**3. Representative examples of ostracod sexual dimorphism in extinct and living species**

Like other crustaceans such as crabs, ostracods grow by moulting (ecdysis; Figure 4). For example, in Podocopida, there are usually eight moult stages between egg and adult, and the last moulting is the first sexually mature stage [20]. The adult stage is termed 'A', whereas juvenile stages are numbered; *e*.*g*., 'A-1' (*i*.*e*., one stage before the adult) and 'A-2' (two stages before the adult), as shown in Figure 4 [20]. Sexual dimorphism (morphological differences between males and females) is commonly found on carapaces and appendages [4, 9, 30], as shown in Figures 5 and 6, and is especially recognisable during the last adult stage (A), and to a lesser degree in the later juvenile stages, such as A-1.

This article introduces representative examples of sexual dimorphism in the last adult stage from the Palaeozoic to Recent, primarily in representative Podocopida from within and around Japan.

#### **3.1. Carapace shape and size**

#### *3.1.1. Palaeozoic example in an extinct group*

A well-known example of ostracod sexual dimorphism of the carapace exists in species of the family Beyrichiidae of the order Palaeocopida from the Ordovician to the Permian (Figure 7) [4] [31]. In many species of this family, distinct sexual dimorphism is seen in the adult stage; *i*.*e*., females possess a large bulbous swelling in the antero-ventral part of each valve. These swellings that open internally into the domicilium found in species of this family are known as brood pouches (or cruminae) [25].

Species of this family were already extinct at the end of Palaeozoic, so various functions for this kind of pouch, including brood care, have been speculated [15]. Valves and carapaces of juvenile stages within this pouch were found in two species of the genus *Beyrichia* (*B. kloedini* and *B. jonesii*) in the family Beyrichiidae [25]. Based on the example of those two species, it became clear that these were probably brood pouches. An alternative (or perhaps double) function as buoyancy aids has been proposed by some researchers; thus firm evidence that all pouch types were used only for brood care has not yet been found [25]. It has been

**Figure 7.** Carapace outlines of female and male of *Beyrichia salteriana* and *Bolbibollia labrosa* in Family Beyrichiidae (Palaeocopida) from Silurian deposits of Sweden and Canada respectively, modifed from Benson *et al.* (1961). Arrows indicate anterior.

inferred that species of this family inhabited Palaeozoic shallow-marine shelf-water environments [7]. Thus, a reasonable interpretation of pouch function in this family is needed to clarify the detailed evolutionary processes of the ecology and reproductive modes of Palaeocopida in Palaeozoic shallow-marine water areas.

#### *3.1.2. Mesozoic example in an extinct species*

58 Sexual Dimorphism

**living species** 

A-1.

around Japan.

**3.1. Carapace shape and size** 

*3.1.1. Palaeozoic example in an extinct group* 

family are known as brood pouches (or cruminae) [25].

oil and gas exploration [4] [27–29].

In particular, the surface ornamentation (reticulation, fossae, muri, eye tubercle, ridges and so on), hingement type, muscle scar morphology (Figures 1 and 2), and pore shape and numbers are useful for ostracod taxonomy [24]. With living specimens, the morphology of male copulatory organs and other appendages are also used for species identification, similar to identification techniques used for other crustaceans, such as decapods [24–26]. Fossil ostracods are commonly utilised by palaeontologists as important palaeoenvironmental and stratigraphic (geological age) indices, and have long been used in

**3. Representative examples of ostracod sexual dimorphism in extinct and** 

Like other crustaceans such as crabs, ostracods grow by moulting (ecdysis; Figure 4). For example, in Podocopida, there are usually eight moult stages between egg and adult, and the last moulting is the first sexually mature stage [20]. The adult stage is termed 'A', whereas juvenile stages are numbered; *e*.*g*., 'A-1' (*i*.*e*., one stage before the adult) and 'A-2' (two stages before the adult), as shown in Figure 4 [20]. Sexual dimorphism (morphological differences between males and females) is commonly found on carapaces and appendages [4, 9, 30], as shown in Figures 5 and 6, and is especially recognisable during the last adult stage (A), and to a lesser degree in the later juvenile stages, such as

This article introduces representative examples of sexual dimorphism in the last adult stage from the Palaeozoic to Recent, primarily in representative Podocopida from within and

A well-known example of ostracod sexual dimorphism of the carapace exists in species of the family Beyrichiidae of the order Palaeocopida from the Ordovician to the Permian (Figure 7) [4] [31]. In many species of this family, distinct sexual dimorphism is seen in the adult stage; *i*.*e*., females possess a large bulbous swelling in the antero-ventral part of each valve. These swellings that open internally into the domicilium found in species of this

Species of this family were already extinct at the end of Palaeozoic, so various functions for this kind of pouch, including brood care, have been speculated [15]. Valves and carapaces of juvenile stages within this pouch were found in two species of the genus *Beyrichia* (*B. kloedini* and *B. jonesii*) in the family Beyrichiidae [25]. Based on the example of those two species, it became clear that these were probably brood pouches. An alternative (or perhaps double) function as buoyancy aids has been proposed by some researchers; thus firm evidence that all pouch types were used only for brood care has not yet been found [25]. It has been Carapace shape and size sexual dimorphism is common in ostracods, and males can be larger or smaller than females [4]. One example from the Mesozoic is a non-marine species of the podocopid genus *Cypridea* [15] [32]. This genus occurred only during the Jurassic and Cretaceous, and is not found in Cenozoic deposits. The shape and size of *Cypridea subvaldensis* (Figure 8) from Cretaceous sediments in northeastern China was studied, and the differences between the two forms were recognised to represent sexual dimorphism within a single species [32].

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 61

many Myodocopida species differ greatly from those of juveniles and females, and females

**Figure 9.** (1) SEM images of carapaces in male (upper) and female (lower) of *Bicornucythere bisanensis* (Podocopida) from Quaternary deposits of central Japan, modified from Ozawa (2009). Arrows indicate anterior. (2) Diagram for valve size (length and height) of this species in adult female, adult male, A-1

**Figure 10.** Carapace outlines of female and male in lateral and dorsal views of *Metacypris digitiformis*  and *Xestoleberis sagamiensis* (Podocopida) from modern water areas of central Japan, redrawn from Smith and Hiruta (2004) and Sato and Kamiya (2007) respectively. Arrows indicate anterior.

The sexual behaviour of ostracods is diverse, and seven different types of brood care have been recognised in various lineages from Palaeozoic to Recent [25] [41]. These various types have arisen independently in several marine and non-marine lineages of ostracods, so diverse carapace shapes acting as brood pouches are found. The ability of the ostracod female to brood eggs or juveniles within the carapace might protect the young from severe

and A-2 juveniles, modified from Ozawa (2009).

are sometimes smaller than males even if there is brood care within the group [10].

**Figure 8.** (1) Carapace outlines of female and male of *Cypridea subvaldensis* (Podocopida) in lateral, ventral and anterior views from Cretaceous deposits in northeastern China, modifed from Hanai (1951). Arrows indicate anterior. (2) Frequency of carapace length/ width ratio (%) for specimens of *Cypridea subvaldensis* in Cretaceous fossil assmblages from northeastern China, modifed from Hanai (1951).

Carapace sizes (length and width) of approximately 100 specimens of this species in one fossil assemblage from sedimentary rocks at one site were measured [32]. Based on the frequency of the carapace length/ width ratio, Hanai suggested that the two different-sized forms represent male and female forms within a single species. He proposed that the male carapace is smaller and narrower than that of the female, with a more arch-shaped outline along dorsal margin. This was based on the carapace shapes of the sexes of the single living species of a related genus, *Chlamydotheca*, in which the male form is slightly smaller and has a more arch-shaped dorsal margin than the female form.

#### *3.1.3. Cenozoic and recent examples*

A number of Japanese Cenozoic and living Podocopida genera, such as *Finmarchinella*, *Loxoconcha, Semicytherura* and *Vestalenula* (*e*.*g*., Figures 5 and 6), can be recognised by their distinct carapace shape and size sexual dimorphism [2, 9, 16, 17, 33]*.* In many cases, the male forms of these podocopids have relatively longer and narrower carapaces than the females (Figures 5 and 6) [34–36]. For example, in one modern species, *Bicornucythere bisanensis*, which is found in shallow brackish water areas in Japan, the male carapace is slightly more slender than that of the female (Figure 9) [8, 37, 38].

Primarily in podocopid species, female carapaces show greater posterior inflation than those of males, as in species of the genera *Metacypris* and *Xestoleberis* from Japan (Figure 10) [26] [39]. This kind of sexual dimorphism is more distinct in the adult than the juvenile stages [9]. In some cases, the male carapaces may be inflated posteriorly to accommodate relatively large copulatory organs, or it may have a more pronounced posterior keel, whereas in podocopid and myodocopid taxa with brood care, female carapaces are larger and more inflated than in the male [25] [40]. Eggs and the first two- or three-stage juveniles are retained within the posterior part of the domicilium. The shape of adult male valves of many Myodocopida species differ greatly from those of juveniles and females, and females are sometimes smaller than males even if there is brood care within the group [10].

60 Sexual Dimorphism

**Figure 8.** (1) Carapace outlines of female and male of *Cypridea subvaldensis* (Podocopida) in lateral, ventral and anterior views from Cretaceous deposits in northeastern China, modifed from Hanai (1951). Arrows indicate anterior. (2) Frequency of carapace length/ width ratio (%) for specimens of *Cypridea subvaldensis* in Cretaceous fossil assmblages from northeastern China, modifed from Hanai (1951).

Carapace sizes (length and width) of approximately 100 specimens of this species in one fossil assemblage from sedimentary rocks at one site were measured [32]. Based on the frequency of the carapace length/ width ratio, Hanai suggested that the two different-sized forms represent male and female forms within a single species. He proposed that the male carapace is smaller and narrower than that of the female, with a more arch-shaped outline along dorsal margin. This was based on the carapace shapes of the sexes of the single living species of a related genus, *Chlamydotheca*, in which the male form is slightly smaller and has

A number of Japanese Cenozoic and living Podocopida genera, such as *Finmarchinella*, *Loxoconcha, Semicytherura* and *Vestalenula* (*e*.*g*., Figures 5 and 6), can be recognised by their distinct carapace shape and size sexual dimorphism [2, 9, 16, 17, 33]*.* In many cases, the male forms of these podocopids have relatively longer and narrower carapaces than the females (Figures 5 and 6) [34–36]. For example, in one modern species, *Bicornucythere bisanensis*, which is found in shallow brackish water areas in Japan, the male carapace is slightly more

Primarily in podocopid species, female carapaces show greater posterior inflation than those of males, as in species of the genera *Metacypris* and *Xestoleberis* from Japan (Figure 10) [26] [39]. This kind of sexual dimorphism is more distinct in the adult than the juvenile stages [9]. In some cases, the male carapaces may be inflated posteriorly to accommodate relatively large copulatory organs, or it may have a more pronounced posterior keel, whereas in podocopid and myodocopid taxa with brood care, female carapaces are larger and more inflated than in the male [25] [40]. Eggs and the first two- or three-stage juveniles are retained within the posterior part of the domicilium. The shape of adult male valves of

a more arch-shaped dorsal margin than the female form.

slender than that of the female (Figure 9) [8, 37, 38].

*3.1.3. Cenozoic and recent examples* 

**Figure 9.** (1) SEM images of carapaces in male (upper) and female (lower) of *Bicornucythere bisanensis* (Podocopida) from Quaternary deposits of central Japan, modified from Ozawa (2009). Arrows indicate anterior. (2) Diagram for valve size (length and height) of this species in adult female, adult male, A-1 and A-2 juveniles, modified from Ozawa (2009).

**Figure 10.** Carapace outlines of female and male in lateral and dorsal views of *Metacypris digitiformis*  and *Xestoleberis sagamiensis* (Podocopida) from modern water areas of central Japan, redrawn from Smith and Hiruta (2004) and Sato and Kamiya (2007) respectively. Arrows indicate anterior.

The sexual behaviour of ostracods is diverse, and seven different types of brood care have been recognised in various lineages from Palaeozoic to Recent [25] [41]. These various types have arisen independently in several marine and non-marine lineages of ostracods, so diverse carapace shapes acting as brood pouches are found. The ability of the ostracod female to brood eggs or juveniles within the carapace might protect the young from severe environmental fluctuations and predation [25]. Brood care may be advantageous for the dispersal of some groups of non-marine water ostracods, such as species in the subfamily Timiriaseviinae (*e*.*g*., genus *Metacypris*), the eggs of which may not be desiccation-resistant. The variety of brood care modes and carapace brood pouches that evolved in unrelated ostracod lineages is one of the most remarkable reproductive characteristics of ostracods, because other crustaceans exhibit a limited number of brood-care solutions [25] [42].

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 63

*Bicornucythere bisanensis* (Podocopida) predominantly inhabits shallow brackish water areas in inner bay areas near river mouths in and around Japan [47]. Sexual dimorphism of the appendages of this species has been known since the 1910s [45, 48, 49]. The right fifth limb (= first thoracic leg) of the male is uniquely thicker and slightly shorter than the left fifth limb [30] [50]. The central podomere of the right fifth limb is very inflated, and is 1.5-times the thickness of that of the female. Thus, the fifth limb of the male in this species shows asymmetry, and this unique morphology has not been observed in the female. This is a

The ecological function of this dimorphism has been clarified by [30] [50]. They observed its mating behaviour under experimental conditions from detailed video recordings. According to their observations (Figure 12), this short and thick podomere in the right fifth limb of the male is an adaptation for courting behaviour, where the male rotates the female's carapace three or four times using the right fifth limb just before mating (Figure 12). Thus, this right limb (especially the central podomere) is thicker and shorter than both the male left limb and the female's fifth limb to facilitate holding and rotating the female carapace. The thick podomere of the male right fifth limb is markedly more muscular than either that of the left

**Figure 12.** Schematic profile of courting behaviour (= rotation of female carapace by male's fifth limb) just before mating behaviour, indicating the location of its fifth limb by black arrows, for modern *Bicornucythere bisanensis* (Podocopida) from modern inner bay area of central Japan, modified from Abe

Several examples of sexual dimorphism of appendages other than the fifth limb in living marine species have been reported for *Vargula hilgendorfi* (Myodocopida) [51]. This species inhabits shallow marine water environments*,* and the relative size of the furca at the posterior part of the soft body compared to the length of the entire carapace differs between the sexes. The male's furca is relatively larger than that of the female. According to detailed

representative example of sexual dimorphism in podocopid appendages.

side and those of the female, facilitating this behaviour [15, 30, 37].

**3.3. Appendages** 

and Vannier (1991).

*3.3.2. The example of Vargula hilgendorfi* 

*3.3.1. The example of Bicornucythere bisanensis* 

#### **3.2. Carapace surface ornamentation**

The podocopid species *Callistocythere pumila* favours very shallow brackish water environments around 1 m depth in inner bay and open lagoon areas near river mouths in Japan [43] [44]. It can be recognised by its conspicuous sexual dimorphism of carapace surface ornamentation (Figure 11). The female form has relatively distinct ornamentations on the entire carapace, such as numerous deep fossae [43] [44]. However, the male form has an extensive weakly ornamented area, especially in the median part of the carapace on a relatively slender valve (Figure 11).

**Figure 11.** (1) Schematic patterns of fossae on carapace surface in female and male of *Callistocythere pumila* (Podocopida) in lateral view from modern brackish water area of central Japan, redrawn from Tsukagoshi (1998). (2) SEM images of carapaces in male and female of this species in lateral view, from modern brackish water area of central Japan, modified from Kamiya *et al.* (2001). Arrows indicate anterior.

The carapace morphology of this species occupies an intermediate taxonomic position between two genera, *Callistocythere* and *Leptocythere*; the female and male forms resemble the former and latter genera, respectively [43]. Due to these morphological differences between males and females, they were originally described as a different species [45] [46]. However, those two forms have recently been considered to represent males and females of a single species, *Callistocythere pumila*, based on the detailed description of soft parts [43]. As yet no reasonable interpretation of the function of sexually dimorphic ornamentation has been postulated. The significance of this dimorphism for the ecology or life-history of *C. pumila* requires investigation to clarify the detailed evolutionary processes of the ecology of Cenozoic brackish water podocopids.

#### **3.3. Appendages**

62 Sexual Dimorphism

**3.2. Carapace surface ornamentation** 

relatively slender valve (Figure 11).

Cenozoic brackish water podocopids.

environmental fluctuations and predation [25]. Brood care may be advantageous for the dispersal of some groups of non-marine water ostracods, such as species in the subfamily Timiriaseviinae (*e*.*g*., genus *Metacypris*), the eggs of which may not be desiccation-resistant. The variety of brood care modes and carapace brood pouches that evolved in unrelated ostracod lineages is one of the most remarkable reproductive characteristics of ostracods,

The podocopid species *Callistocythere pumila* favours very shallow brackish water environments around 1 m depth in inner bay and open lagoon areas near river mouths in Japan [43] [44]. It can be recognised by its conspicuous sexual dimorphism of carapace surface ornamentation (Figure 11). The female form has relatively distinct ornamentations on the entire carapace, such as numerous deep fossae [43] [44]. However, the male form has an extensive weakly ornamented area, especially in the median part of the carapace on a

**Figure 11.** (1) Schematic patterns of fossae on carapace surface in female and male of *Callistocythere pumila* (Podocopida) in lateral view from modern brackish water area of central Japan, redrawn from Tsukagoshi (1998). (2) SEM images of carapaces in male and female of this species in lateral view, from modern brackish water area of central Japan, modified from Kamiya *et al.* (2001). Arrows indicate anterior.

The carapace morphology of this species occupies an intermediate taxonomic position between two genera, *Callistocythere* and *Leptocythere*; the female and male forms resemble the former and latter genera, respectively [43]. Due to these morphological differences between males and females, they were originally described as a different species [45] [46]. However, those two forms have recently been considered to represent males and females of a single species, *Callistocythere pumila*, based on the detailed description of soft parts [43]. As yet no reasonable interpretation of the function of sexually dimorphic ornamentation has been postulated. The significance of this dimorphism for the ecology or life-history of *C. pumila* requires investigation to clarify the detailed evolutionary processes of the ecology of

because other crustaceans exhibit a limited number of brood-care solutions [25] [42].

#### *3.3.1. The example of Bicornucythere bisanensis*

*Bicornucythere bisanensis* (Podocopida) predominantly inhabits shallow brackish water areas in inner bay areas near river mouths in and around Japan [47]. Sexual dimorphism of the appendages of this species has been known since the 1910s [45, 48, 49]. The right fifth limb (= first thoracic leg) of the male is uniquely thicker and slightly shorter than the left fifth limb [30] [50]. The central podomere of the right fifth limb is very inflated, and is 1.5-times the thickness of that of the female. Thus, the fifth limb of the male in this species shows asymmetry, and this unique morphology has not been observed in the female. This is a representative example of sexual dimorphism in podocopid appendages.

The ecological function of this dimorphism has been clarified by [30] [50]. They observed its mating behaviour under experimental conditions from detailed video recordings. According to their observations (Figure 12), this short and thick podomere in the right fifth limb of the male is an adaptation for courting behaviour, where the male rotates the female's carapace three or four times using the right fifth limb just before mating (Figure 12). Thus, this right limb (especially the central podomere) is thicker and shorter than both the male left limb and the female's fifth limb to facilitate holding and rotating the female carapace. The thick podomere of the male right fifth limb is markedly more muscular than either that of the left side and those of the female, facilitating this behaviour [15, 30, 37].

**Figure 12.** Schematic profile of courting behaviour (= rotation of female carapace by male's fifth limb) just before mating behaviour, indicating the location of its fifth limb by black arrows, for modern *Bicornucythere bisanensis* (Podocopida) from modern inner bay area of central Japan, modified from Abe and Vannier (1991).

#### *3.3.2. The example of Vargula hilgendorfi*

Several examples of sexual dimorphism of appendages other than the fifth limb in living marine species have been reported for *Vargula hilgendorfi* (Myodocopida) [51]. This species inhabits shallow marine water environments*,* and the relative size of the furca at the posterior part of the soft body compared to the length of the entire carapace differs between the sexes. The male's furca is relatively larger than that of the female. According to detailed

observations of its ecological behaviour under experimental conditions [51], this species pushes off from sea-bottom sediments just before swimming in water, especially using the furca (Figure 13). Based on observations of video recordings, the male tends to swim around much more actively than the female; thus explaining the function of the relatively large furca in the male [37].

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 65

horn-like protuberance, the carapace appearance of species of this genus, including *Loxocorniculum mutsuense* from Japan, is very similar to that of the genus *Loxoconcha* as noted by Ishii *et al.* [63]. The phylogenetic independence of *Loxocorniculum* in Japan as a genus distinct from *Loxoconcha* has been debated [16]. Therefore this chapter tentatively includes *Loxocorniculum mutsuense*, first proposed as a new species from Japan by [66], in the genus

The ostracod genus *Loxoconcha* (family Loxoconchidae) is widely distributed in shallow marine environments from tropical to subarctic regions [52–54]. This is one of the most diversified ostracod genera, which comprises ca. 600 species [7]. This genus is common in and around Japan [55–59], and about 40 living and fossil species have been described [60].

A new fossil species *Loxoconcha kamiyai* from Pleistocene strata from the eastern coast of the Sea of Japan (Figure 14) was described, and its carapace morphology examined [16]. Palaeobiogeography of *L. kamiyai* was discussed, and its phylogenetic relationship to related

**Figure 14.** SEM images of two *Loxoconcha* species (Podocopida) from Quaternary deposits of central

loxoconchid species was assessed, based on the pore distribution pattern (a type of ostracod sensory organ; Figure 15). The number, distribution, and differentiation of pores on the ostracod carapace surface during ontogeny have been studied to determine phylogenetic relationships among species [61]. The reconstruction of ostracod phylogeny based on pore

Japan, modified from Ozawa (2010). Arrows indicate anterior.

Thus, *Loxoconcha* is one of the most important Japanese ostracod genera.

*Loxoconcha* following the opinion of Ishii *et al.* [63].

**Figure 13.** Schematic profile of 'push-off' behaviour just before swimming for modern *Vargula hilgendorfi* (Myodocopida), indicating the location of its furca by black arrows, modified from Vannier and Abe (1993).

A further four examples of sexually dimorphic appendages and eyes are found on *Vargula hilgendorfi*, as follows: (a) the existence or absence of two suckers on the first appendage (antennule), (b) different numbers of bristles on the first appendage, (c) different sizes of the basal part of the second appendage (antenna), (d) different sizes of compound eyes [37, 51]. The probable function of (a) is support by the male form of the female carapace during mating behaviour. However, the functions of the other sexually dimorphic characteristics (b)–(d) remain unclear. The Myodocopida first appeared during the early Palaeozoic (Ordovician), and still inhabit many marine environments [7]. Therefore, these other sexually dimorphic characteristics are interesting examples of myodocopid ostracod morphology, and indicate the evolutionary processes associated with their ecology, including mating behaviour and reproduction modes, since the Palaeozoic.
