Lynceus Brachyurus Research Articles

Multilocus phylogenetics of smooth clam shrimps (Branchiopoda, Laevicaudata)

AbstractLaevicaudatan branchiopods, also called ‘smooth clam shrimps’ or ’pea shrimps’, are rare crustaceans found exclusively in temporary, small freshwater bodies, which stay dry most of the year. Only 42 laevicaudatan species have been described so far, 90% of which belong to the genus Lynceus. The first multilocus phylogeny of the group is provided here, based on 15 Lynceus species from North and South America, Europe, Africa, Asia, Australia and New Caledonia and using nine molecular markers (two mitochondrial and seven nuclear genes, including newly designed primers). Genetic data suggest populations of Lynceus brachyurus from Europe and North America to represent a complex of cryptic species and sister group to all other laevicaudatans. Species from Thailand, Japan, Mongolia and China formed a distinct East Asian clade. A Southern Hemisphere (Gondwanaland) clade, composed of Chilean, Australian and New Caledonian taxa, was found weakly clustering with an African Lynceus species. Relaxed molecular clock analyses indicate a Pangean origin of Laevicaudata, with further diversification due to vicariance and the continued splitting of continents. Rostrum characters, which are particularly relevant for laevicaudatan systematics, were re‐evaluated and provide morphological evidence supporting molecular clades. Our worldwide overview of Laevicaudata evolution highlights that recent sampling from Africa and South America is scarce, and that further DNA efforts should focus on Paralimnetis and Lynceiopsis species.

Toxic Effects of Fine Plant Powder Impregnated With Avermectins on Mosquito Larvae and Nontarget Aquatic Invertebrates.

The toxic effects of an avermectin-impregnated fine plant powder (AIFP) against larval Aedes aegypti L. (Diptera: Culicidae), Culex modestus Ficalbi (Diptera: Culicidae), and Anopheles messeae Falleroni (Diptera: Culicidae), as well as selected nontarget aquatic invertebrates, were studied under laboratory conditions. The possibility of trophic transfer of avermectins (AVMs) through the food chain and their toxic effects on predaceous species fed AIFP-treated mosquito larvae was also evaluated. Among mosquitoes, Anopheles messeae were the most sensitive to AIFP, while Cx. modestus exhibited the least sensitivity to this formulation. Among nontarget aquatic invertebrates, the greatest toxicity of AIFP was observed for benthic species (larval Chironomus sp. Meigen (Diptera: Chironomidae), whereas predators (dragonflies, water beetles, and water bugs) exhibited the lowest AIFP sensitivity. AIFP sensitivity of the clam shrimp Lynceus brachyurus O. F. Muller (Diplostraca: Lynceidae), the phantom midge Chaoborus crystallinus De Geer (Diptera: Chaoboridae), and the mayfly Caenis robusta Eaton (Ephemeroptera: Caenidae) was intermediate and similar to the sensitivity of the mosquito Cx. modestus. However, these nontarget species were more resistant than An. messeae and Ae. aegypti. Solid-phase extraction of mosquito larvae treated with AIFP and subsequent high-performance liquid chromatography (HPLC) analysis of the extracts revealed an AVM concentration of up to 2.1 ± 0.3 μg/g. Feeding the creeping water bug Ilyocoris cimicoides L. (Hemiptera: Naucoridae) on the AIFP-treated mosquito larvae resulted in 51% mortality of the predaceous species. But no toxicity was observed for Aeshna mixta Latreille (Odonata: Aeshnidae) dragonfly larvae fed those mosquito larvae. The results of this work showed that this AVM formulation can be effective against mosquito larvae.

Key environmental factors for the conservation of large branchiopods in farmland vernal pools — a case from a Central European diversity hotspot

Abstract Large branchiopod crustaceans are among the most threatened freshwater invertebrates of the world. They are regarded as a flagship group in the conservation of temporary waters, and knowledge of their ecology and conservation has developed rapidly in recent years, especially in the Mediterranean region. Little is known, however, about population functioning in species associated with vernal pools in cooler temperate areas, despite recent data showing a significant decline in their localities. The present study was conducted in western Poland, on a cluster of vernal pools known to host a Central European diversity hotspot of large branchiopods. We collected data on water and sediment quality, as well as hydroperiod length, to determine their effect on the occurrence of four large branchiopod species present in the ponds during our samplings: Eubranchipus grubii (Anostraca), Lynceus brachyurus (Laevicaudata), Cyzicus tetracerus (Spinicaudata) and Lepidurus apus (Notostraca). According to the statistical analyses, the presence or absence of particular species was affected by hydroperiod length, heavy metal content in the sediments of the marginal zone of the pond, total water phosphorus content, and pH value. Since previous studies show that deep and relatively small kettle-hole ponds seem to be the most vulnerable to human-induced changes in water nutrient contents and acidification, large branchiopod populations of this type of pools should be the first target for monitoring and active forms of protection, with the aim of maintaining the good water quality. Shallow shore-bursting type ponds are subject to increased heavy metal contamination and require activities reducing the influx of these elements from their agricultural catchment. Possible forms of protection are suggested, and the use of large branchiopods as an umbrella group in temporary waters of temperate climate is also discussed.

New Records of Clam Shrimp (Laevicaudata, Spinicaudata) from New York

Abstract We present records of 3 clam shrimp species from New York: 2 spinicaudatan species— Eulimnadia agassizii (Agassiz Clam Shrimp; the first record for the state) and Cyzicus sp. (the second record of the genus from the state)—and Lynceus brachyurus (Laevicaudata; Holarctic Clam Shrimp; first reported from New York in 1883, is herein documented from the Hudson Valley). Some of the temporary waters that these species inhabit are protected by wetland regulations, but rain-puddle habitat on or along unimproved roads has no such protection.

Laevicaudata catalogus (Crustacea: Branchiopoda): an overview of diversity and terminology

The Laevicaudata (smooth clam shrimp) are a small group of freshwater bivalved branchiopod crustaceans in need of taxonomic revision. Here the extant Laevicaudata are defined and diagnosed according to modern standards, and synapomorphies are listed, discussed, and illustrated. A catalogue of the Laevicaudata is presented with synonyms and some taxa are partially revised. One hundred and three recent laevicaudatan taxa are presented, of which 39 are considered valid species. Chresonyms are provided for taxa redescribed according to modern standards. Furthermore we designate a neotype for Lynceus brachyurus Müller, 1776. This species catalogue will provide a basis for further taxonomic revision and phylogenetic work within the Laevicaudata.

Cyst morphology of large branchiopod crustaceans (Anostraca, Notostraca, Laevicaudata, Spinicaudata) in western Poland

AbstractThe morphology of resting eggs was studied in Polish populations of 7 large branchiopod species:Branchipus schaefferi,Chirocephalus shadini,Eubranchipus grubii(Anostraca),Lepidurus apus,Triops cancriformis(Notostraca),Lynceus brachyurus(Laevicaudata), andCyzicus tetracerus(Spinicaudata). Scanning electron microscope photographs for each species are also provided. In some species (especiallyE. grubii), the ornamentation and size of cysts from Polish populations significantly differ from literature data for populations from other parts of Europe. We also present a key to determination of the studied species based on cyst morphology and discuss whether those traits are useful in surveys of new localities of endangered large branchiopods.

On the natural history and functional morphology of the clam shrimp, Lynceus brachyurus Müller (Branchiopoda: Laevicaudata)

The small bivalved clam shrimp, Lynceus brachyurus Muller, 1776, is seasonally abundant in two long-duration vernal pools in central Ohio, USA. Previously unrecognized anatomical features include dorsal patches of spines or filaments on posterior trunk segments, diverse medial spines on the dorsal lobes of the leg exopods, two-segmented dorso-lateral appendages on the male eleventh segment, and brown coloration on portions of male legs. It can swim actively, but is basically benthic. Females are sometimes totally quiescent. Gut contents, observation, and experiments showed a diet of substrate scrapings, suspended detritus, and small zooplankton. Females ovulate following male stimulation and produce new egg batches without an intervening molt. Repeated matings are needed to produce the large egg masses often carried. Larvae were first collected in late February, and adults in late April. Lynceus was found in nine of the eleven years studied, being absent in the years that followed two years of high rainfall.

Ultrastructural analysis of the ovary and oogenesis in Spinicaudata and Laevicaudata (Branchiopoda) and its phylogenetic implications

Recent molecular studies have indicated a close relationship between Crustacea and Hexapoda and postulated their unification into the Pancrustacea/Tetraconata clade. Certain molecular analyses have also suggested that the crustacean lineage, which includes the Branchiopoda, might be the sister group of Hexapoda. We test this hypothesis by analyzing the structure of the ovary and the ultrastructural features of oogenesis in two branchiopod species, Cyzicus tetracerus and Lynceus brachyurus, representing two separate orders, Spinicaudata and Laevicaudata, respectively. The female gonads of these species have not been investigated before. Here, we demonstrate that in both studied species the ovarian follicles develop inside characteristic ovarian protrusions and comprise a germline cyst surrounded by a simple somatic (follicular) epithelium, supported by a thin basal lamina. Each germline cyst consists of one oocyte and three supporting nurse cells, and the oocyte differentiates relatively late during ovarian follicle development. The synthesis of oocyte reserve materials involves rough endoplasmic reticulum and Golgi complexes. The follicular cells are penetrated by a complex canal system and there is no external epithelial sheath covering the ovarian follicles. The structure of the ovary and the ultrastructural characteristics of oogenesis are not only remarkably similar in both Cyzicus and Lynceus, but also share morphological similarities with Notostraca as well as the basal hexapods Campodeina and Collembola. Possible phylogenetic implications of these findings are discussed.

Mating behaviour in laevicaudatan clam shrimp (Crustacea, Branchiopoda) and functional morphology of male claspers in a phylogenetic context: a video-based analysis.

Clam shrimps are freshwater branchiopod crustaceans which often present complicated breeding systems including asexual reproduction (parthenogenesis) and mixed mating systems (in androdioecious species both selfing and outcrossing occurs due to the co-presence of hermaphrodites and males). Reproductive patterns of Spinicaudata, which contains most clam shrimp species, have received much attention. Another group of clam shrimps, Laevicaudata, which holds a key position in branchiopod phylogeny, has practically not been studied. As a part of the mating process, males clasp to the carapace margin of the females with a pair (or two pairs) of anterior trunk limbs modified as claspers. Previous studies have shown that clasper morphology is important in a phylogenetic context, and that some parts of the claspers in Spinicaudata and Laevicaudata may have undergone a remarkable parallel evolution. Here we have used video microscopy to study aspects of the mating behaviour, egg extrusion, and fertilization in Lynceus brachyurus (Laevicaudata). It is shown that fertilization is likely to be external and that the peculiar tri-lobed lateral lamellae of female's hind body assist in guiding the egg mass to the exopodal egg carriers where they are collected by their distal setation. The functional morphology of the male claspers was studied in detail by close-up video recordings. The movable “finger” of the clasper bends around the female's carapace edge and serves to hold the female during mating. The larger palp grasps around the female carapace margin in a way very similar to the movable “finger”, possibly indirectly providing sensory input on the “finger” position. A brief comparative study of the claspers of a spinicaudatan clam shrimp showed both similarities and differences to the laevicaudatan claspers. The presence of two pairs of claspers in Spinicaudata seems to give males a better hold of the female which may play a role during extended mate guarding.

НОВЫЕ МАТЕРИАЛЫ К ФАУНЕ ANOSTRACA, NOTOSTRACA, CONCHOSTRACA УДМУРТСКОЙ РЕСПУБЛИКИ

The list of species of Anostraca, Notostraca and Conchostraca of Udmurtia is compiled for the first time. The list contains 6 species: Anostraca – 1, Conchostraca – 3, and Notostraca – 3 species. Lepidurus couesii Packard, 1875, Lynceus brachyurus Müller, 1776 and Branchipodopsis affinis G.O. Sars, 1901 are recorded in the Udmurt Republic for the first time. This is the first record of Lepidurus couesii Packard, 1875 in the European part of Russia, and the second Russian record since 1876.

Larval development of japanese “conchostracans”: Part 3, larval development of lynceus biformis (crustacea, branchiopoda, laevicaudata) based on scanning electron microscopy and fluorescence microscopy

For comparison with the remarkable larvae of the laevicaudatan (clam shrimp) Lynceus brachyurus, a basic description of the larval sequence of another laevicaudatan branchiopod, the Japanese Lynceus biformis, is provided. Four larval stages have been identified, ranging in size from 258 to 560 μm in length. The first stage has no flattened dorsal shield, in contrast to the three following stages, in which such a shield is present. During development, the only significant changes to the naupliar appendages occur in the antenna at the molt from stage 1 to 2, with the addition of a fourth apical seta to the endopod and a change in the form of the naupliar process, used for food manipulation, from a long, unbranched, pointed spine to a bifid structure. In addition, buds of trunk limbs (five pairs) first appear externally in stage 4 but can be recognized through the cuticle in the previous stage. The larval sequence and larval morphology of L. biformis differ from those of L. brachyurus in at least two respects. L. brachyurus has a dorsal shield in the earliest known stages, but such a shield is lacking in the first stage of L. biformis. Another difference is that L. brachyurus has a huge, flattened, kidney-shaped labrum, whereas that of L. biformis is smaller and bears four robust, denticulate spines on the distal margin. Based on out-group comparison, the morphology of L. biformis, at least in these respects, is likely to represent the ancestral morphology. Despite the partly peculiar morphology of the larvae of Lynceus species, they share many similarities with other branchiopod larvae, at least two of which, the naupliar swimming/feeding apparatus and the mode of development of the trunk limbs, could be considered synapomorphies for the Branchiopoda.

Diversity and conservation status of large branchiopods (Crustacea) in ponds of western Poland

A survey on temporary ponds has been conducted in search for large branchiopod crustaceans (Anostraca, Notostraca, Spinicaudata and Laevicaudata) in Wielkopolska province (western Poland). 728 pools have been studied and large branchiopods have been found in 221 of them. Seven species have been recorded, including three anostracans: Branchipus schaefferi , Chirocephalus shadini and Eubranchipus grubii ; two notostracans: Lepidurus apus and Triops cancriformis ; one spinicaudatan, Cyzicus tetracerus and one laevicaudatan, Lynceus brachyurus . According to the analysis of co-occurrence, the species form three groups, differing in habitat preferences and conservation status. The number of species shows that the diversity of globally threatened large branchiopods is still relatively high in the region. On the other hand, their conservation status is highly diverse and in most species unfavourable. Distribution of all species is highly clustered: large branchiopods have been generally found in 33 UTM squares (10 × 10 km) of 96 squares studied. However, only two species, i.e. E. grubii and L. apus occurred in more than five such squares and could be assessed as moderately widespread. Most water bodies inhabited by large branchiopods occur in groups forming patches of suitable habitats which are dispersed among prevailing seemingly unsuitable areas. Sustaining the existence of large metapopulations seems, therefore, to be essential for conservation of branchiopod species diversity. Field observations also bring some examples of human activities unintentionally supporting the branchiopod conservation.

The development of the nervous system in Laevicaudata (Crustacea, Branchiopoda): insights into the evolution and homologies of branchiopod limbs and ‘frontal organs’

We investigated the development of the external morphology and of the nervous system in Lynceus biformis and Lynceus brachyurus (Laevicaudata, Branchiopoda), by using immunohistochemical methods in combination with a confocal laser scanning analysis. In both Lynceus species, a free-swimming nauplius larva, equipped with three appendages, hatches from resting eggs. Despite their close phylogenetic relationship to each other, considerable differences are present in their external morphology. Hatching L. brachyurus larvae are equipped with a large and flattened labrum, where in contrast, the L. biformis larvae possess a smaller labrum with four conspicuous posteriorly directed spines at its margin. Despite these differences, the development of the nervous system is quite similar in both species. The hatching larvae are equipped with a naupliar nervous system, and only in the more advanced stages, the development of the ventral nerve cord starts. Furthermore, our investigation into the nervous system provided insights into architecture and evolution of protocerebral sensory organs, the dorsal setae field and the dorsal frontal organ, only present in Laevicaudata. The identification of frontal filaments with an associated frontal filament organ in Lynceus revealed—after a comprehensive comparison with other branchiopods—that these organs exist throughout Branchiopoda and are comparable to those in other crustaceans. Additionally, our results of the peripheral nervous system analysis showed that the innervation pattern of the naupliar appendages (antenna and mandible) and the trunk appendages could be serially homologized, despite much difference in gross morphology of these. Based on the innervation pattern of limbs, we suggest that the larval uniramous mandibular palp, found in the larvae of all ‘large’ branchiopods, is largely exopodal of nature (contrary to most earlier statements) and that the endopodite of the trunk limbs consists of only one distal endite-like segment (confirming some earlier statements) and not of three as proposed by others.

Resting eggs of conchostracans and their adaptive possibilities

The dimensional specifications and morphology of the resting eggs of ten conchostracan species (Cyzicus tetracerus, Eocyzicus orientalis, Caenestheria davidi, C. perrieri, Leptestheria dahalacensis, Eoleptestheria ticinensis, Limnadia lenticularis, Imnadia yeyetta, Lynceus brachyurus, and L. dauricus) have been studied. The data are presented on the number of eggs produced by females. Unlike other crustaceans, no correlation between the eggs size and their quantity was found. Although the number of eggs can vary one to two times, their size is relatively stable (variation coefficient <5%). The wet (1.5–5.4 μg) and dry (0.41–0.93 μg) weights of eggs and the share of clutch (11.4–21.6%) in the female’s wet weight were determined for eight species. The polyfunctionality of the external envelope of the egg was considered under anhydrobiosis conditions.

Larval development ofLynceus brachyurus (Crustacea, Branchiopoda, Laevicaudata): Redescription of unusual crustacean nauplii, with special attention to the molt between last nauplius and first juvenile

The larval development of "conchostracans" has received only scattered attention. Here I present the results of a study on the larval (naupliar) development and the metamorphosis of Lynceus brachyurus, a member of the bivalved branchiopod order the Laevicaudata. Lynceus brachyurus is the only species of the "Conchostraca" in Denmark. The phylogenetic position of the Laevicaudata has traditionally been a source of controversy, and this study does not solve the question completely. This work focuses on features potentially important for phylogeny. The general appearance of the larvae of L. brachyurus has been known for more than a century and a half, and some of its unique features include a large, larval dorsal shield; a huge, plate-like labrum; and a pair of immovable, horn-like antennules. However, many details relating to limb morphology, potentially important for phylogeny, have not been studied previously. Based on size categories, five or six larval stages can be recognized. The larvae approximately double their length and width during development (length: 230-520 microm). Most morphological features stay largely unchanged during development, but the antennal coxal masticatory spines are significant exceptions: they become bifid after one of the first molts. In all larval stages only the antennae and the mandibles actively move. In late naupliar stages the trunk limbs become visible as rows of laterally placed, undeveloped, and still immovable lobes. Swimming is performed by the antennae, whereas the mandibles appear to be involved mainly in feeding, as in other branchiopod larvae. The last naupliar stage undergoes a small metamorphosis to the first juvenile stage, the details of which in part were studied by following the premolt juvenile condition through the cuticle of the last stage nauplius. Among other changes there is a characteristic change in the shape and morphology of the univalved dorsal naupliar shield to a bivalved juvenile carapace. The general morphologies of the antennae and the mandibles are very similar to those of other branchiopod larvae and fall well within the "branchiopod naupliar feeding apparatus" recognized as a branchiopod synapomorphy by Olesen (2003), but some specific features shared with the larvae of other "conchostracans" are also identified. These special "conchostracan" features include: 1) a similar antennular setation; 2) a similar comb-like setulation of the bifid antennal coxal processes; and 3) mandibular palpsetae with setules condensed. In light of recent suggestions concerning branchiopod phylogeny (Cyclestheria as a sister group to the Cladocera), these similarities probably do not support a monophyletic "Conchostraca" but rather are symplesiomorphies of this taxon. A final decision must await a phylogenetic analysis of a more complete set of characters.

Physical-chemical influences on vernal zooplankton community structure in small lakes and wetlands of Wisconsin, U.S.A.

We sampled zooplankton communities from 54 small water bodies distributed throughout Wisconsin to evaluate whether a ‘snap-shot’ of zooplankton community structure during early spring could be used for the purpose of differentiating lakes from wetlands. We collected a single set of zooplankton and water chemistry data during a one-month time window (synchronized from south to north across the state) from an open water site in each basin as a means to minimize and standardize sampling effort and to minimize cascading effects arising from predator–prey interactions with resident and immigrant aquatic insect communities. We identified 53 taxa of zooplankton from 54 sites sampled across Wisconsin. There was an average of 6.83 taxa per site. The zooplankton species were distributed with a great deal of independence. We did not detect significant correlations between number of taxa and geographic region or waterbody size. There was a significant inverse correlation between number of taxa and the concentration of calcium ion, alkalinity and conductivity. One pair of taxa, Lynceus brachyurus and Chaoborus americanus, showed a significant difference in average duration of sites of their respective occurrence. All other pairs of taxa had no significant difference in average latitude, waterbody surface area, total phosphorus, total Kjeldahl nitrogen, alkalinity, conductivity, calcium ion, sulfate, nitrate, silicate or chloride. Taxa were distributed at random among the sites – there were no statistically significant pairs of taxa occurring together or avoiding each other. Multivariate analysis of zooplankton associations showed no evidence of distinct associations that could be used to distinguish lakes from wetlands. Zooplankton community structure appears to be a poor tool for distinguishing between lakes and wetlands, especially at the relatively large scale of Wisconsin (dimension of about 500 km). The data suggest that a small body of water in Wisconsin could be classified as a wetland if it persists in the spring and summer for only about 4 months, and if it is inhabited by Lynceus brachyurus, Eubranchipus bundyi, and if Chaoborus americanus and Chydorus brevilabris are absent.

Relative availability and utilization of algae in two subarctic rivers

The ash free dry weight of algae in the plankton, epilithon and gut of predominant herbivores was determined between June 1975 and July 1976 in 2 subarctic Canadian rivers (Baker Creek, Yellowknife River). Algae usually represented < 1% of suspended solids in both rivers and up to 8o% of material attached to rocks. While they were never found in the gut of rotifers and copepod nauplii in Baker Creek, they accounted for 0.5% of the contents of the planktonic conchostracon Lynceus brachyurus. Due to the inpalatable colonial structure of most attached algal species, the zoobenthos (mainly mayfly nymphs) contained only a few cells. Size selection against large fragments of detritus resulted in increased consumption of algae (5–13%) by Simulium venustum, S. decorum and S. arcticum. In the Yellowknife River, Diaptomus ashlandi (CIII–CVI stages) and Holopedium gibberum contained only a few (30–50) algal cells during summer. Because of size selection, large algae were not ingested by these species. The inpalatability of colonial algae greatly reduced consumption in the predominant zoobenthic species, Ephemerella coxalis and E. margarita. While all species digested diatoms, the Chlorophyta usually passed through the gut unharmed. It is concluded that algae are not an important energy source for invertebrates in either stream.

The occurrence, life cycle, and pathogenicity of Echinuria uncinata (Rudolphi, 1819) Soloviev, 1912 (Spirurida, Nematoda) in waterfowl at Delta, Manitoba.

In the Delta Marsh, third-stage Echinuria uncinata juveniles were found in Daphnia magna, D. pulex, Simocephalus vetulus, and Gammarus lacustris. Daphnia magna, the major host, were found infected from late May to early November with a peak of 108 parasites per 100 Daphnia in early August. Experimentally, D. magna, D. pulex, Ceriodaphnia reticulata, C. acanthina, S. vetulus, Moina macrocopa, Eurycercus lamellatus, G. lacustris, Hyallela azteca, Chirocephalopsis bundyi, and Lynceus brachyurus became infected when exposed to E. uncinata eggs. Parasites developed to the infective stage in D. magna and D. pulex in 30 days at 15 °C and in 10 days at 20–24 °C.In mallard ducks, E. uncinata completed the fourth molt 20 days after infection; male worms were sexually mature after 30 days and females oviposited 40 days after infection. Parasites grew faster in 1-week-old Delta mallards than in 2- and 3-month-old birds. Adult nematodes were located beneath the mucosal layer at the junction of the proventriculus and gizzard where granulomas formed after 30 days. The number of granulomas was correlated with the number of parasites. Mallards, pintails, gadwalls, lesser scaup, common eiders, and domestic geese were more susceptible to Echinuria infection than were shovellers, blue-winged teal, redheads, ruddy ducks, and American coots. Parasite eggs died when frozen but 50% survived 85 days when dried on filter paper. Echinuria uncinata can survive winter in resident mallards.

A New Estheriid Conchostracan with a Review of the Other North American Forms

The order Conchostraca (subclass Branchiopoda) was established by G. 0. Sars in 1867. The terms Phyllopoda and Eubranchiopoda have been used to designate a Division including the three orders Anostraca (fairy shrimp,), Notostraca (tadpole shrimp), and the Conchostraca (the clam or claw shrimp). Conchostracans are frequently encountered in temporary, fresh water pools or ponds, and just as frequently present problems in proper identification. The history of the classification of the conchostracans is a devious and confusing one. In the order there are now recognized five families. However, only four families are known from North America, the Lynceidae, Limnadiidae, Leptestheriidae, and the Cyzicidae. The family Lynceidae, Stebbing 1902, is represented by the genus Lynceus 0. F. Muller, 1785, an easily recognized group with a wide North American distribution. Lynceus brachyurus 0. F. Muller, with the widest distribution, is found in Europe and Asia as well as scattered over most of North America. The subspherical, bivalved shell (the two valves form a sphere) with no lines of growth (fig. 1) is characteristic of the genus as is the presence of but a sinigle pair of claspers or claws in the male. The truncated rostrum of the male and the twosegmented first antennae (fig. 11) are also unique for this group which is represented in North America by four species: L. brachyurus 0. F. Muller, 1785, L. gracilicornis (Packard) 1871, L. mucronatus (Packard) 1875, and L. brevifrons (Packard) 1877. Another family with distinctive characters is the Limnadiidae Baird, 1849, including the North American genera Limnadia Brongniart, 1820, and Eulimnadia Packard, 1883. The presence of a pyriform frontal organ on the dorsal surface of the head is diagnostic for the limnadiids (figs. 12, 13). The shell of the limnadiids is extremely variable and is characterized by the absence of or poorly developed umbo region (figs. 2, 3). The members of the genus Eulimnadia possess a conspicuous spine, at the base of the cercopoci, on the ventral surface of the telson. This spine is absent in the genus Limnadia. The North American Eulimnadia have been reviewed by Mattox (1945a). Limnadia lenticularis (Linne) (=L. amiericana Morse) is the only representative of that genus in North America. The remaining types referred to as the estheriids are those that present the problems confrontiag us at this time. These are the conchostracans most frequently encountered and are the largest in the order, some grow to 16 mm in shell length. The name Estheria, as a generic name for the group, was first applied by Ruppell in 1837. This namie was incorrectly used for many years, since seven years earlier Estheria had been established for a genus of parasitic Diptera by Robineau-Desvoidy (1830). The name is currently used for this