, and conflict is key to determining the validity of ecological hypotheses of sexual dimorphism. This is particularly true of dimorphisms such as heterodonty, in which the morphological character in question plays a clear role in both reproduction (Pratt 1979) and prey acquisition (Whitenack 2008). Ecology, through natural selection, is often thought to counteract the evolution of traits linked to sexual selection and conflict (Tobias and Seddon 2009;Okada et al. 2021), as such traits may increase predation risk, reduce foraging success, or impede efficient locomotion. Where this is the case, the trait optima and adaptive landscapes favoured by each form of selection differ substantially, with the resultant phenotype potentially exhibiting reduced or even no dimorphism. Importantly, this outcome is not inevitable and assumes that the strength of natural selection outweighs that of sexual selection or conflict. Where this is not the case, sexual dimorphism may prevail even in the presence of ecological constraint. It is also plausible that the adaptive landscapes and trait optima favoured by both natural and sexual selection/conflict are similar (Tobias and Seddon 2009), in which case the resultant strength of selection would be increased, and observed sexual dimorphisms may be more pronounced than would be expected in the presence of only one form of selection. Considering such multivariate selection is crucial when investigating the evolution of specific cases of sexual dimorphism, yet studies concerning chondrichthyans frequently ignore potential interactions between natural selection, sexual selection, and sexual conflict. All future studies considering the adaptive basis of sexual dimorphism in chondrichthyans should take into account the potential for multivariate selection, particularly when attempting to provide selection-based explanations for the presence and nature of such dimorphism.

Understanding evolution: ecology, morphology, and genetics
As with the abundance and variety of sexual dimorphisms seen in chondrichthyan taxa, our understanding of the evolution of such dimorphisms is limited by the number of taxa and morphological structures upon which existing studies have focussed. Besides these limitations, two more fundamental issues complicate our ability to understand the nature of evolutionary processes operating in extant chondrichthyan populations, and how they might influence sexual dimorphism. The first derives from the slow generation time of chondrichthyans (Smith et al. 2004), which makes direct studies of selection through experimental evolution essentially impossible. For this reason, when studying the selective drivers of phenotypic traits (including sexual dimorphism) we are restricted to inference and post hoc probabilistic approaches. Whilst experimental evolution studies may fail to capture the full extent of ecological variation present in natural systems (Bailey and Bataillon 2016), they provide the only direct method by which the genetic and phenotypic consequences of specific selection pressures can be established (Garland Jr. and Rose 2009). The second fundamental issue is uncertainty regarding the genetic architectures underlying sexually dimorphic traits. Genetic architecture refers to the number, genomic location, frequencies, and effects of quantitative trait loci, as well as interactions between them for any given quantitative trait (Zeng et al. 1999). The selection-based models discussed previously (both sexual and ecological) are based upon traits with a simple genetic basis, consisting of a relatively small number of loci under nearidentical selective regimes. Whilst some morphological traits do indeed have such a basis (Boyko et al. 2010), the majority are likely to have more complex architectures (Mackay 2004), potentially resulting in phenomena such as genetic hitchhiking and background selection (Stephan 2010). Some theories such as the phenotypic gambit argue that the genetic architecture of traits can be ignored when considering their long-term evolution (Hadfield et al. 2007), and Slatkin (1984) demonstrated mathematically that sexual dimorphisms can still evolve in the presence of genetic correlations. However, where both multivariate selection and complex architectures are present, inferring the action of a single selective pressure upon a quantitative trait when the underlying complexities of the architecture are unknown is likely to be extremely challenging, as the response to selection at any given locus may depend upon that of other loci, which in turn may be influenced by other, unrelated selective pressures (Barton 2000).
Whilst these issues do indeed present significant barriers to our understanding of chondrichthyan sexual dimorphism and evolution more broadly, these barriers are by no means insurmountable. If research effort increases accordingly, our understanding of sexual dimorphism evolution and the extent to which it is influenced by genetic architecture should become less uncertain. In lieu of direct selection experiments, comparative phylogenetic methods allow inferences to be made regarding the selective pressures contributing to the evolution of specific traits (Vincent et al. 2006;Slater and Harmon 2013). Additional studies considering sexual dimorphism in a phylogenetically and ecologically disparate range of chondrichthyan taxa are required such that we might evaluate the extent to which sexual or ecological factors are responsible for the evolution of specific dimorphisms. Such an approach could also be utilised to quantify the nature of the relationship between genetic polyandry and sexual dimorphism. Post hoc quantitative genetic methods are also capable of detecting different forms of selection acting at specific loci in natural populations (Kreitman 2003;Momigliano et al. 2017). Thus, when combined with evolutionarydevelopmental studies unravelling the likely genetic basis of morphological structures in chondrichthyans (e.g. Cole and Currie 2007), such studies will likely provide one route by which the significance of genetic architectures to the evolution of sexual dimorphisms can be recovered.

Conclusions and future directions
The phenomenon of sexual dimorphism has been known of in the literature for decades (Slatkin 1984); however, it is only relatively recently that the taxonomic and morphological range of dimorphisms known from studies of chondrichthyan taxa has begun to increase. This rise is likely due (at least in part) to the geometric morphometrics 'revolution' (Rohlf and Marcus 1993) enabling investigation of sex-based differences in overall shape of a morphological structure, rather than specific linear measurements alone. Sexual dimorphism arises where there exist differences in the selective regimes to which the sexes are subjected (Connallon et al. 2010), and there appear to be links between sexual dimorphism and phenomena such as sexual conflict and genetic polyandry. Increasingly, it is apparent that ecology may play a role, although the majority of such hypotheses remain difficult to prove empirically (Shine 1989).
Despite these advances, several important uncertainties remain regarding our understanding of sexual dimorphism, particularly in chondrichthyan taxa: to what extent to sexual and ecological factors play a role in the evolution of dimorphism, and how do these factors interact through multivariate selection? Are dimorphisms genetically entrained, and if so, what genetic architectures underlie them? To what extent is the evolution of sexual dimorphism linked to that of other, apparently unrelated traits? Each of these questions has been considered -to varying extents -in other taxa, yet have been largely ignored in the context of chondrichthyans. Further studies are warranted not only because chondrichthyans provide a fascinating case study through which to study evolutionary phenomena but also because they represent an important component of marine and freshwater ecosystems (Heithaus et al. 2022) that is intrinsically vulnerable to rapid environmental change (Frisk et al. 2001). For these reasons, I argue that studies addressing the above questions are urgently required. Ultimately, the vulnerability of declining chondrichthyan populations cannot be fully evaluated without an understanding of the interplay between evolutionary genetics and functional ecomorphology. Incorporating future studies that explicitly consider this complex relationship through the lens of sexual dimorphism is essential if we are to fully understand the ecological and evolutionary significance of sexual dimorphism in these charismatic organisms.
Author contribution JHG conceptualised the study and prepared the entirety of the manuscript.

Declarations
Ethics and animal welfare No experimental work was carried out at any point during this study, either involving animals or otherwise.

Competing interests The author declares no competing interests.
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