The ontogeny of the cypridid ostracod Eucypris virens (Jurine, 1820) (Crustacea,Ostracoda)

The chaetotaxy (shape, structure and distribution of setae) of appendages and valve allometry during the post embryonic ontogeny of the cyprididine ostracod Eucypris virens are described. It is shown that the basic ontogenetic development of E. virens is very similar to that of other species of the family Cyprididae. During ontogeny, the chaetotaxy shows continual development on all podomeres of the limbs with the exception of the last podomere on the antennulae. The long setae on the exopodite and protopodite of the antennae have a natatory function until the actual natatory setae develop in later instars. Aesthetascs (presumed chemoreceptors) ya and y3 are the first to develop and may have an important function in the first instars. Cyprididae require a pediform limb in the posterior of the body presumably to help them to attach to substrates and this is reflected by the pediform nature of one limb at all times throughout all instars. This study has also shown that the fifth limb is most probably of thoracic origin and hence ostracods have only one pair of maxillae.     

Homology and homoeomorphy in ostracod limbs

The functional modifications of myodocopan and podocopan ostracod limbs constitute a rich data set with which to carry out phylogenetic analyses, but efforts are hindered by lack of consensus on homologies. Homoeomorphy presents particular difficulties; for example, the furca is post-anal in Myodocopa but pre-anal in Podocopa, suggesting homoeomorphy, not homology. Homoeomorphies also exist between ostracod appendages and those of other Crustacea, for example the oral cone and styliform mandibulae of Paradoxostomatidae (Ostracoda) and Siphonostomatoida (Copepoda), both adaptations to commensal or parasitic lifestyles. Such clear manifestations of homoeomorphy, arising independently in different lineages as a result of similar functional requirements imposed on plesiomorphic appendage structures, warn of the possibility of more subtle examples which, if unrecognized, would lead to misinterpretations of character states used in phylogenetic analysis. For instance, the branchial plates found on third, fourth and fifth limbs of podocopans may not be homologous with the branchial plates on the fifth and sixth limbs of myodocopans. Limb homologies of podocopan ostracods (primarily as represented by various podocopid taxa) are investigated. Evidence is presented, based on studies of morphology and musculature, that podocopid branchial plates are exopodites (arising from the basis), while those of myodocopans are epipodites (arising from the coxa or precoxa). In Podocopida, moreover, the protopodites of post-mandibular limbs appear to be undifferentiated, comprising only a basis, while those of Myodocopa clearly exhibit a basis, coxa and often a precoxa. These differences argue against monophyly of the Ostracoda. The absence of epipodites, combined with the lack of a coxa in post-mandibular limbs, is potentially indicative of closer affinities between podocopans and Cambrian stem-group crustaceans (including Phosphatocopida) than between podocopans and myodocopans. The possible derivation of podocopid third, fourth and fifth limbs from a stem-group crustacean limb is demonstrated. The hypothesis is advanced that podocopan ostracods (represented today by Podocopida, Platycopida and Palaeocopida) are derived from much nearer the base of the crown-group Crustacea than myodocopans.     

On the origin of the putative furca of the Ostracoda (Crustacea)

The posterior end of body of the extant ostracods exhibits a pair of variously shaped appendages, commonly designated as furca(e), uropods or caudal rami, used for feeding and/or locomotion. It is here shown that the so-called furca of all extant ostracods has evolved from the (probably epipodal) vibratory plates of a pair of uropods. The transformation comprised the following steps: (a) complete reduction of the uropodal protopodite and endopodite; (b) sclerotisation of the lateral walls of the vibratory plates; (c) transformation of the branchial filaments into spines and/or claws; (d) re-orientation of the plates from posterodorsal to posteroventral. These modifications are suggested to have evolved in parallel with a change in function, from respiratory to locomotory and/or feeding. The most primitive condition, reminiscent of the ancestral state of character, is seen in the Platycopida: the ‘furca’ still appears similar in shape to the vibratory plates of the pair of sixth limbs. In the Podocopida the uropodal plates have been modified into plate-like, more often into rod-shaped rami mainly used for locomotion. In both the Platycopida and Podocopida the anus has remained in its original place, posterior to the ‘furcal’ plates or rami. In the Myodocopida and Halocyprida the uropodal vibratory plates are transformed into heavily developed lamellae bearing sturdy spines. They are activated by a complex apparatus of muscles and sclerites, the development of which necessitated the displacement of the anus from the end of the body towards its present place, anterior to the ‘furca’. The furca of the Ostracoda being not a ‘true’ furca, a change in terminology is proposed: uropodal plates or lamellae in the Platycopida, Palaeocopida and Myodocopida/Halocyprida; uropodal rami in the Podocopida. The so-called furcae of the Ostracoda being homologous structures, it is concluded that all extant ostracods belong to a monophyletic lineage.     

Preface: The phylogeny, fossil record and ecological diversity of ostracod crustaceans

After more than two centuries of research, more than 65,000 living and fossil ostracod species have been described and studied, yet much remains to be learned about this ancient, widespread and diverse group of bivalved arthropods. Their higher classification and phylogeny are subjects of vigorous debate, as is their position in the broader picture of crustacean phylogeny. At the same time, major advances in our understanding of ostracod lineages and their relationships are resulting from the application of innovative approaches and techniques. This preface provides a contextual overview of the 15 contributions to this volume, which resulted from the 14th International Symposium on Ostracoda (ISO2001) held in 2001at Shizuoka, Japan. As such it provides a cross-section of topics at the forefront of research on the evolution and diversity of Ostracoda, and indicates directions for future work.     

介形类动物系统发育的研究现状及其趋势

近年来,世界各国的研究者越来越多地关注节肢动物系统进化问题,作为节肢动物一员的介形类,其系统进化问题也相应地引起了研究者的注意,这主要涉及介形类动物系统学定位问题,单系性问题,以及介形类动物各类群间的系统学关系等。就目前这几个问题的研究现状进行简单的介绍,同时对介形类动物系统学研究的趋势作了分析。     

Preface: Ostracoda and the four pillars of evolutionary wisdom

Morphology, palaeontology, genetics and ecology are the main scientific domains contributing theories, concepts and new data to evolutionary biology. Ostracods are potentially very good model organisms for evolutionary studies because they combine an excellent fossil record with a wide extant distribution and, therefore, allow studies on both patterns and processes leading to extant diversity. This preface provides an overview of the 15 contributions to the present volume and concludes that this set of papers supports the claim that ostracod studies are situated in all main evolutionary domains.     

Evidence of brooding in Permian non-marine Ostracoda

The discovery of some exceptional non-marine fossil Ostracoda, showing larval stages within the carapaces of females, demonstrates that incubation was an ontogenetical adaptation which appeared at least as early as the Permian. Thus, it seems, the superfamilies Danvinulacea and Cytheracea diversified in Permian lakes where the Carbonitacea had flourished in the Carboniferous. The Cypridacea accomplished an evolutionary radiation in continental environments after a permanent invasion from the sea in the Middle Jurassic, thanks to the resistance of their eggs to dessication and heezing and the evolution of parthenogenetic reproduction.     

Freshwater ostracods (Crustacea: Ostracoda) of the subfamily Dolerocypridinae Triebel, 1961 from Southeast Asia

The systematics of the freshwater ostracods of the subfamily Dolerocypridinae in Southeast Asia is reviewed: A total of five species are known and they are assigned to three genera. Astenocypris papyracea (Sars) 1903 and Dolerocypris sinensis Sars, 1903 are redescribed. Dolerocypris fasciata (Müller) 1776, from Indonesia is figured and the hitherto observed variations are discussed. Dolerocypris pellucida Klie, 1932 is transferred to the genus Tanycypris Triebel, 1959.     

Myodocopa (Crustacea: Ostracoda) as models for evolutionary studies of light and vision: multiple origins of bioluminescence and extreme sexual dimorphism

Evolutionists often use phylogeny to examine independent evolutionary events in search of generality. Therefore, groups of organisms rich in such independent character transitions are particularly valuable for the study of evolution. With respect to eyes, vision, and light-related characters, one such group is Ostracoda (Crustacea). Phylogenies of ostracods, derived from DNA sequence data and morphological characters, are presented. These inferred relationships largely agree with previous assessments of ostracod phylogeny, with the exception of paraphyletic Philomedidae. Based on methods of character reconstruction using these inferred relationships, different groups of ostracods probably evolved both bioluminescence and extreme sexual dimorphism (females lack eyes, males have large eyes) multiple times. Furthermore, myodocopid ostracods may have evolved compound eyes independently of other arthropods. For these and other reasons, it is proposed that the Ostracoda are an exceptionally important group for studying the evolution of vision- and light-related characters.     

Pycnogonid affinities: a review

Early authors regarded Pycnogonida (sea spiders) either as aquatic arachnids, ‘degraded’ crustaceans or as some sort of intermediate form between the two. Subsequently, pycnogonids were either placed among the Chelicerata or considered as an isolated group, unrelated to other arthropods. The latter model is untenable under phylogenetic systematics and recent cladistic studies have supported one of two alternative hypotheses. The first is the traditional Chelicerata s.lat. concept, i.e. (Pycnogonida + Euchelicerata). This, however, has only one really convincing synapomorphy: chelate chelicerae. The second hypothesis recognizes (Pycnogonida + all other Euarthropoda) and has been recovered in various ‘total evidence’ studies. Morphologically some characters – the presence of gonopores on the trunk and absence of a labrum, nephridia and intersegmental tendons – support Cormogonida (Euarthropoda excluding pycnogonids). Advances in developmental biology have proposed clear interpretations of segmentation homologies. However, so far there is also a confrontation of the two hypotheses depending on whether the last walking leg segment is considered part of the prosoma. In this case pycnogonids have too many prosomal segments compared with Euchelicerata; perhaps implying they are not sister groups. Alternatively, if part of the postprosomal region, the last leg pair could correspond to the chilarial segment in euchelicerates and its uniramous state could be apomorphic with respect to other euarthropods. Molecular phylogenies need to be more rigorously analysed, better supported by data from different sources and technique‐sensitive aspects need to be explored. Chelicerata s.lat. may emerge as the more convincing model, yet even the putative autapomorphy of chelicerae needs to be treated with caution as there are fossil ‘great appendage’ arthropods in the early Palaeozoic which also have a robust, food‐gathering, pair of head limbs and which may lie on the chelicerate, or even the euarthropod, stem lineage.     

The Ontogeny of Neonesidea oligodentata (Bairdioidea, Ostracoda, Crustacea)

This is the first detailed ontogenetic study of the appendages and carapace of a bairdioidean ostracod. This paper uses the development of the appendages and changes in the pore systems of the carapace through ontogeny to help determine the relationship between the Bairdioidea and other podocope groups. Neonesidea oligodentata has eight post-embryonic stages: one fewer than the Cypridoidea, Cytheroidea and Darwinuloidea. The first instar of N. oligodentata resembles that of the second instar of the Cypridoidea and Cytheroidea in terms of appendages, and it is postulated that there is an additional instar stage of N. oligodentata that molts within the egg. The general sequence of appearance of the limbs from instar A-7 onwards is similar to that of the Cypridoidea and Cytheroidea, but different from that of the Darwinuloidea. Like the Cypridoidea and Cytheroidea, N. oligodentata has a gap in its ontogenetic development during instar A-6, where no new Anlage is added. Pore system analysis of A-7 instars suggests that the Bairdioidea may be more closely related to the Cypridoidea than to the Cytheroidea.     

Limb morphogenesis in the branchiopod crustacean, Thamnocephalus platyurus, and the evolution of proximal limb lobes within Anostraca

Crustacean limbs exhibit highly diverse morphologies. One major route of diversification is in the number and position of branches arising from the proximal part of the limb. Here I describe development of larvae of the branchiopod crustacean, Thamnocephalus platyurus and describe in detail the development of the thoracic limbs. The thoracic limbs bear proximal branches both medially and laterally. The most proximal branches on either side (gnathobase and pre-epipod) show a similar developmental history: they develop via fusion of two rudiments into a single adult branch. However, phylogenetic analysis suggests that the developmental fusions have distinct evolutionary histories. In one case (gnathobase), the developmental rudiments reflect the ancestral adult morphology of two distinct branches. In the other (pre-epipod), the rudiments are an apparent novelty within the Anostraca and develop into two adult structures in only a single derived family.     

Phylogeny and evolution of Loxoconcha (Ostracoda, Crustacea) species around Japan

The pore-systems of 17 extant species of Loxoconcha around Japan were studied in order to understand their phylogeny and evolution. The phylogeny was estimated by two steps. First, the 17 species were divided into two groups, Group A (12 species) and Group B (five species) by Pore pattern Below Eye tubercle (PBE) analysis. Then, intragroup relationships were estimated by Differentiation of Distributional pattern of Pore-system (DDP) analysis. PBE analysis reveals that species of Groups A and B have on average different ecological preferences. Species of Group A, which appeared in the late Pliocene, are more diverse, have both phytal and bottom-dwelling modes of life, possess fewer pore-systems in the ventral area, and inhabit normal marine environments. Species of Group B, whose oldest fossil record is the lower Miocene, are less diverse, have only bottom-dwelling species, possess more pore-systems in the ventral area, and tend to inhabit brackish water environments. The results of this study suggest that the differences in ecology may have had an impact on the late Cenozoic diversification around Japan. The primary invasion of Group B occurred before the lower Miocene,with no subsequent diversification. Group A invaded after the late Pliocene and immediately diversified, which created the present abundance of Loxoconcha species around Japan in both species diversity and variety of modes of life.     

Carapace formation of the podocopid ostracode Semicytherura species (Crustacea: Ostracoda)

Details of ostracode carapace structures were examined by SEM and TEM. The podocopine ostracode Semicytherura kazahana has major ridges on the carapace surface and develops its prismatic layer inside the adult carapace. Electron microscopy at the final molt reveals that the major ridges arise from the highly dense formation of pits within the underlying swollen epidermis, and that disappearance of the epidermis in the presumptive area of the prismatic layer occurs after the calcification of the outer lamella cuticle, and just before synthesis of the membranous layer. These facts suggest that the formation of the carapace in Semicytherura takes place via a more complex process than that of the other podocopid ostracodes.     

The ontogeny of Loxoconcha japonica Ishizaki, 1968 (Cytheroidea, Ostracoda, Crustacea)

The ontogeny of the cytheroidean species Loxoconcha japonica is documented from the earliest instar to the adult. The first instar (instar A-8) of L. japonica is different from that of cypridoidean species in that it has an additional appendage, the furca, present. From instar A-7 onwards, the appearance of the appendages is similar to that of cypridoidean and bairdioidean species. The furca is well developed in instars A-8 to A-5, and is probably an important appendage in these early instars, despite its reduced form in the adults. Some appendages of L. japonica (e.g. the antennae) gain very few setae and claws through ontogeny, compared with species from other superfamilies. This possibly reflects paedomorphic evolution of this species.     

Anchialine podocopid Ostracoda of the Galapagos Islands

Water-filled fissures and lava tubes on Isla Santa Cruz and Isla Isabela provide anchialine habitats for 11 species of podocopid and platycopid Ostracoda, most with Atlantic and West Indian affinities. Their ancestors may have been carried to earlier islands in the Galapagos system by trans-Isthmian currents, on vegetation or larger animals, or by migrating waterfowl. None are derived from present-day Pacific coastal faunas of North, Central or South America. Two species of Cytheracea, a new species of Anchistrocheles (Bairdiacea), and one of Cytherella (Platycopida) are described.     

The osmoregulatory capacity of the Ostracoda

All ostracods that inhabit inland waters are osmoregulators. Freshwater ostracods must be hyperosmotic regulators while ostracods that live in hyperhaline water are hypoosmotic regulators. Some euryhaline species are hypoosmotic regulators in salinities above 8 g·l^-1 and hyperosmotic below. Hyperosmotic regulation in ostracods is partly dependent on salt consumed in the food but hypoosmotic regulation is dependent on the excretion of salt brought about by special cells located on the inside of the carapace.