**1. Introduction**

Studies of the origin and history of sex in organisms are important for elucidating lifehistory strategies and reproductive modes, and are an essential component of the study of evolutionary biology [1]. Sexual dimorphism (*i*.*e*., morphological differences between males and females) and its relationship to reproductive modes in both living and extinct/fossil organisms is a key aspect of such studies [2] [3].

Ostracods (Arthropoda) are the only organisms useful for investigations of the long-term history of sexual dimorphism during the last ca. 500 million years since the early Palaeozoic, *i.e.*, Ordovician (ca. 490 Ma = 490 million years ago) [4]. Ostracods are a class of small crustaceans (Figures 1 and 2) of which the adult form is around 1.0 mm in length, that inhabit most aquatic areas; *e*.*g*., marine, brackish, and freshwater conditions (Figure 3) [5] [6]. Most ostracods have the ability to reproduce sexually, except for a part of species capable of reproducing asexually (parthenogenesis). The most distinctive feature of ostracods is the calcareous carapace (Figures 1 and 2). Species with strongly calcified carapaces are relatively easily fossilised, and ostracods are abundant in sediments globally starting from the early Ordovician [7]. In contrast, the proteinaceous (= 'chitinous') soft body with appendages (Figure 2) is rarely fossilised due to a lack of mineralised parts, and as a result an ostracod fossil typically consists only of the hard carapace. However, this is sufficient for both recent and fossil specimens to be identified to the species level, based on various carapace morphological characteristics (Figures 1 and 2).

Similar to other crustaceans such as decapods, ostracods grow by moulting (ecdysis; Figure 4). For example, in one ostracod order, the Podocopida, there are usually eight moult stages between egg and adult, with the last moulting being the first sexually mature stage. Carapace and appendage sexual dimorphism can be recognised during the last adult stage (Figures 5 and 6), and to a lesser degree in late juvenile stages [8–10].

© 2013 Ozawa, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 53

**Figure 2.** Morphology of ostracod male, based on *Hemicythere villosa* (Podocopida), modified from

Horne *et al*. (2002)

**Figure 1.** SEM images of fifteen ostracod species (Podocopida) from modern coast of southern Okhotsk Sea, modified from Ozawa (2012). 1: *Baffinicythere ishizakii,* RV, 2: *Baffinicythere robusticostata,* LV, 3: *Cornucoquimba alata*, RV, 4: *Howeina camptocytheroidea*, RV, 5: *Johnnealella nopporensis*, LV, 6: *Pectocythere* sp., RV, 7: *Laperousecythere robusta*, LV, 8: *Rabilimis septentrionalis*, LV, 9: *Robertsonites hanaii*, LV, 10: *Acanthocythereis dunelmensis* s.l., LV, 11: *Argilloecia toyamaensis*, LV, 12: *Cytheropteron carolae*, LV, 13: *Palmenella limicola*, RV, 14: *Elofsonella* cf. *concinna*, LV, 15: *Krithe* sp, LV. Arrows indicate anterior. LV: left valve, RV: right valve.

valve, RV: right valve.

**Figure 1.** SEM images of fifteen ostracod species (Podocopida) from modern coast of southern Okhotsk Sea, modified from Ozawa (2012). 1: *Baffinicythere ishizakii,* RV, 2: *Baffinicythere robusticostata,* LV, 3: *Cornucoquimba alata*, RV, 4: *Howeina camptocytheroidea*, RV, 5: *Johnnealella nopporensis*, LV, 6: *Pectocythere* sp., RV, 7: *Laperousecythere robusta*, LV, 8: *Rabilimis septentrionalis*, LV, 9: *Robertsonites hanaii*, LV, 10: *Acanthocythereis dunelmensis* s.l., LV, 11: *Argilloecia toyamaensis*, LV, 12: *Cytheropteron carolae*, LV, 13: *Palmenella limicola*, RV, 14: *Elofsonella* cf. *concinna*, LV, 15: *Krithe* sp, LV. Arrows indicate anterior. LV: left

**Figure 2.** Morphology of ostracod male, based on *Hemicythere villosa* (Podocopida), modified from Horne *et al*. (2002)

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 55

**Figure 5.** SEM images of carapaces in male and female of *Loxoconcha kamiyai* (upper two rows) and *Finmarchinella hanaii* (lower two rows) (Podocopida) from Quaternary deposits of central Japan,

In organisms—other from ostracods—with abundant fossil records, there are very few cases of likely sexual dimorphism. One such case is the speculated "sexual dimorphism" of extinct ammonites (molluscs) discovered since the 1860s [11]. Many occurrences of ammonite fossils have been reported globally, and with respect to shell size, two different forms exist in particular groups of ammonites: the larger 'macroconch' and smaller 'microconch' within a probable single species from such genera as *Graphoceras, Ludwigina, Perisphinctes,* and

However, ammonite sexual dimorphism has long been debated. In addition, whether microconchs of ammonites are males or females remains speculative [11] [15]; therefore,

modified from Ozawa (2012). Arrows indicate anterior.

*Yokoyamaceras* from Jurassic and Cretaceous strata [12–14].

**Figure 3.** Schematic hypothetical profile of terrestrial aquatic to abyssal habitats of living ostracods progressing toward coastline, modified from Benson (2003).

**Figure 4.** SEM images of last three molting stgaes in *Aurila* sp. of Ozawa and Kamiya (2009) (Podocopida) from the modern coast of northeastern Japan Sea, modified from Ozawa (2012). Arrows indicate anterior.

indicate anterior.

**Figure 3.** Schematic hypothetical profile of terrestrial aquatic to abyssal habitats of living ostracods

**Figure 4.** SEM images of last three molting stgaes in *Aurila* sp. of Ozawa and Kamiya (2009)

(Podocopida) from the modern coast of northeastern Japan Sea, modified from Ozawa (2012). Arrows

progressing toward coastline, modified from Benson (2003).

**Figure 5.** SEM images of carapaces in male and female of *Loxoconcha kamiyai* (upper two rows) and *Finmarchinella hanaii* (lower two rows) (Podocopida) from Quaternary deposits of central Japan, modified from Ozawa (2012). Arrows indicate anterior.

In organisms—other from ostracods—with abundant fossil records, there are very few cases of likely sexual dimorphism. One such case is the speculated "sexual dimorphism" of extinct ammonites (molluscs) discovered since the 1860s [11]. Many occurrences of ammonite fossils have been reported globally, and with respect to shell size, two different forms exist in particular groups of ammonites: the larger 'macroconch' and smaller 'microconch' within a probable single species from such genera as *Graphoceras, Ludwigina, Perisphinctes,* and *Yokoyamaceras* from Jurassic and Cretaceous strata [12–14].

However, ammonite sexual dimorphism has long been debated. In addition, whether microconchs of ammonites are males or females remains speculative [11] [15]; therefore, which ammonite form constitutes the male shell remains uncertain. In contrast, because ostracods are living organisms, we can examine their soft body and appendages in detail, including rarely fossilised parts such as the copulatory organs of males and females. Furthermore, we have excellent an fossil record of ostracods extending back to the early Palaeozoic, so we can compare modes of sexual dimorphism by comparing living to extinct fossil species. Therefore, we are able to determine the detailed characteristics associated with sexual dimorphism and their function in ostracods more easily than in extinct organisms such as ammonites.

The History of Sexual Dimorphism in Ostracoda (Arthropoda, Crustacea) Since the Palaeozoic 57

Palaeozoic to Recent, reviewing another case of ostracod sexual dimorphism with

**2. General features of ostracods for taxonomy, ecology, and morphology** 

significance compared to other crustaceans; *e*.*g*., shrimps and crabs.

ostracod but belongs to another order, Myodocopida.

particles, are also distributed globally [18] [19].

[20–22].

morphological characteristics.

Ostracods are a class of small crustaceans (Figures 1 and 2), the adult form of which is typically ca. 1.0 mm in length. They are not generally well known to many people except for researchers of fossils or living crustaceans due to their small size and lack of commercial

Class Ostracoda consists of the following six orders: Podocopida, Platycopida, Palaeocopida, Leperditicopida, Myodocopida, and Halocypridina [10]. This chapter focuses mainly on ostracod species belonging to the order Podocopida, which consists of more than 20,000 named living and fossil species distributed globally, because Podocopida is the most diversified taxonomic group in class Ostracoda (Figure 3) [10]. The well-known bioluminescent organism 'the sea firefly' (called 'umi-hotaru' in Japanese) is a kind of

Ostracods occur in most aquatic environments on earth (Figure 3), such as the deep sea to a depth of several thousands of meters, through to shallow seas on the continental shelf [5]. They also live in rock pools in intertidal zones, brackish water areas at river mouths, lagoons and estuaries, freshwater lakes, ponds, irrigated rice fields, and temporary puddles. Most species are benthic throughout their lives, and crawl on or through the surface sediment and among aquatic plants [10]. A number of interstitial species, which live between sediment

The most distinctive feature of ostracods is the calcareous bivalved carapace (or shell), consisting of two valves that completely envelop the soft body and all appendages (Figures 1 and 2) [10]. Various types of appendage are protruded between opened valves for locomotion, feeding, and reproduction. The two valves (termed right and left) are connected by a hingement running along the dorsal margin (Figure 2). The word ostracod (or 'ostracode') is derived from Greek word 'ostrakon', which means 'a shell'. This carapace (or shell) has various morphological characteristics (Figures 1 and 2) that allow detailed taxonomic and phylogenetic studies to be performed on both living and fossil specimens

Like other crustaceans such as decapods, ostracods grow by moulting (ecdysis; Figure 4). For example, in Podocopida, there are usually eight moult stages between egg and adult [20]. Species with strongly calcified carapaces, such as most marine species, are relatively easily fossilised. Ostracods are abundant in sediments globally, beginning in the early Ordovician (ca. 490 Ma) [7]. The proteinaceous (= 'chitinous') soft body and appendages are rarely fossilised due to a lack of mineralised parts, with rare exceptions, such as preservation of the soft parts of Silurian fossils [23]. Therefore, an ostracod fossil typically consists only of the hard-calcified carapace; however this is sufficient for identification of both modern and fossil specimens to the species level based on various carapace

paedomorphosis reported by [17].

**Figure 6.** Morphology of female and male with right valve removed (one of each pair of appendages drawn for clarity) of *Vestalenula cornelia* (Podocopida) from modern springs in Yaku-shima Island of southwestern Japan, modified from Smith *et al.* (2006). Arrows indicate anterior.

Due to these unique traits, ostracods are the only organisms on earth that are useful for studying the history of sexual dimorphism since the Ordovician. Representative examples of sexual dimorphism within ostracods, mainly in species from Japan, are first introduced. This chapter then introduces one example of a sexually dimorphic feature accompanied by heterochrony (paedomorphosis). Recently, our research group found a rare case of sexual dimorphism in ostracod hingements with a paedomorphic character within only one phylogenetic group of one family. This morphology could be an important characteristic for evaluation of the history of sexual dimorphism in ostracods since the early Palaeozoic [16]. The author discusses this together with the history of ostracod sexual dimorphism from the Palaeozoic to Recent, reviewing another case of ostracod sexual dimorphism with paedomorphosis reported by [17].

56 Sexual Dimorphism

organisms such as ammonites.

which ammonite form constitutes the male shell remains uncertain. In contrast, because ostracods are living organisms, we can examine their soft body and appendages in detail, including rarely fossilised parts such as the copulatory organs of males and females. Furthermore, we have excellent an fossil record of ostracods extending back to the early Palaeozoic, so we can compare modes of sexual dimorphism by comparing living to extinct fossil species. Therefore, we are able to determine the detailed characteristics associated with sexual dimorphism and their function in ostracods more easily than in extinct

**Figure 6.** Morphology of female and male with right valve removed (one of each pair of appendages drawn for clarity) of *Vestalenula cornelia* (Podocopida) from modern springs in Yaku-shima Island of

Due to these unique traits, ostracods are the only organisms on earth that are useful for studying the history of sexual dimorphism since the Ordovician. Representative examples of sexual dimorphism within ostracods, mainly in species from Japan, are first introduced. This chapter then introduces one example of a sexually dimorphic feature accompanied by heterochrony (paedomorphosis). Recently, our research group found a rare case of sexual dimorphism in ostracod hingements with a paedomorphic character within only one phylogenetic group of one family. This morphology could be an important characteristic for evaluation of the history of sexual dimorphism in ostracods since the early Palaeozoic [16]. The author discusses this together with the history of ostracod sexual dimorphism from the

southwestern Japan, modified from Smith *et al.* (2006). Arrows indicate anterior.
