Evolutionary transitions among dioecy,androdioecy and hermaphroditism in limnadiid clam shrimp (Branchiopoda: Spinicaudata)

Examinations of breeding system transitions have primarily concentrated on the transition from hermaphroditism to dioecy, likely because of the preponderance of this transition within flowering plants. Fewer studies have considered the reverse transition: dioecy to hermaphroditism. A fruitful approach to studying this latter transition can be sought by studying clades in which transitions between dioecy and hermaphroditism have occurred multiple times. Freshwater crustaceans in the family Limnadiidae comprise dioecious, hermaphroditic and androdioecious (males + hermaphrodites) species, and thus this family represents an excellent model system for the assessment of the evolutionary transitions between these related breeding systems. Herein we report a phylogenetic assessment of breeding system transitions within the family using a total evidence comparative approach. We find that dioecy is the ancestral breeding system for the Limnadiidae and that a minimum of two independent transitions from dioecy to hermaphroditism occurred within this family, leading to (1) a Holarctic, all‐hermaphrodite species, Limnadia lenticularis and (2) mixtures of hermaphrodites and males in the genus Eulimnadia. Both hermaphroditic derivatives are essentially females with only a small amount of energy allocated to male function. Within Eulimnadia, we find several all‐hermaphrodite populations/species that have been independently derived at least twice from androdioecious progenitors within this genus. We discuss two adaptive (based on the notion of ‘reproductive assurance’) and one nonadaptive explanations for the derivation of all‐hermaphroditism from androdioecy. We propose that L. lenticularis likely represents an all‐hermaphrodite species that was derived from an androdioecious ancestor, much like the all‐hermaphrodite populations derived from androdioecy currently observed within the Eulimnadia. Finally, we note that the proposed hypotheses for the dioecy to hermaphroditism transition are unable to explain the derivation of a fully functional, outcrossing hermaphroditic species from a dioecious progenitor.     

Biostratigraphy and geometric morphometrics of conchostracans (Crustacea,Branchiopoda) from the Late Triassic fissure deposits of Cromhall Quarry,UK

The enigmatic fissure deposits of south‐western England and southern Wales are famous for their unique assemblage of Late Triassic vertebrates, although their age is contentious. While recent studies of palynomorphs have dated some as Rhaetian, their conchostracan (Crustacea, Branchiopoda) assemblages have not been described in detail nor used in biostratigraphy. We find that species determination of British Late Triassic conchostracans requires detailed observations of size, shape and ornamentation. We provide evidence that although Euestheria brodieana is invariably smaller than E. minuta, with some slight differences in carapace ornamentation, the traditional view that they are very similar is upheld. The use of conchostracans as a biostratigraphical tool is here tested by application to the British Triassic fissures at Cromhall quarry where the usual stratigraphical evidence provided by superposition is absent. We find no distinction between conchostracans from bedded Rhaetian deposits of the UK and specimens collected from the fissure deposits of Cromhall Quarry, Gloucestershire, supporting a late Rhaetian age for these deposits.     

Breeding systems in the clam shrimp family Limnadiidae (Branchiopoda, Spinicaudata)

Abstract. Crustaceans in the class Branchiopoda exhibit a wide range of breeding systems, including dioecy (gonochorism), androdioecy, parthenogenesis, cyclic parthenogenesis, and hermaphroditism. The largest subgroup of the Branchiopods, the Diplostraca, is reported to encompass all five of these breeding systems. However, many of these reports are based primarily on simple observations of sex ratios in natural populations. Herein we report the beginnings of a more rigorous approach to breeding system determination in the Diplostraca, starting with the family Limnadiidae. We combine measurements of sex ratio, offspring rearings, and behavior to identify three breeding systems within the Limnadiidae: dioecy, androdioecy, and selfing hermaphroditism. To date, no instances of parthenogenetic reproduction have been identified in this family. Comparisons of breeding system determination via simple population sex ratios with our more controlled studies show that simple sex ratios can be useful when these sex ratios are ∼50% males (=dioecy) or 5–30% males (androdioecy). However, population sex ratios of 0–5% males or 35–45% males necessitate further investigation because estimates in these ranges cannot distinguish selfing hermaphroditism from androdioecy or androdioecy from dioecy, respectively. We conclude by noting that the genetic sex-determining system outlined for one of these limnadiid species, Eulimnadia texana , provides a parsimonious framework to describe the evolution of the three breeding systems observed within the Limnadiidae.     

Rediscovery of Leptestheria dahalacensis and Eoleptestheria ticinensis (Crustacea: Branchiopoda: Spinicaudata): an overview on presence and conservation of clam shrimps in Austria

According to recent literature, five of the six known Austrian conchostracan species are extinct. However, interim results of a current study on large freshwater branchiopods in Austria show that five species still occur at a restricted number of sites in the Pannonian region of Lower Austria. The clam shrimps Leptestheria dahalacensis and Eoleptestheria ticinensis were rediscovered in May 1994 in the flood plains of the river Morava near Marchegg. Imnadia yeyetta and Cyzicus tetracerus have been known to the authors in the same region since 1981, and 1992, respectively. Limnadia lenticularis occurs in the flood plains of the rivers Morava and Danube. Lynceus brachyurus, the only Austrian representative of the Laevicaudata, was not found and most probably got extinct. All Austrian clam shrimp species are considered to be endangered. Main threats are agricultural activities and artificial changes of the hydrologic conditions. Conservational measures are discussed for their effectivity.     

SEM investigations of the larval development of Imnadia yeyetta and Leptestheria dahalacensis (Crustacea: Branchiopoda: Spinicaudata)

This paper presents a SEM documentation of the larval development of the two most abundant Austrian conchostracan species, Imnadia yeyetta (Limnadiidae) and Leptestheria dahalacensis (Leptestheriidae). As in several previously examined spinicaudatan species, five larval stages were documented: Nauplius 1, Nauplius 2, Metanauplius, Peltatulus, and Heilophore. Additionally, three postlarval stages of L. dahalacensis and the first larval instars of Eoleptestheria ticinensis and Limnadia lenticularis are shown and compared with the examined stages. Species identification of conchostracan larval stages is possible by using surface structures, and using SEM methods, except for L. lenticularis which can be identified more easily on the characteristic shape of the labrum.     

Production of intersexes and the evolution of androdioecy in the clam shrimp Eulimnadia texana (Crustacea,Branchiopoda, Spinicaudata)

Summary

The production of low numbers of offspring that exhibit a mixture of male and female traits (termed “intersexes”) is commonly reported for crustaceans. The production of intersexes has been ascribed to both genetic and non-genetic (e.g., parasitic infections and environmental pollutants) causes. Herein we report on two observed types of intersexes in the clam shrimp Eulimnadia texana: (1) a “morphological” intersex, possessing secondary male characteristics (e.g., claspers) and an eggproducing gonad, and (2) a “gonadal” intersex, possessing primarily male traits (e.g., male secondary sexual characters and male gamete production) but also producing low levels of abortive female gametes. We propose that these intersexes are likely the products of low frequencies of crossing over between the sex determining chromosomes that result in the array of observed mixed sexual phenotypes. Additionally, we suggest that the low-level production of intersexes, combined with the ephemeral nature of the habitats occupied by these shrimp, may explain the preponderance of androdioecy (mixtures of males and hermaphrodites) found in these clam shrimp, and possibly branchiopods more generally.     

The larval development of an Australian limnadiid clam shrimp (Crustacea, Branchiopoda, Spinicaudata), and a comparison with other Limnadiidae

The adult morphology of the Australian Limnadopsis shows some remarkable differences to that of other Limnadiidae. These differences are not reflected in its larval development. In Limnadopsis parvispinus, larval development comprises six stages. In stages I and II only the three naupliar appendages are present: the antennule as a small bud, the biramous antenna as the main swimming organ, and the mandible. The antennal protopod bears two endites, the proximal naupliar process and a more distal endite. In stage III a bifid naupliar process (in earlier stages not bifid) and the first signs of the carapace and trunk limb anlagen (undifferentiated rudiments) appear. In stage IV the carapace anlagen become more pronounced. The number of trunk limb anlagens increases to five, and differentiation has commenced. In stage V the first five pairs of trunk limbs are differentiated to varying degrees. The anterior-most four pairs of trunk limbs are subdivided into five endites, a small endopod, an exopod and an epipod. The bivalved carapace covers the anterior-most limbs. In larval stage VI the carapace is larger and the trunk limbs are further differentiated. A general pattern in the sequence of larval stages is the increasing number of sensilla on the antennules. From the last larval to the first postlarval stage, a significant change in morphology takes place. The trunk limbs are now used for swimming. Typical larval organs are much smaller than in the last larval stage. A comparison with other representatives of the Limnadiidae shows a high degree of correspondence, with most differences explained by the heterochronous appearance of characters during development. Five to seven stages are described for all studied Limnadiidae, including one particular stage in which four fully developed setae, a bifid naupliar process and the first signs of carapace anlagen are present. These characters are found in stage III in L. parvispinus, Limnadia stanleyana, Eulimnadia texana, and Imnadia yeyetta but in stage IV in E. braueriana and L. lenticularis. Based on a comparison of the larval stages of six limnadiid and one cyzicid species, we conclude that at least six naupliar stages belong to the limnadiid ground pattern.     

Evidence for a dioecious mating system in Early Jurassic Hardapestheria maxwelli gen. et sp. nov. (Crustacea,Branchiopoda, Spinicaudata) from the Kalkrand Formation of Namibia

A new spinicaudatan (clam shrimp), Hardapestheria maxwelli gen. et sp. nov., is described from the Jurassic Kalkrand Formation of central Namibia. Specimens were collected from a sedimentary interbed within a succession of flood basalts. These are the first spinicaudatans to be described from the Jurassic of south‐western Africa. The new taxon is assigned to the family Eosestheriidae based on the combination of punctae and radial ornamentation on the carapace. Ornamentation on the growth bands in H. maxwelli differs from other eosestheriid genera because the punctate ornamentation is not restricted to the dorsal region of the carapace. Instead, all growth bands include a proximal region with punctae even though the distal portion of each growth band may exhibit anastomosing lirae. Among well‐documented Mesozoic spinicaudatan genera, Hardapestheria is most closely related to Carapacestheria from the contemporaneous Kirkpatrick Basalt of Antarctica and Yanjiestheria from the Early Cretaceous of China. Hardapestheria maxwelli displays sexual dimorphism, which can be definitively related to a dioecious mating system with discrete male and female individuals. Review of additional early members of the Eosestheriidae suggests that the maintenance of two discrete sexes was the ancestral state for the clade. The ability to discriminate sexual mating system type unambiguously is rare in fossils, and this new species sheds light on the relationship between environmental stability and mating system evolution.     

The status of Anostraca,Notostraca and Conchostraca (Crustacea: Branchiopoda) in Yugoslavia

The investigations of Anostraca, Notostraca and Conchostraca in Yugoslavia began after a great delay compared with other European countries. Intensive systematic investigations date back only to the 1970s.To date, 19 species have been recorded. All species, with the exception of Artemia salina, are faunal elements of the Pannonian region. Seven species, including Artemia salina, Branchinecta orientalis, Branchipus serbicus, Leptestheria dahalacensis, Limnadia lenticularis, Imnadia cristata and Imnadia panonica are known from single localities. Six species including Branchinecta ferox, Chirocephalus brevipalpis, Chirocephalus carnuntanus, Lepidurus apus, Eoleptestheria spinosa and Imnadia banatica have restricted distributions. Streptocephalus torvicornis and Cyzicus tetracerus are known from several localities, while Branchipus schaefferi, Chirocephalus diaphanus, Triops cancriformis and Leptestheria saetosaare common. Large branchiopods are mainly confined to anthropogenic landscapes, especially in the northern part of the country. Rare species inhabiting a single pond, or those with rather restricted distributions, are the most endangered and such species account for about 70% of the fauna. Limnadia lenticularis, Imnadia cristata and I. panonica have not been found for more than 20 years and they are probably extinct in Yugoslavia. Branchipus serbicus, Imnadia cristata and panonica have not been documented since being originally described. Eoleptestheria spinosa is currently the most endangered species since it only appears irregularly in small populations. At present, Branchinecta ferox, Chirocephalus carnuntanus and Imnadia banatica are the safest of the rare species as they regularly appear in large numbers. There are no large branchiopods on the List of protected animals in Yugoslavia. This revised version was published online in August 2006 with corrections to the Cover Date.     

Ultrastructure of the male gonad and male gametogenesis in the clam shrimp Eulimnadia texana (Crustacea, Branchiopoda, Spinicaudata)

Abstract. The ultrastructure of the male gonad of Eulimnadia texana (Branchiopoda, Spinicaudata) has been observed for the first time to investigate the sexuality of a well-studied case of androdioecy in the animal kingdom. The male gonad is a double structure located in the hemocoel throughout the entire body length on each side of the midgut. Male gametes originate from the wall and mature centripetally toward the lumen; the proliferative activity is very high and continuous and therefore the mature gonad is full of numerous germ cells. Inside the lumen several degenerative stages are found mixed with sperm cells and spermatids, the latter two being not easily distinguishable because of the slight differences between them. The evolutionary meaning of the degenerative process in E. texana male gametes is difficult to explain, and we propose some hypotheses about its possible role or cause in the studied population: (a) to help build spermatophores, (b) to act as a trophic component for viable sperm, (c) as a manifestation of inbreeding depression, and/or (d) to regulate the number of sperm cells.     

Ducts of the labral glands of Leptestheria dahalacensis (Crustacea: Branchiopoda: Spinicaudata)

Inside the labrum of Leptestheria dahalacensis are situated three types of large epidermal gland cells, whose ducts open onto the outer dorsal surface of the labrum. SEM revealed that the thin ducts of the A-type gland cells open out behind the epipharynx at the end of small, conically shaped protuberances, the two paired ducts of the B-type gland cells lead into the distal portion of the labrum, and the external opening of the single duct of the C-type gland cells lies on the dorsal lobe of the labrum. The ducts of the three different gland cell types have the same fundamental constitution, but vary in diameter. Each secretory unit consists of a pair of gland cells (A, B, or C) and a secretory duct. The duct is formed by ring-shaped folding of one anteroposteriorly elongated epidermal cell (duct cell), whose ends adhere closely to one another. A further ring-folded epidermal cell (accessory cell), but flattened in shape, is interposed, like a sleeve-connection, between the gland cells and the duct cell. The reservoirs of gland cells open into the lumen of the duct. Discontinuous deposits of highly electron-dense matter are present on the plasma membrane of the accessory cell delimiting the initial part of the duct lumen, while the plasma membrane of the duct cell facing the lumen is cuticularized. The cytoplasm of the accessory cell, on examination by TEM, appears quite similar to that of the duct cell, except for the different distribution and greater abundance of microtubules. Similarly organized tricellular tegumental glands also commonly occur in other Crustacea, both Malacostraca and non-Malacostraca. Possible functions of secretions from the three different types of gland cells present in the labrum of L. dahalacensis are discussed.     

Sperm number assessed by flow cytometry in species of Daphnia (Crustacea,Cladocera)

Daphnia magna and Daphnia pulex are two important model species in ecotoxicology. In daphniids, studies of the effects of contaminants have mostly focused on female life history traits, yet it would also be important to examine male reproductive traits, particularly in relation to endocrine disruptors. In this study, we developed a protocol that uses flow cytometry to measure sperm number in individual males of different species of Daphnia. We tested our protocol on 114 males from several clones of three common species of Daphnia. Sperm count varied widely among individuals and reached high numbers (up to 1.45 × 10⁵). Positive relationships between male length and sperm number were observed in D. pulex and Daphnia pulicaria, but not in D. magna. Important inter‐clonal differences in sperm production were observed in all species, with some clones producing very little sperm. Duplicated sperm samples showed on average only 6% difference in sperm counts. Sperm counts were stable at least over a 2‐hr period and up to 5 hr for most samples. This sperm isolation protocol and flow cytometric enumeration approach will be of major interest to ecotoxicologists.     

Larval and post-larval development of the branchiopod clam shrimp Cyclestheria hislopi (Baird, 1859) (Crustacea, Branchiopoda, Conchostraca, Spinicaudata)

The larval and post-larval development of Cyclestheria hislopi is examined by SEM. There are at least nine stages (excluding the adult) – six larval and three post-larval stages. The first four stages are passed within the egg-membrane. The larval and the post-larval phase are separated by a profound change in morphology that takes place between stages VI and VII. The larva shifts from a dorso-ventrally flattened 'larval' appearance up to stage VI to a laterally flattened, more 'adult' appearance from stage VII. New morphological data have been revealed by this study, including (1) a large and globular larval dorsal organ; (2) the carapace starts its development from the segments of the first and second maxillae; (3) the anterior ramus of the second antenna in adult Cyclestheria hislopi is the endopod, and the posterior ramus the exopod. Direct development of the brood in Cyclestheria hislopi – unique among conchostracans – is compared with that of the Cladocera. If Cyclestheria is the sister group to the Cladocera, as favoured in this work, the classical neoteny theory of the Cladocera must be reconsidered, as there is no particular similarity between any adults of the Cladocera and any of the larval stages of Cyclestheria . It is suggested that Cyclestheria displays the type of development present in a cladoceran ancestor. A comparison between Cyclestheria and the Upper Cambrian 'Orsten' fossil Rehbachiella kinnekullensis reveals a remarkable similarity in the endite morphology of the trunk limbs.     

Comparative morphology among trunk limbs of Caenestheriella gifuensis and Leptestheria kawachiensis (Crustacea: Branchiopoda: Spinicaudata)

Morphological differences among groups of the 24 trunk limbs of Caenestheriella gifuensis (Ishikawa, 1895) and differences between males and females are described and illustrated. A setose attenuate lobe located proximally near enditic lobe 1 and a discoid lobe covered with small setae proximal to enditic lobe 1 are newly described. The five ventral enditic lobes, endopod, exopod, and dorsal exite of traditional spinicaudatan morphology are redescribed. Trunk limbs 1–4 of females bear a palp on enditic lobe 5 and trunk limbs 1–15 of males bear a similar palp. A second, articulating palp is associated with the base of the endopod of trunk limbs 1–2 of males. The proximal part of trunk limbs 19–24, bearing enditic lobe 1, articulates by an arthrodial membrane with the remainder of the limb, and the exite is distal to this arthrodial membrane. Development of trunk limbs, ascertained through an examination of early juvenile instars of Leptestheria kawachiensis Uéno, 1927, includes an asetose limb followed in time by a series of setose limbs that increase in morphological complexity with age. The number of lobes on the asetose limb varies from seven (corresponding to five enditic lobes, an endopod, and an exopod) on anterior limbs to five on trunk limb 24, which lacks the lobes corresponding to enditic lobe 4 and the endopod; these two structures are added later to setose limbs. The attenuate lobe, the discoid lobe, the exite of all trunk limbs, and the palps of the anterior trunk limbs are added to the setose limbs. Development of anterior limbs is accelerated relative to that of posterior limbs, and development of the more posterior limbs is truncated relative to that of limbs immediately anterior to them. Enditic lobe 4 and the endopod of limbs like trunk limb 24 develop from, or are patterned by, enditic lobe 5; the articulating palp of male trunk limbs 1–2 also may be added in this way. A comparison of these observations with development of the copepod maxilliped suggests that the spinicaudatan trunk limb is composed of a praecoxa with three lobes, a coxa and a basis each with one lobe, and an endopod of three segments in females and four in males. This is similar to the homology scheme previously proposed by Hansen in 1925. A critique is given of attempts to homologize parts of arthropod limbs based on developmental gene expression patterns. Stenopodal to phyllopodal transformations of maxillipeds in copepods provide a model at least partly applicable to spinicaudatans, and a ‘multibranched’ interpretation of spinicaudatan (and by extension branchiopodan) limb morphology is rejected. There is nothing intrinsic to the structure of the adult trunk limbs suggesting that they are similar to the adult limbs of the ancestral branchiopod or the ancestral crustacean, but early developmental steps of more posterior limbs are good matches for the morphology of an ancestral crustacean biramal limb predicted by a hypothesis of duplication of the proximo‐distal axis. © 2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 139 , 547–564. No claim to original US government works.     

宽网叶肢介属重要分类特征的发现

通过扫描电镜重新研究了产自俄罗斯维特奴河流域下三叠统维特奴宽网叶肢介(Loxomegaglypta wetlugiana Novo-jilov, 1958)的正模标本。扫描电镜成像揭示了光学照相无法分辨的重要分类学新特征, 其中包括: 生长带饰有圆形、角状或椭圆形小筛坑状装饰(直径15–25 μm), 筛坑被不同形状的隆起包围。均匀分布的针孔(直径4–6 μm)不但出现在筛坑里, 而且布满了筛坑周围的隆起。这种装饰与该属建立时描述的多角状大网格装饰区别较大。在此基础上,本研究修订了宽网叶肢介属生长带的装饰特征。     

Isolation and characterization of 13 polymorphic microsatellite loci from the clam shrimp Eulimnadia texana (Crustacea: Spinicaudata)

Thirteen polymorphic microsatellite loci were isolated and characterized from the clam shrimp Eulimnadia texana. In analyses of 20–50 individuals from two populations the number of alleles ranged from two to seven with observed heterozygosity ranging between 0.00 and 0.37. The low values for heterozygosity were not unexpected for a group characterized by its unusual androdioecious mating system, in which males compete with self‐compatible hermaphrodites for offspring production. These microsatellites are likely to be useful for further evolutionary investigations of this rare mating system in these crustaceans.     

A new species of Cyzicidae (Crustacea: Conchostraca) from Iraq

A new species of Conchostraca, Eocyzicus mesopotamiensis sp. nov., from a small pond in Iraq is described and illustrated. The account includes structural details and means of recognition. Information on the habitat is also given.     

Hidden diversity and cryptic speciation refute cosmopolitan distribution in Caprella penantis (Crustacea: Amphipoda: Caprellidae)

Caprella penantis is considered a cosmopolitan species and one of the most challenging caprellids in taxonomic terms because of its remarkable intraspecific morphological variation. This study examined DNA sequences from mitochondrial (COI) and nuclear (18S) markers together with morphological data from 25 localities of C. penantis, and closely related species Caprella dilatata and Caprella andreae, all traditionally considered part of the old ‘acutifrons’ complex. The large genetic divergence and reciprocally allopatric distributions point to the existence of a species complex of at least four species, of which one is reported as a cryptic species. This study provides the first evidence of cryptic speciation in the family Caprellidae, and questions the validity of some traditional morphological characters used to delimit species in the genus Caprella. Our results are consistent with the idea that main factors were probably isolation by distance and ecological traits, promoting diversification in C. penantis. The strong genetic structure reported for this species in the Iberian Peninsula and Moroccan coasts also suggests restriction to dispersal as well as the presence of refugial areas. These results highlight the utility of the COI and 18S genes in combination with morphological characters for shedding light on systematic questions in caprellids, and patterns of genetic connectivity.     

Evolutionary systematics of the Australian Eocyzicus fauna (Crustacea: Branchiopoda: Spinicaudata) reveals hidden diversity and phylogeographic structure

Although ‘large branchiopods’ are an important faunal element of the temporary water bodies in Australia's vast (semi)arid regions, knowledge of their diversity, distribution and ecology is still poor. Here, on the basis of one mitochondrial [cytochrome oxidase subunit I (COI)] and three nuclear (EF1α, ITS2 and 28S) markers, we present new data relating to the diversity and phylogeography of eastern and central Australian Eocyzicus (Spinicaudata) fauna. Using a combination of phylogenetic, haplotype network and DNA barcoding analyses of COI, 312 individuals were grouped into eleven main lineages. To infer whether these lineages are reproductively isolated from each other (the prerequisite for species delineation according to the Biological or Hennigian Species Concepts), separate analyses of each nuclear marker were performed on a subset of specimens. Although some lineages are non‐monophyletic in the analysis of one nuclear marker, this is mostly attributed to processes such as incomplete lineage sorting rather than ongoing reproduction. The eleven lineages translate into at least seven species whose reproductive isolation is additionally indicated by sympatry, including both Australian Eocyzicus species previously described. Another three lineages may constitute further species, but their clear allopatric distribution rendered the test for reproductive isolation inapplicable. One lineage appears not to be reproductively isolated and is therefore considered a genetically distinct lineage within one of the other species, and one divergent lineage within E. argillaquus may constitute an additional species. Although sympatry is very common – six species occur in the central Paroo River catchment in eastern Australia, for instance – syntopic occurrence is rare. It is possible that a combination of differing habitat preferences and priority effects inhibits the presence of more than one Eocyzicus species per water body. There is little to no genetic differentiation between certain populations of the species found in eastern and central Australia (e.g. the Murray–Darling Basin, the Bulloo River catchment and the eastern and northern Lake Eyre Basin; LEB), suggesting high dispersal rates within this large area. Between the central Australian populations themselves, however (e.g. those inhabiting the central and western LEB), genetic differentiation is pronounced, probably as a result of the lack of abundance of important dispersal vectors (aquatic birds) and the lower diversity and density of suitable habitats in the area. The most prominent biogeographical break exists towards north‐eastern Australia (north‐east LEB), which does not share species with any other region studied.     

Functional morphology of amplexus (clasping) in spinicaudatan clam shrimps (Crustacea,Branchiopoda) and its evolution in bivalved branchiopods: A video‐based analysis

Male clam shrimps (Crustacea: Branchiopoda: Laevicaudata, Spinicaudata, and Cyclestherida) have their first one or two trunk limb pairs modified as “claspers,” which are used to hold the female during mating and mate guarding. Clasper morphology has traditionally been important for clam shrimp taxonomy and classification, but little is known about how the males actually use the claspers during amplexus (clasping). Homologies of the various clasper parts (“movable finger,” “large palp,” “palm,” “gripping area,” and “small palp”) have long been discussed between the three clam shrimp taxa, and studies have shown that only some structures are homologous while others are convergent (“partial homology”). We studied the clasper functionality in four spinicaudatan species using video recordings and scanning electron microscopy, and compared our results with other clam shrimp groups. General mating behavior and carapace morphology was also studied. Generally, spinicaudatan and laevicaudatan claspers function similarly despite some parts being nonhomologous. We mapped clasper morphology and functionality aspects on a branchiopod phylogeny. We suggest that the claspers of the three groups were adapted from an original, simpler clasper, each for a “stronger” grip on the female's carapace margin: 1) Spinicaudata have two clasper pairs bearing an elongated apical club/gripping area with one setal type; 2); Cyclestherida have one clasper pair with clusters of molariform setae on the gripping area and at the movable finger apex; and 3) Laevicaudata have one clasper pair, but have incorporated an additional limb portion into the clasper palm and bear a diverse set of setae. J. Morphol. 278:523–546, 2017. © 2017 Wiley Periodicals, Inc.