Free-swimming Larval Stage Research Articles

On the genetic diversity and protoconch variability of snails of the genus Caspiohydrobia Starobogatov, 1970 (Caenogastropoda: Hydrobiidae)

The genus Caspiohydrobia belongs to the less-known taxa of the family Hydrobiidae. According to a conchologically-based system, developed in the 1970-1980s by Yaroslav I. Starobogatov and his co-authors, the genus includes around 30 living species. However, the most recent data based on molecular evidence indicate that most (if not all) of these morpho-species should be considered junior synonyms of a widely distributed species, Ecrobia grimmi. However, the vast majority of nominal species described in the genus Caspiohydrobia have never been reassessed by molecular taxonomic methods. In this work, the genetic diversity of Caspiohydrobia spp. was studied based on a large sample taken in Lake Elanach in the Kurgan region (Trans-Urals, Russia). Conchological analysis made it possible to identify 7 nominal species of the genus Caspiohydrobia in the sample, whereas genetically all the studied individuals were the same, sharing a single COI haplotype. A phylogenetic tree of Ecrobia spp. has shown that all studied genetically specimens of Caspiohydrobia (including newly sequenced ones) forms a separate cluster on a cladogram, which can be identified as Ecrobia grimmi s. lato. However, a higher genetic diversity was revealed in a population of Caspiohydrobia spp., from a lake adjacent to the Caspian Sea (West Kazakhstan). Distinct morphotypes in the structure of the protoconch were found in samples of Caspiohydrobia spp. from the Caspian and Aral Seas. Although the characteristics of the protoconch (size, sculpture) do not give grounds to distinguish ‘good’ species within the group, the genetic and morphological diversity described in this work makes it possible to assume the ongoing microevolutionary processes within the discussed group and, possibly, the existence of young (incipient) species in Central Asia. Based on the study of the morphology of the protoconch, it has been suggested that the development of molluscs of the genus Caspiohydrobia (= Ecrobia, partim) can include the free-swimming larval stage; this hypothesis is, however, not supported by other evidence and needs to be carefully verified.

Microalgal biofilm induces larval settlement in the model marine worm Platynereis dumerilii.

A free-swimming larval stage features in many marine invertebrate life cycles. To transition to a seafloor-dwelling juvenile stage, larvae need to settle out of the plankton, guided by specific environmental cues that lead them to an ideal habitat for their future life on the seafloor. Although the marine annelid Platynereis dumerilii has been cultured in research laboratories since the 1950s and has a free-swimming larval stage, specific environmental cues that induce settlement in this nereid worm are yet to be identified. Here, we demonstrate that microalgal biofilm is a key settlement cue for P. dumerilii larvae, inducing earlier onset of settlement and enhancing subsequent juvenile growth as a primary food source. We tested the settlement response of P. dumerilii to 40 different strains of microalgae, predominantly diatom species, finding that P. dumerilii have species-specific preferences in their choice of settlement substrate. The most effective diatom species for inducing P. dumerilii larval settlement were benthic pennate species including Grammatophora marina, Achnanthes brevipes and Nitzschia ovalis. The identification of specific environmental cues for P. dumerilii settlement enables a link between its ecology and the sensory and nervous system signalling that regulates larval behaviour and development. Incorporation of diatoms into P. dumerilii culture practices will improve the husbandry of this marine invertebrate model.

A draft genome of the neritid snail Theodoxus fluviatilis.

The neritid snail Theodoxus fluviatilis is found across habitats differing in salinity, from shallow waters along the coast of the Baltic Sea to lakes throughout Europe. Living close to the water surface makes this species vulnerable to changes in salinity in their natural habitat, and the lack of a free-swimming larval stage limits this species' dispersal. Together, these factors have resulted in a patchy distribution of quite isolated populations differing in their salinity tolerances. In preparation for investigating the mechanisms underlying the physiological differences in osmoregulation between populations that cannot be explained solely by phenotypic plasticity, we present here an annotated draft genome assembly for T. fluviatilis, generated using PacBio long reads, Illumina short reads, and transcriptomic data. While the total assembly size (1045 kb) is similar to those of related species, it remains highly fragmented (N scaffolds = 35,695; N50 = 74 kb) though moderately high in complete gene content (BUSCO single copy complete: 74.3%, duplicate: 2.6%, fragmented: 10.6%, missing: 12.5% using metazoa n = 954). Nevertheless, we were able to generate gene annotations of 21,220 protein-coding genes (BUSCO single copy complete: 65.1%, duplicate: 16.7%, fragmented: 9.1%, missing: 9.1% using metazoa n = 954). Not only will this genome facilitate comparative evolutionary studies across Gastropoda, as this is the first genome assembly for the basal snail family Neritidae, it will also greatly facilitate the study of salinity tolerance in this species. Additionally, we discuss the challenges of working with a species where high molecular weight DNA isolation is very difficult.

The versatility of simplicity: Structures of Cardiocephaloides longicollis used for different purposes during cercarial transmission.

Transmission and infection strategies are critical for completing the life cycles of trematode parasites, which are characterized by complex life cycles involving multiple hosts and stages. Transmission between the first and second intermediate hosts typically relies on cercariae, a free-swimming larval stage that displays a series of behaviors to efficiently disperse, locate, attach to, and infect the next host. The aim of this study is to provide detailed information on behaviors used by furcocercariae (bifurcated tail) during its transmission from the snail to the fish host, using the laboratory-established model of Cardiocephaloides longicollis (Strigeidae). These cercariae are released from snails into seawater, where they swim, locate, penetrate the skin of fish, and encyst as metacercariae in their brain. In a series of in vivo assays, freshly-emerged cercariae were used to visually study their behavior and locomotion. Histopathology of experimentally infected gilthead seabreams with C. longicollis, taken at sequential post-infections times, were analysed to localize the migrating cercariae to the fish brain. Our results show that simplicity and versatility are the key features for the success of cercariae transmission by using their organs for different purposes. While 80% of the behavior was spent in a resting position, the most common swimming behavior was with tail-first, which is commonly described in furcocercariae to reach the host microhabitat. However, C. longicollis relies more on the furcae of the tail by using them as a propeller providing thrust and guidance when they swim, instead of using the tail stem. After attaching to the fish skin, cercariae rapidly creep on it using the oral- and ventral-suckers simulating a leech-like movement until they find a suitable penetration site. To penetrate, cercariae press the cephalic structures against the skin, while the ventral sucker anchors the cercariae to it. After this, they switch their locomotion to a slow peristaltic movement, opening the path through tissues with the help of their cephalic structures and anchoring their body with their surface spines. This is consistent with the post-penetration histological analyses, which suggested that C. longicollis cercariae move between the cells of the connective tissue and muscle fibers when migrating towards the fish's brain, without provoking relevant tissue damage or host responses. Understanding the versatility of cercarial structures to adapt to external conditions enriches our knowledge on parasites and their transmission ecology, opening the door to the design of avoidance methods in fish farms struggling with harmful parasites.

Ultrastructural observations on the oncomiracidium epidermis and adult tegument of Discocotyle sagittata, a monogenean gill parasite of salmonids

During their different life stages, parasites undergo remarkable morphological, physiological, and behavioral “metamorphoses” to meet the needs of their changing habitats. This is even true for ectoparasites, such as the monogeneans, which typically have a free-swimming larval stage (oncomiracidium) that seeks out and attaches to the external surfaces of fish where they mature. Before any obvious changes occur, there are ultrastructural differences in the oncomiracidium’s outer surface that prepare it for a parasitic existence. The present findings suggest a distinct variation in timing of the switch from oncomiracidia epidermis to the syncytial structure of the adult tegument and so, to date, there are three such categories within the Monogenea: (1) Nuclei of both ciliated cells and interciliary cytoplasm are shed from the surface layer and the epidermis becomes a syncytial layer during the later stages of embryogenesis; (2) nuclei of both ciliated cells and interciliary syncytium remain distinct and the switch occurs later after the oncomiracidia hatch (as in the present study); and (3) the nuclei remain distinct in the ciliated epidermis but those of the interciliary epidermis are lost during embryonic development. Here we describe how the epidermis of the oncomiracidium of Discocotyle sagittata is differentiated into two regions, a ciliated cell layer and an interciliary, syncytial cytoplasm, both of which are nucleated. The interciliary syncytium extends in-between and underneath the ciliated cells and sometimes covers part of their apical surfaces, possibly the start of their shedding process. The presence of membranous whorls and pyknotic nuclei over the surface are indicative of membrane turnover suggesting that the switch in epidermis morphology is already initiated at this stage. The body tegument and associated putative sensory receptors of subadult and adult D. sagittata are similar to those in other monogeneans.

Molluscicidal and parasiticidal activities of Eryngium triquetrum essential oil on Schistosoma mansoni and its intermediate snail host Biomphalaria glabrata, a double impact

BackgroundFreshwater snails are the intermediate hosts of a large variety of trematode flukes such as Schistosoma mansoni responsible for one of the most important parasitic diseases caused by helminths, affecting 67 million people worldwide. Recently, the WHO Global Vector Control Response 2017–2030 (GVCR) programme reinforced its message for safer molluscicides as part of required strategies to strengthen vector control worldwide. Here, we present the essential oil from Eryngium triquetrum as a powerful product with molluscicide and parasiticide effect against S. mansoni and the snail intermediate host Biomphalaria glabrata.MethodsIn the present study, we describe using several experimental approaches, the chemical composition of E. triquetrum essential oil extract and its biological effects against the snail B. glabrata and its parasite S. mansoni. Vector and the free-swimming larval stages of the parasite were exposed to different oil concentrations to determine the lethal concentration required to produce a mortality of 50% (LC50) and 90% (LC90). In addition, toxic activity of this essential oil was analyzed against embryos of B. glabrata snails by monitoring egg hatching and snail development. Also, short-time exposure to sublethal molluscicide concentrations on S. mansoni miracidia was performed to test a potential effect on parasite infectivity on snails. Mortality of miracidia and cercariae of S. mansoni is complete for 5, 1 and 0.5 ppm of oil extract after 1 and 4 h exposure.ResultsThe major chemical component found in E. triquetrum oil determined by GC-FID and GC/MS analyses is an aliphatic polyacetylene molecule, the falcarinol with 86.9–93.1% of the total composition. The LC50 and LC90 values for uninfected snails were 0.61 and 1.02 ppm respectively for 24 h exposure. At 0.5 ppm, the essential oil was two times more toxic to parasitized snails with a mortality rate of 88.8 ± 4.8%. Moderate embryonic lethal effects were observed at the concentration of 1 ppm. Severe surface damage in miracidia was observed with a general loss of cilia that probably cause their immobility. Miracidia exposed 30 min to low concentration of plant extract (0.1 ppm) were less infective with 3.3% of prevalence compare to untreated with a prevalence of 44%.ConclusionsEssential oil extracted from E. triquetrum and falcarinol must be considered as a promising product for the development of new interventions for schistosomiasis control and could proceed to be tested on Phase II according to the WHO requirements.

Sea urchin larvae show resilience to ocean acidification at the time of settlement and metamorphosis

Extensive research has shown that the early life stages of marine organisms are sensitive to ocean acidification (OA). Less is known, however, on whether larval settlement and metamorphosis may be affected, or by which mechanisms. These are key processes in the life cycle of most marine benthic organisms, since they mark the transition between the free swimming larval stage to benthic life. We investigated whether OA could affect the larval settlement success of the sea urchin Evechinus chloroticus, a key coastal species with ecological, economic and cultural importance in New Zealand. We performed four settlement experiments to test whether reduced seawater pH (ranging from 8.1 to 7.0, at an interval of ~0.2 pH units) alters larval settlement and metamorphosis success. Our results show that settlement success was not significantly reduced when the larvae were exposed to a range of reduced seawater pH treatments (8.1–7.0) at time of settlement (on direct effects). Similarly, when presented with crustose coralline algae (CCA) pre-conditioned in seawater pH of either pH 8.1 or 7.7 for 28 days, larval settlement success remained unaltered (on indirect effects). We conclude that competent larvae in this species are resilient to OA at time of settlement. Further research on a range of taxa that vary in settlement selectivity and behaviour is needed in order to fully understand the effects of OA on the life cycle of marine invertebrates and the consequences it might have for future coastal marine ecosystems.

Schistosome migration in the definitive host.

Schistosomes are parasitic blood flukes that infect >200 million people around the world. Free-swimming larval stages penetrate the skin, invade a blood vessel, and migrate through the heart and lungs to the vasculature of the liver, where maturation and mating occurs. From here, the parasite couples migrate to their preferred egg laying sites. Here, we compare and contrast what is known about the migration patterns within the definitive host of the three major species of human schistosome: Schistosoma mansoni, S. japonicum, and S. haematobium. We conclude that intravascular schistosomes are inexorable colonizers whose migration and egg laying strategy is profligate; all three species (and their eggs) can be found throughout the mesenteric venules, the rectal venous plexus, and, to a greater or lesser extent, the urogenital venous plexuses. In addition, it is common for parasite eggs to be deposited in locations that lack easy access to the exterior, further demonstrating the relentless exploratory nature of these intravascular worms.

Middle Triassic gastropods from the Besano Formation of Monte San Giorgio, Switzerland

For the first time gastropods from the Besano Formation (Anisian/Ladinian boundary) are documented. The material was collected from three different outcrops at Monte San Giorgio (Southern Alps, Ticino, Switzerland). The taxa here described are Worthenia (Humiliworthenia)? aff. microstriata, Frederikella cf. cancellata, ?Trachynerita sp., ?Omphaloptycha sp. 1 and ?Omphaloptycha sp. 2. They represent the best preserved specimens of a larger collection and document the presence in this formation of the clades Vetigastropoda, Neritimorpha and Caenogastropoda that were widespread on the Alpine Triassic carbonate platforms. True benthic molluscs are very rarely documented in the Besano Formation, which is interpreted as intra-platform basin sediments deposited in usually anoxic condition. Small and juvenile gastropods could have been lived as pseudoplankton attached to floating algae or as free-swimming veliger planktotrophic larval stages. Accumulations of larval specimens suggest unfavorable living conditions with prevailing disturbance in the planktic realm or mass mortality events. However, larger gastropods more probably were washed in with sediments disturbed by slumping and turbidite currents along the basin edge or storm activity across the platform of the time equivalent Middle San Salvatore Dolomite.

Host-pathogen interaction for larvae oysters with salinity dependent transmission

Mathematical models of host-pathogen interactions are proposed and analyzed. Here hosts are oyster population in a free-swimming larval stage and assumably live in the closed homogeneous environment. In terms of an epidemic, they are classified into two states, namely susceptible and infectious hosts. The epidemic model of oyster hosts with seasonal forced transmission is firstly described by the SIS model where the region of attraction, the existence of equilibrium points, their stability conditions, and upper and lower bounds on the attack rate are investigated. Then free-living pathogen is introduced in the oyster area. Numerical simulations are finally carried out by making use of the various salinity-dependent transmissions in support of the hypothesis that the lower the salinity level, the lower oyster’s immunity.

Evaluation of targeted copper sulfate (CuSO4) application for controlling swimmer's itch at a freshwater recreation site in Michigan.

Swimmer's itch has historically been controlled by applying copper sulfate (CuSO4) to lakes as a way to eliminate snails that serve as the intermediate hosts for swimmer's itch-causing parasites. CuSO4 is still sometimes applied specifically to areas of lakes where swimmer's itch severity is high. It is unclear whether targeted application of chemical molluscicides like CuSO4 is effective for controlling swimmer's itch. Previous research has found that the larval stage of the parasites responsible for swimmer's itch are released from infected snails and are concentrated by onshore and alongshore winds, and thus, may not be affected by such focused applications. In this study, we evaluated the impact of targeted CuSO4 application to a specific recreational swimming area in a lake in Michigan. We measured the effect on snail populations, as well as on the presence/abundance of swimmer's itch-causing parasites using qPCR. Ultimately, while CuSO4 was confirmed to significantly reduce populations of snails within the treatment area, it was found to have no significant impact on swimmer's itch-causing parasites in the water, likely due to the free-swimming larval stages (cercariae) moving into the treatment area from surrounding regions.

The Pronephros; a Fresh Perspective.

Contemporary papers and book chapters on nephrology open with the assumption that human kidney development passes through three morphological stages: pronephros, mesonephros, and metanephros. Current knowledge of the human pronephros, however, appears to be based on only a hand full of human specimens. The ongoing use of variations in the definition of a pronephros hampers the interpretation of study results. Because of the increased interest in the anamniote pronephros as a genetic model for kidney organogenesis we aimed to provide an overview of the literature concerning kidney development and to clarify the existence of a pronephros in human embryos. We performed an extensive literature survey regarding vertebrate renal morphology and we investigated histological sections of human embryos between 2 and 8 weeks of development. To facilitate better understanding of the literature about kidney development, a referenced glossary with short definitions was composed. The most striking difference between pronephros versus meso- and metanephros is found in nephron architecture. The pronephros consists exclusively of non-integrated nephrons with external glomeruli, whereas meso- and metanephros are composed of integrated nephrons with internal glomeruli. Animals whose embryos have comparatively little yolk at their disposal and hence have a free-swimming larval stage do develop a pronephros that is dedicated to survival in aquatic environments. Species in which embryos do not have a free-swimming larval stage have embryos that are supplied with a large amount of yolk or that develop within the body of the parent. In those species the pronephros is usually absent, incompletely developed, and apparently functionless. Non-integrated nephrons were not identified in histological sections of human embryos. Therefore, we conclude that a true pronephros is not detectable in human embryos although the most cranial part of the amniote excretory organ is often confusingly referred to as pronephros. The term pronephros should be avoided in amniotes unless all elements for a functional pronephros are undeniably present.

Genome size and lifestyle in gnesiotrochan rotifers.

Gnesiotrochan rotifers display a variety of life styles ranging from taxa with free-swimming larval and sessile adult stages to those with motile adult stages and colonial habits. Several explanations for the C- value enigma posits that genome size is correlated with lifestyle. To investigate this, 13 gnesiotrochan species representing nine genera were measured by flow cytometry. Genome sizes (1C) within Gnesiotrocha ranged from 0.05 pg (Hexarthra mira and Hexarthra fennica) to 0.25 pg (Sinantherina ariprepes). Genome sizes varied within genera and species; e.g., H. fennica (El Huérfano, Mexico) was estimated to be 15% larger than that of H. mira and H. fennica (Keystone Wetland, TX, USA). Gnesiotrochan genome sizes are similar to those reported within Ploima, which range from 0.06 pg (Brachionus rotundiformis, B. dimidiatus) to 0.46 pg (B. asplanchnoidis). Within Gnesiotrocha, genome size was found to be significantly smaller in sessile versus motile species as well as in solitary versus colonial species. To account for phylogenetic background, Linear Mixed Models with hierarchical taxonomic ranks showed that there is a taxonomic component underlying genome size. This study provides the first estimates of genome size within the superorder, providing a baseline for genomic and evolutionary studies within the group.

Noncanonical Hox, Etv4, and Gli3 gene activities give insight into unique limb patterning in salamanders.

Limb development in salamanders is unique among tetrapods in significant ways. Not only can salamanders regenerate lost limbs repeatedly and throughout their lives, but also the preaxial zeugopodial element and digits form before the postaxial ones and, hence, with a reversed polarity compared to all other tetrapods. Moreover, in salamanders with free-swimming larval stages, as exemplified by the axolotl (Ambystoma mexicanum), each digit buds independently, instead of undergoing a paddle stage. Here, we report gene expression patterns of Hoxa and d clusters, and other crucial transcription factors during axolotl limb development. During early phases of limb development, expression patterns are mostly similar to those reported for amniotes and frogs. Likewise, Hoxd and Shh regulatory landscapes are largely conserved. However, during late digit-budding phases, remarkable differences are present: (i) the Hoxd13 expression domain excludes developing digits I and IV, (ii) we expand upon previous observation that Hoxa11 expression, which traditionally marks the zeugopodium, extends distally into the developing digits, and (iii) Gli3 and Etv4 show prolonged expression in developing digits. Our findings identify derived patterns in the expression of key transcription factors during late phases of salamander limb development, and provide the basis for a better understanding of the unique patterning of salamander limbs.

A new species of trogloplacine crab of the genus Australocarcinus Davie, 1988 from a freshwater stream in Mahé, Seychelles (Crustacea, Brachyura, Chasmocarcinidae).

A new species of freshwater chasmocarcinid crab, Australocarcinus insperatus sp. n., is described from the Seychelles Islands in the Indian Ocean. This is the first record of the genus and the subfamily Trogloplacinae Guinot, 1986, from the Indian Ocean, with all other members previously recorded from Australia, New Britain, New Caledonia, and Palau in the Pacific Ocean. The disjunct distribution of Australocarcinus is unexpected considering all trogoplacines are believed to practice direct development, lacking free-swimming larval stages. The new species is morphologically most similar to A. riparius Davie, 1988, from Queensland, Australia, but can be distinguished from its three congeners on the basis of the structures of its carapace, ambulatory legs and male first gonopod.

Use of artificial seawater in the rearing of the fluvial prawn Macrobrachium yui larvae

The fluvial prawn Macrobrachium yui lives in caves during the free-swimming zoea larval stages, and the hatched larvae require cave stream water for normal development. To develop larval culture techniques for M. yui in the absence of cave stream water, we determined the relationship between salinity tolerance of hatched larvae and egg size in Macrobrachium species and tested the effect of rearing-water salinity (0, 1.5, 3.5, and 10.5 ppt) on larval survival during free-swimming zoea larval stages. When reared at 3.5-ppt salinity, the hatched larvae successfully settled on the bottom and had the highest survival, with more than 90% of the hatched larvae developing into postlarvae. This indicates that we can successfully rear the free-swimming zoea larvae using 3.5-ppt seawater in place of cave stream water (0.18- to 0.27-ppt salinities). To test the acclimation of the postlarvae to freshwater, we reared the postlarvae for 2 weeks after settling, under three treatment conditions (I: rearing at 0 ppt for 2 weeks; II: rearing at 1.7 ppt for the first week and 0 ppt for the second week; III: rearing at 3.5 ppt for 2 weeks), and compared their survival under the different conditions. The postlarvae had the highest survival (78%) in treatment II. This suggests that M. yui has the ability to gradually adapt to freshwater, after the free-swimming zoea larvae have settled on the bottom and developed into postlarvae. These results would be critically useful for larval culture of M. yui, helping to restock the wild with the larval prawns from captive stocks to maintain the endangered local population.

QPCR Detection of Early Life History StageChrysaora quinquecirrha(Sea Nettles) in Barnegat Bay, New Jersey

ABSTRACT Gaynor, J.J.; Bologna, P.A.X.; Restaino, D.J., and Barry, C.L., 2017. qPCR detection of early life history stage Chrysaora quinquecirrha (sea nettles) in Barnegat Bay, New Jersey. In: Buchanan, G.A.; Belton, T.J., and Paudel, B. (eds.), A Comprehensive Assessment of Barnegat Bay–Little Egg Harbor, New Jersey. The sea nettle Chrysaora quinquecirrha has become abundant in the Barnegat Bay estuary and frequently blooms in warm summer months. Various factors have been attributed to the increasing localized appearance of sea nettles and other jellyfish including eutrophication, overfishing, global warming, construction, and species introduction. Despite its abundance and frequent distribution within estuarine systems, very little work has been done to detect and quantify the early life history stages of this organism. Free-swimming larval stages of C. quinquecirrha can be detected and quantified using a quantitative polymerase chain reaction assay specific for the C. quinquecirrha 16S ribosomal (r)DNA...

Paternity Outcomes in the Freshwater Gastropod, Chilina dombeiana in the Biobío River, Chile.

Studying the mating system of obligate aquatic organisms that inhabit river ecosystems is important for understanding its evolution as well as the role of biological and environmental factors in modulating population dynamics and species distributional patterns. Here, we studied the reproductive strategy of the Chilean endemic freshwater snail, Chilina dombeiana, in the Biobío River, one of the largest rivers in Chile. This species has a low potential for dispersal given the absence of a free-swimming larval stage (benthic larval development) and given that adults have a low capacity for mobility. We hypothesized that: 1. Females would mate with different males (polyandry) resulting in intrabrood multiple paternity, 2. Individuals from closer sites would be more related than individuals from distant sites, and 3. Male parental contributions would be unevenly distributed within broods. Individuals from three different sites were sampled along the river: upper, mid, and river mouth. In the laboratory, hatching juveniles from a total of 15 broods were collected for paternity analyses. We used microsatellite markers and the programs GERUD and COLONY to determine whether multiple paternity exists and to estimate the contribution of different males to the brood. We found that multiple paternity was very common at all of the sites analyzed with as many as 8 males fertilizing a single female and a mean of 4.2 fathers per brood estimated by COLONY. Sire contribution was skewed to particular males in several broods. In addition, overall relatedness among broods for the three sites ranged from 0.17 to 0.45 with evidence of many half-siblings. Relatedness differed among the three sites. Particularly in upstream sites or in anthropogenically disturbed populations, the high levels of multiple paternity observed in C. dombeiana may be an efficient strategy to avoid inbreeding and prevent the loss of genetic diversity within populations.

Refined stratified-worm-burden models that incorporate specific biological features of human and snail hosts provide better estimates of Schistosoma diagnosis, transmission, and control.

BackgroundSchistosoma parasites sustain a complex transmission process that cycles between a definitive human host, two free-swimming larval stages, and an intermediate snail host. Multiple factors modify their transmission and affect their control, including heterogeneity in host populations and environment, the aggregated distribution of human worm burdens, and features of parasite reproduction and host snail biology. Because these factors serve to enhance local transmission, their inclusion is important in attempting accurate quantitative prediction of the outcomes of schistosomiasis control programs. However, their inclusion raises many mathematical and computational challenges. To address these, we have recently developed a tractable stratified worm burden (SWB) model that occupies an intermediate place between simpler deterministic mean worm burden models and the very computationally-intensive, autonomous agent models.MethodsTo refine the accuracy of model predictions, we modified an earlier version of the SWB by incorporating factors representing essential in-host biology (parasite mating, aggregation, density-dependent fecundity, and random egg-release) into demographically structured host communities. We also revised the snail component of the transmission model to reflect a saturable form of human-to-snail transmission. The new model allowed us to realistically simulate overdispersed egg-test results observed in individual-level field data. We further developed a Bayesian-type calibration methodology that accounted for model and data uncertainties.ResultsThe new model methodology was applied to multi-year, individual-level field data on S. haematobium infections in coastal Kenya. We successfully derived age-specific estimates of worm burden distributions and worm fecundity and crowding functions for children and adults. Estimates from the new SWB model were compared with those from the older, simpler SWB with some substantial differences noted. We validated our new SWB estimates in prediction of drug treatment-based control outcomes for a typical Kenyan community.ConclusionsThe new version of the SWB model provides a better tool to predict the outcomes of ongoing schistosomiasis control programs. It reflects parasite features that augment and perpetuate transmission, while it also readily incorporates differences in diagnostic testing and human sub-population differences in treatment coverage. Once extended to other Schistosoma species and transmission environments, it will provide a useful and efficient tool for planning control and elimination strategies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-016-1681-4) contains supplementary material, which is available to authorized users.

Cranial muscle development in frogs with different developmental modes: Direct development versus biphasic development

Normal development in anurans includes a free swimming larva that goes through metamorphosis to develop into the adult frog. We have investigated cranial muscle development and adult cranial muscle morphology in three different anuran species. Xenopus laevis is obligate aquatic throughout lifetime, Rana(Lithobates) pipiens has an aquatic larvae and a terrestrial adult form, and Eleutherodactylus coqui has direct developing juveniles that hatch from eggs deposited on leaves (terrestrial). The adult morphology shows hardly any differences between the investigated species. Cranial muscle development of E. coqui shows many similarities and only few differences to the development of Rana (Lithobates) and Xenopus. The differences are missing muscles of the branchial arches (which disappear during metamorphosis of biphasic anurans) and a few heterochronic changes. The development of the mandibular arch (adductor mandibulae) and hyoid arch (depressor mandibulae) muscles is similar to that observed in Xenopus and Rana (Lithobates), although the first appearance of these muscles displays a midmetamorphic pattern in E. coqui. We show that the mix of characters observed in E. coqui indicates that the larval stage is not completely lost even without a free swimming larval stage. Cryptic metamorphosis is the process in which morphological changes in the larva/embryo take place that are not as obvious as in normal metamorphosing anurans with a clear biphasic lifestyle. During cryptic metamorphosis, a normal adult frog develops, indicating that the majority of developmental mechanisms towards the functional adult cranial muscles are preserved.