2.6 Reproductive System

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EDITH COWAN UNIVERSITY EVOLUTION,SYSTEMATICS & GEOGRAPHIC PARTHENOGENESIS OF Ilyodromus (CRUSTACEA,OSTRACODA) by Rylan James Shearn, BSc Hons A thesis submitted to Edith Cowan University in accordance with requirements for the degree of Doctor of Philosophy Submitted 27th February 2015 HAPTER C 2 AN INTRODUCTION TO MORPHOLOGY OFTHE CYPRIDOIDEA (CRUSTACEA,OSTRACODA) 2.1 Abstract Several chapters of this thesis frequently use specialist terminology to iden- tify and describe morphological features of ostracods from the superfamily Cypridoidea. To assist the non-specialist reader, this chapter guides the reader through a series of illustrations that serve to explain the fundamen- tal terms and concepts required to interpret these descriptive sections. It is hoped that this chapter will serve as a basic guide, or atlas from which readers can refer back to while reading the remainder of the thesis. 36 CHAPTER 2. THE CYPRIDOIDEA 2.2 Introduction Ostracods are small shrimp-like crustaceans enclosed by a calcitic bivalved carapace, essentially resembling a mussel with a shrimp inside (De Deckker, 1995). They have been discovered in nearly all aquatic environments (marine and non-marine), deep subterranean waters (Karanovic, 2007), and semi- terrestrial habitats (moist leaf litter; Pinto et al., 2003, 2004), while some are commensal on other aquatic organisms (Horne et al., 2002). Many species are generalists, having mainly detrital and herbivorous feeding habits (Martens, 2001), but some predate other small crustaceans (Martens, 2001) or filter feed (Karanovic, 2012). They are also known to serve as food for fish (Kornicker and Sohn, 1971; Vinyard, 1979) and waterbirds (Green et al., 2008). Most marine ostracods reproduce sexually (Cohen and Morin 1990), while in non-marine Ostracoda, there are three known reproductive modes accord- ing to Martens et al. (1998): 1. Sexual; whereby females of a species require insemination by a male in order to reproduce, this life cycle involving an alternation between meiosis (producing haploid male and female gametes) and syngamy (fusion of the two gametes to restore a diploid zygote). 2. Parthenogenetic (here and throughout this thesis also referred to as asexual, or asexuality); whereby females produce offspring without male fertilisation or syngamy. 3. Mixed reproduction; whereby populations can consist of sexual, parthenogenetic, or a mixture of both modes (sexual males, sexual females, and parthenogenetic females). Usually, parthenogenetic females and sexual females of the same species are morphologically indistinguishable. However, sex ratios can be indicative of reproductive mode, whereby any ratio close to 50:50 is likely to be sexual, while populations with a mixture of both modes are likely to have female- skewed ratios (Martens et al., 1998). Females lay eggs on plants, at the sediment surface, or in some species are kept in brood cavities within the carapace until juveniles become fully independent (Martens 2001). For many species, eggs are resistant to desiccation, and can be viable for over 50 years (Martens, 1994a,b), enabling them to persist in highly temporal habitats (See Chapter 6). This also enables ex- cellent dispersal ability for some species, as desiccation resistant eggs can be swept long distances by the wind (Vanschoenwinkel et al., 2008a), go against 2.2. INTRODUCTION 37 wind direction by surviving a passage through the gut of travelling waterbirds (Green et al., 2008), or attaching externally to travelling birds (De Deckker, 1977; Horne and Smith, 2004) or mud wallowing animals (Vanschoenwinkel et al., 2008b, 2011). There are three main superfamilies of non-marine Ostracoda (Cytheroidea, Darwinuloidea and Cypridoidea). Cypridoidea represents over 75 % of the approximately 2000 known extant species of non-marine Ostracoda (Martens et al., 2008), including species of Ilyodromus. Many taxa within the superfamily Cypridoidea, including Ilyodomus, are discussed and described throughout this thesis using specialised terminology. To assist non-specialist readers, this chapter aims to briefly explain basic terminology used in describing extant cypridoidean ostracods. As most of the information given in this chapter is attributed to only a small number of key texts, a different referencing style has been adopted to avoid repetition and to give an overall improvement of readability. Through- out this chapter, unless specific references are given, explanations of terminology were derived from the following sources; Meisch (2000) and Van Morkhoven (1962) for valve structure, Broodbakker and Danielopol (1982), Karanovic (2012), Martens (1987), and Meisch (2000) for appendage structure and layout, and McGregor and Kesling (1969a,b) for reproductive systems. This guide is largely intended to be a visual experience, and the reader is encouraged to interpret morphological concepts mainly through examina- tion of the figures listed at the end, under the guidance of the main text. Some illustrations were made specifically for this chapter, others were adapted from existing illustrations in other chapters or other literature, and have been refer- enced accordingly. Readers can refer to several other useful texts for compar- ative morphology among the other superfamilies of non-marine Ostracoda in Karanovic (2012) and Meisch (2000), and among other taxa of marine Ostra- coda in Horne et al. (2002). 2.3 Orientation Like all ostracods, cypridoideans are small bivalved crustaceans, being completely enclosed between two calcite valves. For orientation, these animals are best examined in lateral view, and often an eye (ocular lens) can be observed beneath the carapace in the antero-dorsal region (Figures 2.1 and 2.4), otherwise an antennule may be seen protruding between the two valves, also in the antero-dorsal region. Features toward the core of the animal are referred to as ‘medial’ or ‘proximal’ while features toward the extremities 38 CHAPTER 2. THE CYPRIDOIDEA are usually termed ‘distal’ (Figure 2.1). Carapace length, height and width measurements are usually taken as the maximum distance between parallel points at the anterior and posterior, dorsal and ventral, and right lateral and left lateral margins respectively (Figure 2.1). 2.4 Carapace and valves During dissection, the left and right valves that constitute the carapace are separated and the inner structures can be observed, which are important tax- onomic features (Figure 2.2). Both valves have an inner and outer lamella, that are separated by the vestibulum (Figure 2.3). Some of the inner lamella is calcified around the margin, and the boundary between this calcified and non-calcified inner lamella is well defined and termed the inner margin (Fig- ures 2.2 and 2.3). The calcified part can have inner lists and selvages (Fig- ures 2.2 and 2.3) that have a gasket-like function, often completely sealing the carapace when the valves are closed together, and the variation of these structures are also important taxonomically. The outer margin refers to the outer most edge of the valve in lateral view. Toward the valve outer margin, the outer and inner lamellae may be fused, and here, radial pore canals can often be observed (Figures 2.2 and 2.3). The outer lamella also has normal pore canals that protrude setae on the outer side of the valve (Figure 2.3). The left and right valves are attached to one another by a dorsal hinge and the ostracod closes the valves together with centrally and dorsally positioned adductor muscles (Figure 2.4). These muscles leave attachment scars on the valves (Figure 2.2) called central muscle scars, and dorsal muscle scars, both of which are important taxonomically. 2.5 Appendage layout and terminology Cypridoidean ostracods have eight sets of paired appendages (Table 2.1 and Figure 2.4) not including copulatory organs (the male hemipenis or female genital lobe). From anterior to posterior, they are the Antennule (A1), Antenna (A2), Mandibula (Md), Maxillula (Mx), Fifth limb (L5), Sixth limb (L6), Seventh limb (L7) and Caudal rami or ramus (CR). Although these names are generally accepted in current literature, other terms are used for the same appendages of cypridoidean ostracods, or for homologous appendages in other ostracod taxa (Table 2.1). The Antennule is the most anterior appendage and is mainly used for lo- comotion, bearing many natatory (swimming) setae (Figure 2.5), but it also 2.4. CARAPACE AND VALVES 39 Table 2.1: Terminology used throughout this chapter and the thesis to refer to appendages of species of Cypridoidea, listed from anterior to posterior, also showing abbreviations used throughout the thesis, and other synonymous terminology used either for different superfamilies

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