Embryonic development clarifies polyphyly in ostracod crustaceans

The present study describes the embryonic developmental process of the bioluminescent ostracod crustacean Vargula hilgendorfii . Optical microscopy, scanning electron microscopy, DAPI staining and video recording were used for observations. This study is the first detailed report of the embryonic development of a myodocopid ostracod. Contrary to previous studies, cleavage occurred in the yolk sphere and no evident cleavage furrow was found. No nauplius stage was found, and five pairs of appendages developed simultaneously. A bivalved carapace developed from two independent buds of the carapace valves. The buds of the left and right valves are enlarged, and become combined. The combined 'one-piece' carapace was divided by the formation of a hinge, and the usual bivalved carapace was formed. On the 16th day, embryos hatched as juveniles with six pairs of appendages, a pair of immature appendages, a pair of compound eyes, a median eye and a bivalved carapace. An important suggestion for the classification of Ostracoda is derived from the observed development of appendages and carapace, because the subclass Ostracoda is defined mainly by the similarities of appendages and the bivalved carapace. The present observations clearly show that the developmental process of Myodocopa differs from that of Podocopa, and supports polyphyly of the Ostracoda.     

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.     

Origin of the Ostracoda and their maxillopodan and hexapodan affinities

There are Cambrian fossils attributed to the Ostracoda but the extant subclasses Podocopa and Myodocopa do not appear until the Ordovician. At this time the morphologically similar, free-living ancestors of the now sedentary Thecostraca (Ascothoracida, Acrothoracica and Cirripedia) may have still been extant, and from an ecological point of view it seems likely that, by and large, ostracods replaced them. However, living ostracods have an abbreviated, direct development, and some key aspects of their morphology, such as the nature of the maxillary segment and abdomen, are conjectural. Thus the affinities between these and related taxa remain uncertain; e.g., while some contemporary carcinologists place Ostracoda as a taxon coordinate with the Branchiopoda, Remipedia, Cephalocarida, Maxillopoda, Malacostraca, others tentatively or unequivocally ally them with the Maxillopoda (generally Mystacocarida, Copepoda, Tantulocarida and Thecostraca, and sometimes Branchiura and Pentastomida). Others, largely involved with fossils, have stretched the definition of the Maxillopoda even further, to the point where it seems even less likely a monophyletic taxon. Until recently cladistic analyses utilizing genetic (largely 18S rDNA) as well traditional morphological characteristics have given confusing results regarding the affinities between these taxa, and an important one suggested the Ostracoda might even be diphyletic. Furthermore, a very recent genetic study utilizing protein encoding genes places a podocopine ostracod among the most primitive of the extant crustaceans (Branchiopoda, Cephalocarida Remipedia and Mystacocarida), and then generally at the base of a lineage leading to the Malacostraca, a lineage giving rise to copepods and cirripeds along the way. This indicates these so-called maxillopodan taxa evolved independently from a malacostracan-like ancestor, and if so they are convergent. And finally, from genetic studies it is not only becoming well documented the Crustacea rather than Myriapoda gave rise to the Hexapoda, but it appears the Hexapoda stem from among the lower rather than the higher crustaceans, possibly even from the Ostracoda. Whether there were terrestrial ostracods at the time hexapods appeared in the Lower Ordovician is unknown, but the modest diversity of terrestrial ostracods today are podocopines which also first appeared in the Lower Ordovician. Thus, if current interpretations of living ostracodan and fossil hexapodan body plans are largely correct, it can be hypothesized the Ostracoda are close to the ancestor of the Hexapoda.     

Morphological design and fossil record of the podocopid ostracod naupliar eye

The naupliar eye of podocopid ostracods is a useful character for considering evolution in photic environments. Based on external morphologies and histological observations, naupliar eyes are here categorized into six types. The fossil record demonstrates that the major evolutionary changes in podocopid naupliar eyes occured after the Ordovician. Eye types 1 and 2 are not joined to the carapace by the hypodermal cells, and these two types are found in Palaeozoic ostracods. Eye type 3, 4, 5 and 6 are extended on eye stalks, tightly joined to the carapace by the hypodermal cells, and use the carapace as the refractive cuticle lens. Eye type 3 appeared in the Permian, and eye types 4, 5 and 6 appeared in the Early Jurassic. The design of the podocopid naupliar eye diversified in the Early Jurassic.     

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.     

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。     

A Silurian myodocope with preserved soft-parts: cautioning the interpretation of the shell-based ostracod record

Ostracod crustaceans are the most abundant fossil arthropods. The Silurian Pauline avibella gen. et sp. nov., from the Herefordshire Lagerstätte, UK, is an extremely rare Palaeozoic example with soft-part preservation. Based on its soft-part morphology, especially the exceptionally preserved limbs and presence of lateral eyes, it is assigned to the myodocopid myodocopes. The ostracod is very large, with an epipod on the fifth limb pair, as well as gills implying the presence of a heart and an integrated respiratory–circulatory system as in living cylindroleberidid myodocopids. Features of its shell morphology, however, recall halocyprid myodocopes and palaeocopes, encouraging caution in classifying ostracods based on the carapace alone and querying the interpretation of their shell-based fossil record, especially for the Palaeozoic, where some 500 genera are presently assigned to the Palaeocopida.     

Longitudinal changes in protistan bacterivory and bacterial production in two canyon-shaped reservoirs of different trophic status

The drastic interactions of weather as El Niño events with catchment and hydrological processes can cause unexpected changes in physical, chemical and biological properties of freshwater aquatic ecosystems. The severe drought during 1998–1999 in the northeastern region of Brazil induced ecological changes in numerous reservoirs as in Tapacurá reservoir, one of the biggest drinking-water suppliers in Pernambuco state. Investigations were based on monthly sampling over 2 years (May 1998–May 2000) conducted at 3 representative stations with 3 sampled depths through the water column (0.5 m, middle and 0.5 m above the bottom). Temporal changes in ecological processes, especially stratification, were driven by two major precipitation patterns, with an initial marked dry period (period 1) followed by a rainy season (period 2). Dissolved oxygen and pH variations, higher conductivity and alkalinity values, higher concentrations of particulate organic material (carbon, nitrogen and phosphorus) and higher levels of algal biomass (chlorophyll a) characterized the dry period (May 1998–May 1999). During this phase of low water level when the reservoir storage capacity reached a minimum of 3.9%, the concentrations of chlorophyll a gradually increased with a cyanobacterial bloom (Cylindrospermopsis raciborskii) noted in April 1999. The decline in chlorophyll a and particulate organic matter were observed as a result of the first rains in May–June 1999, with the drastic changes of quality of matter (higher particulate C/N ratio). After a phase characterized by the entire water column turning anoxic, a second phase in the stratification process could be identified from June 1999 with the pronounced rainfalls accompanied by an overturn event. Annual rainfall deficit and lack of reservoir water renewal in 1998–1999 linked to the 1997 El Niño consequences were important determinants of high eutrophication levels and drastic ecological modifications in Tapacurá reservoir.     

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.     

The clonal composition of biramous and uniramous arthropod limbs

We present the first comparative cell lineage analysis of uniramous and biramous limbs of an arthropod, the crustacean Orchestia cavimana. Via single cell labelling of the cells that are involved in limb development, we are able to present the first complete clonal composition of an arthropod limb. We show that the two main branches of crustacean limbs, exopod and endopod, are formed by a secondary subdivision of the growth zone of the main limb axis. Additional limb outgrowths such as exites result from the establishment of new axes. In contrast to general belief, uniramous limbs in Orchestia are not formed by the loss of the exopod but by suppression of the split into exopod and endopod. Our results offer a developmental approach to discriminate between the different kinds of branches of arthropod appendages. This leads to the conclusion that a 'true' biramous limb comprising an endopod and an exopod might have occurred much later in euarthropod evolution than has previously been thought, probably either in the lineage of the Mandibulata or that of the Tetraconata.     

On Indiacypris luxata (Brady), a freshwater ostracod (Crustacea: Entomostraca) from Sri Lanka

Indiacypris luxata (Brady), a freshwater ostracod from Sri Lanka is described and both the valves and the limbs and soft parts are figured.     

Life history of a lacustrine ostracod

The ostracod Cypria turneri has one generation per year in Mirror Lake, New Hampshire. Juveniles hatch in January through May and reach maturity by August. The life history of this ostracod suggests that the turnover rate of the population is low.     

The Cambrian origin of the circulatory system of crustaceans

The carapace of Recent crustaceans such as myodocope ostracodes and phyllocarids is pervaded with well-developed anastomosing sinuses conveying hemolymph from the metabolizing organs to the dorsal heart. The inner lamella cuticle, which separates the sinuses from seawater, is thin enough to allow gaseous diffusion (e.g., O₂ uptake) over its surface. Comparable radiating and/or anastomosing features, of possible vascular origin, are herein recognized in several possible Crustacea from the Cambrian: cambriid, svealutiid, hipponicharionid and beyrichonid Bradoriida and in Carnarvonia from the middle Cambrian Burgess Shale. The vascular network is basically the same in these groups, consisting of sinuses radiating from supposed adductorial areas or from inferred areas of dorsal attachment of the body. The integumental (carapace sinuses) and branchial (gills) systems of respiration in crustaceans and crustacean-like animals were probably already differentiated by the middle Cambrian. The oldest record of probable integumental circulation is in the bradoriid Petrianna from the early Cambrian of Greenland. Similar circulatory systems may be represented by radiating ridges on the cephalon of other Cambrian arthropod groups such as the arachnomorphs ( Burgessia ) and trilobites ( Naraioa ) and may also be manifest in the carapaces of Ordovician-Devonian leperditicope ostracodes. Organs on the thoracopods of Cambrian supposed crustaceans, such as Canadaspis , resemble the foliaceous thoracic gills of Recent nebaliid phyllocarids and therefore may have served the same (respiratory) function.     

Appendage Homologies and the First Record of Eyes in Platycopid Ostracods, with the Description of a New Species of Keijcyoidea (Crustacea: Ostracoda) from Japan

A median nauplius eye is reported for the first time in a platycopid ostracod, a group hitherto considered to be blind. A new species of the platycopid ostracod genus Keijcyoidea is described from coastal rocky marine habitats on the Pacific coast of Japan. Observations of living specimens in the laboratory show that it is capable of burrowing to a depth of several millimeters in sandy sediment, using the first two head appendages (antennulae and antennae) and the furca. Females brooded a maximum of five eggs in the posterior brood space of the carapace. The homologies and phylogenetic implications of the trunk segmentation and limbs are discussed, paying particular attention to the sexually dimorphic fifth and sixth limbs; the copulatory appendages of both sexes are interpreted as being attached to trunk segments T6–T7 (counting from the posterior; T1 = posteriormost segment).     

Gradual evolutionary transformations of ontogeny in an Ordovician ostracod lineage

Carapace morphology was studied biometrically for each instar in a series of populations of Mojczella Olempska 1988 from the Ordovician strata exposed in Mójcza, Holy Cross Mountains, Poland. Phosphatic coatings of ostracod valves occur there in abundance from the top of the Arenig to the Late Caradoc. The lineage shows smooth acceleration in ontogenetic expression of the junction of crests C1 and C3, which originally were separated throughout the ontogeny, while in the latest populations the separation was preserved only in the earliest moult stage. Moreover, the velar structure, being originally developed as a wide, slightly convex, flange-like dolon in heteromorphs, became transformed into a convex dolon.