The decline of kilkas, sturgeons and seals in the Caspian Sea: The potential of qualitative loop analysis for the cumulative assessment of multiple drivers of stress

Multiple stressors often act concomitantly on ecosystems but detection of species responses follows the "single species-single driver" strategy, and cumulative impacts are seldom considered. During 1990-2010, multiple perturbations in the Caspian Sea, led to the decline of kilka, sturgeon and Caspian seal populations. Specific causes for their collapse were identified but a cumulative assessment has never been carried out. Using loop analysis, a qualitative modelling technique suitable in poor-data contexts, we show how multiple drivers can be combined to assess their cumulative impact. We confirm that the decline of kilka, sturgeon and Caspian seal populations is compatible with a net effect of the concomitant perturbations. Kilkas collapse was certainly due to the outburst of M. leidyi and overfishing. In addition, the excess nutrient might have conspired to reduce these populations. The interplay between concurrent drivers produces trade-offs between opposite effects and ecosystem management must face this challenge.

Prediction of ctenophore Beroe ovata Bruguiere, 1789 habitat expansion and control of invading Mnemiopsis leidyi Agassiz, 1865 in the Ponto-Caspian basin associated with climate change

The seas of Ponto-Caspian basin (Black, Azov and Caspian) are exposed to species invasions, including harmful ctenophore Mnemiopsis leidyi A. Agassiz, 1865 and its predator Beroe ovata Bruguière, 1789. Current environmental conditions of invasive ctenophores M. leidyi and B. ovata occurrence, development and variability in the Ponto-Caspian basin have been compiled, based on own field observations and published data. Analysis of climatological data on basic environmental parameters (water temperature, salinity, chlorophyll concentration) and modeling approach were used to predict favourable for both ctenophores conditions, and changes of those conditions, associated with the climate variations. The role of B. ovata as a bio-controller of M. leidyi population has been assessed. Several climate change scenarios have been considered in this study i.e., uniform increase/decrease of SST throughout the year by 2 °C and only spring temperatures increase/decrease by 2 °C. The most sensitive to increase in SST in terms of M. leidyi reproduction duration are the western coast of the Black Sea and the Southern and Middle Caspian. On the other hand, B. ovata reproduction duration is expected to increase in several areas, including the northern coast of the Black Sea and the Southern and Middle Caspian. The coastal areas of the Black Sea and the transitional regions between the Southern and Middle Caspian are exposed to an earlier start of M. leidyi reproduction during warmer springs. Regarding B. ovata, the whole Black Sea is vulnerable to spring SST changes, but in the Caspian Sea B. ovata extends its reproduction duration only in the Middle Caspian during warmer springs. Since B. ovata consumes mostly M. leidyi, it is an important biocontrolling agent of M. leidyi, harmless for the ecosystem.

An impact of non-native species invasions on the Caspian Sea biota

The Caspian Sea is a large inland brackish basin, vulnerable to invaders due to its long isolation and considerable endemism among its native biota. A brief description of Caspian biota evolution until its modern state is given. The pathways and vectors of invasion and the ways of establishment of non-native species since the early 20th century are summarized. The newly established species are euryphilic, with high ecological plasticity, able to adapt to new environments and to affect their biodiversity. This review is based on unpublished field data, collected in 1999-2019 in the Northern, Middle and Southern Caspian, and on relevant published information. The arrival of non-native species occurred in three periods: (1) in the 1930s, deliberate introductions aimed at enriching commercial stocks and edible resources, (2) since 1952, the construction of the Volga-Don Canal led to the arrival of benthic foulers and macrophytes from ships; (3) since the early 1980s to present, ballast water tanks were mounted on ships, favoring the arrival of phyto- and zooplankton species. Most established non-native species reached the Caspian Sea via the Black Sea. They include both Black Sea native species and non-native species from the North Atlantic areas, which first arrived and established in the Black Sea. Few established non-native species came from brackish water; fresh water fishes were deliberately introduced to develop aquaculture. Though not numerous, these species became dominant in both benthos and plankton communities, where they replaced native Caspian species. Among them, the invading ctenophore Mnemiopsis leidyi, which had no predators, continues to thrive in the Caspian ecosystem, impoverishing its biodiversity and bio-resources. However, lately its natural predator, the ctenophore Beroe ovata, arrived and established in the Southern and Middle Caspian providing a chance for ecosystem recovery, as has already happened in the Black Sea.

Ctenophores and the evolutionary origin(s) of neurons

Ctenophores (commonly known as comb jellies) are among the earliest branching extant lineages of the animal kingdom. Here, I present a brief overview of the ctenophore nervous system, discussing its cellular architecture and molecular composition, as well as insights it offers into the early evolution of neurons and chemical neurotransmission.

Ctenophores are direct developers that reproduce continuously beginning very early after hatching

A substantial body of literature reports that ctenophores exhibit an apparently unique life history characterized by biphasic sexual reproduction, the first phase of which is called larval reproduction or dissogeny. Whether this strategy is plastically deployed or a typical part of these species’ life history was unknown. In contrast to previous reports, we show that the ctenophore Mnemiopsis leidyi does not have separate phases of early and adult reproduction, regardless of the morphological transition to what has been considered the adult form. Rather, these ctenophores begin to reproduce at a small body size and spawn continuously from this point onward under adequate environmental conditions. They do not display a gap in productivity for metamorphosis or other physiological transition at a certain body size. Furthermore, nutritional and environmental constraints on fecundity are similar in both small and large animals. Our results provide critical parameters for understanding resource partitioning between growth and reproduction in this taxon, with implications for management of this species in its invaded range. Finally, we report an observation of similarly small-size spawning in a beroid ctenophore, which is morphologically, ecologically, and phylogenetically distinct from other ctenophores reported to spawn at small sizes. We conclude that spawning at small body size should be considered as the default, on-time developmental trajectory rather than as precocious, stress-induced, or otherwise unusual for ctenophores. The ancestral ctenophore was likely a direct developer, consistent with the hypothesis that multiphasic life cycles were introduced after the divergence of the ctenophore lineage.

Studying Ctenophora WBR Using Mnemiopsis leidyi

Ctenophores, also known as comb jellies, are a clade of fragile holopelagic, carnivorous marine invertebrates, that represent one of the most ancient extant groups of multicellular animals. Ctenophores show a remarkable ability to regenerate in the adult form, being capable of replacing all body parts (i.e., whole-body regeneration) after loss/amputation. With many favorable experimental features (optical clarity, stereotyped cell lineage, multiple cell types), a full genome sequence available and their early branching phylogenetic position, ctenophores are well placed to provide information about the evolution of regenerative ability throughout the Metazoa. Here, we provide a collection of detailed protocols for use of the lobate ctenophore Mnemiopsis leidyi to study whole-body regeneration, including specimen collection, husbandry, surgical manipulation, and imaging techniques.

Isolation and Maintenance of In Vitro Cell Cultures from the Ctenophore Mnemiopsis leidyi

The ability to isolate, monitor, and examine specific cells of interest enables targeted experimental manipulations that would otherwise be difficult to perform and interpret in the context of the whole organism. In vitro primary cell cultures derived from ctenophores thus serve as an important tool for understanding complex cellular and molecular interactions that take place both within and between various ctenophore cell types. Here we describe methods for reliably generating and maintaining primary cell cultures derived from the lobate ctenophore Mnemiopsis leidyi that can be used for a wide variety of experimental applications.

Invasion genomics uncover contrasting scenarios of genetic diversity in a widespread marine invader

Invasion rates have increased in the past 100 y irrespective of international conventions. What characterizes a successful invasion event? And how does genetic diversity translate into invasion success? Employing a whole-genome perspective using one of the most successful marine invasive species world-wide as a model, we resolve temporal invasion dynamics during independent invasion events in Eurasia. We reveal complex regionally independent invasion histories including cases of recurrent translocations, time-limited translocations, and stepping-stone range expansions with severe bottlenecks within the same species. Irrespective of these different invasion dynamics, which lead to contrasting patterns of genetic diversity, all nonindigenous populations are similarly successful. This illustrates that genetic diversity, per se, is not necessarily the driving force behind invasion success. Other factors such as propagule pressure and repeated introductions are an important contribution to facilitate successful invasions. This calls into question the dominant paradigm of the genetic paradox of invasions, i.e., the successful establishment of nonindigenous populations with low levels of genetic diversity.

Neuropeptide repertoire and 3D anatomy of the ctenophore nervous system

Ctenophores are gelatinous marine animals famous for locomotion by ciliary combs. Due to the uncertainties of the phylogenetic placement of ctenophores and the absence of some key bilaterian neuronal genes, it has been hypothesized that their neurons evolved independently. Additionally, recent whole-body, single-cell RNA sequencing (scRNA-seq) analysis failed to identify ctenophore neurons using any of the known neuronal molecular markers. To reveal the molecular machinery of ctenophore neurons, we have characterized the neuropeptide repertoire of the ctenophore Mnemiopsis leidyi. Using the machine learning NeuroPID tool, we predicted 129 new putative neuropeptide precursors. Sixteen of them were localized to the subepithelial nerve net (SNN), sensory aboral organ (AO), and epithelial sensory cells (ESCs), providing evidence that they are neuropeptide precursors. Four of these putative neuropeptides had a behavioral effect and increased the animals' swimming speed. Intriguingly, these putative neuropeptides finally allowed us to identify neuronal cell types in single-cell transcriptomic data and reveal the molecular identity of ctenophore neurons. High-resolution electron microscopy and 3D reconstructions of the nerve net underlying the comb plates confirmed a more than 100-year-old hypothesis of anastomoses between neurites of the same cell in ctenophores and revealed that they occur through a continuous membrane. Our work demonstrates the unique ultrastructure of the peptidergic nerve net and a rich neuropeptide repertoire of ctenophores, supporting the hypothesis that the first nervous system(s) evolved as nets of peptidergic cells.

The IS630/Tc1/mariner transposons in three ctenophore genomes

Transposable elements (TEs) exert a significant effect on the structure and functioning of the genomes and also serve as a source of the new genes. The study of the TE diversity and evolution in different taxa is indispensable for the fundamental understanding of their roles in the genomes. IS630/Tc1/mariner (ITm) transposable elements represent the most prevalent and diverse group of DNA transposons. In this work, we studied the diversity, evolutionary dynamics and the phylogenetic relationships of the ITm transposons found in three ctenophore species: Mnemiopsis leidyi, Pleurobrachia bachei, Beroe ovata. We identified 29 ITm transposons, seven of which possess the terminal inverted repeats (TIRs) and an intact transposase, and, thus, are, presumably, active. Four other ITm transposons have the features of domesticated TEs. According to the results of the phylogenetic analysis, the ITm transposons of the ctenophores represent five groups - MLE/DD34D, TLE/DD34-38E, mosquito/DD37E, Visiror/DD41D and pogo/DDxD. Pogo/DDxD superfamily turnes out to be the most diverse and prevalent, since it accounts for more than 40% of the TEs identified. The data obtained in this research will fill the gap of knowledge of the diversity and evolution of the ITm transposons in the multicellular genomes and will lay the ground for the study of the TE effects on the evolution of the ctenophores.

Improved histological fixation of gelatinous marine invertebrates

Background

Gelatinous zooplankton can be difficult to preserve morphologically due to unique physical properties of their cellular and acellular components. The relatively large volume of mesoglea leads to distortion of the delicate morphology and poor sample integrity in specimens prepared with standard aldehyde or alcohol fixation techniques. Similar challenges have made it difficult to extend standard laboratory methods such as in situ hybridization to larger juvenile ctenophores, hampering studies of late development.

Results

We have found that a household water repellant glass treatment product commonly used in laboratories, Rain-X®, alone or in combination with standard aldehyde fixatives, greatly improves morphological preservation of such delicate samples. We present detailed methods for preservation of ctenophores of diverse sizes compatible with long-term storage or detection and localization of target molecules such as with immunohistochemistry and in situ hybridization and show that this fixation might be broadly useful for preservation of other delicate marine specimens.

Conclusion

This new method will enable superior preservation of morphology in gelatinous specimens for a variety of downstream goals. Extending this method may improve the morphological fidelity and durability of museum and laboratory specimens for other delicate sample types.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00414-z.

Whole-Body Regeneration in the Lobate Ctenophore Mnemiopsis leidyi

Ctenophores (a.k.a. comb jellies) are one of the earliest branching extant metazoan phyla. Adult regenerative ability varies greatly within the group, with platyctenes undergoing both sexual and asexual reproduction by fission while others in the genus Beroe having completely lost the ability to replace missing body parts. We focus on the unique regenerative aspects of the lobate ctenophore, Mnemiopsis leidyi, which has become a popular model for its rapid wound healing and tissue replacement, optical clarity, and sequenced genome. M. leidyi’s highly mosaic, stereotyped development has been leveraged to reveal the polar coordinate system that directs whole-body regeneration as well as lineage restriction of replacement cells in various regenerating organs. Several cell signaling pathways known to function in regeneration in other animals are absent from the ctenophore’s genome. Further research will either reveal ancient principles of the regenerative process common to all animals or reveal novel solutions to the stability of cell fates and whole-body regeneration.

Adaptive strategies of Mnemiopsis leidyi A. Agassiz 1865 in different environments of the Eurasian seas

Comprehensive synthesis of the harmful invader ctenophore Mnemiopsis leidyi adaptive strategies and its validation as of a single polymorphic species has been presented. Its high morphological and physiological variability in different environments were demonstrated. M. leidyi being native for eastern coasts of Americas, since the early 1980s began to invade in the Eurasian seas and now it is recorded in a wide range of recipient habitats. Analysis of M. leidyi morphological and eco-physiological variability, phenology and rate of reproduction was performed for different environments based on author's data and published sources. Prominent morphological features of M. leidyi, previously used to subdivide it in three species, in fact are a phenotypical variability, associated with environmental conditions. In recipient environments, M. leidyi pre-adapts for rapid colonization, due to a high metabolism and reproduction rates. It created extensive populations with the various patterns of annual cycle and distribution and heavily impacted the ecosystems.

Par protein localization during the early development of Mnemiopsis leidyi suggests different modes of epithelial organization in the metazoa

In bilaterians and cnidarians, epithelial cell-polarity is regulated by the interactions between Par proteins, Wnt/PCP signaling pathway, and cell-cell adhesion. Par proteins are highly conserved across Metazoa, including ctenophores. But strikingly, ctenophore genomes lack components of the Wnt/PCP pathway and cell-cell adhesion complexes raising the question if ctenophore cells are polarized by mechanisms involving Par proteins. Here, by using immunohistochemistry and live-cell imaging of specific mRNAs, we describe for the first time the subcellular localization of selected Par proteins in blastomeres and epithelial cells during the embryogenesis of the ctenophore Mnemiopsis leidyi. We show that these proteins distribute differently compared to what has been described for other animals, even though they segregate in a host-specific fashion when expressed in cnidarian embryos. This differential localization might be related to the emergence of different junctional complexes during metazoan evolution.

Cultivable microbiota associated with Aurelia aurita and Mnemiopsis leidyi

The associated microbiota of marine invertebrates plays an important role to the host in relation to fitness, health, and homeostasis. Cooperative and competitive interactions between bacteria, due to release of, for example, antibacterial substances and quorum sensing (QS)/quorum quenching (QQ) molecules, ultimately affect the establishment and dynamics of the associated microbial community. Aiming to address interspecies competition of cultivable microbes associated with emerging model species of the basal animal phyla Cnidaria (Aurelia aurita) and Ctenophora (Mnemiopsis leidyi), we performed a classical isolation approach. Overall, 84 bacteria were isolated from A. aurita medusae and polyps, 64 bacteria from M. leidyi, and 83 bacteria from ambient seawater, followed by taxonomically classification by 16S rRNA gene analysis. The results show that A. aurita and M. leidyi harbor a cultivable core microbiome consisting of typical marine ubiquitous bacteria also found in the ambient seawater. However, several bacteria were restricted to one host suggesting host‐specific microbial community patterns. Interbacterial interactions were assessed by (a) a growth inhibition assay and (b) QS interference screening assay. Out of 231 isolates, 4 bacterial isolates inhibited growth of 17 isolates on agar plates. Moreover, 121 of the 231 isolates showed QS‐interfering activities. They interfered with the acyl‐homoserine lactone (AHL)‐based communication, of which 21 showed simultaneous interference with autoinducer 2. Overall, this study provides insights into the cultivable part of the microbiota associated with two environmentally important marine non‐model organisms and into interbacterial interactions, which are most likely considerably involved in shaping a healthy and resilient microbiota.

Population genomic structure of the gelatinous zooplankton species Mnemiopsis leidyi in its nonindigenous range in the North Sea

Nonindigenous species pose a major threat for coastal and estuarine ecosystems. Risk management requires genetic information to establish appropriate management units and infer introduction and dispersal routes. We investigated one of the most successful marine invaders, the ctenophore Mnemiopsis leidyi, and used genotyping-by-sequencing (GBS) to explore the spatial population structure in its nonindigenous range in the North Sea. We analyzed 140 specimens collected in different environments, including coastal and estuarine areas, and ports along the coast. Single nucleotide polymorphisms (SNPs) were called in approximately 40 k GBS loci. Population structure based on the neutral SNP panel was significant (F ST .02; p < .01), and a distinct genetic cluster was identified in a port along the Belgian coast (Ostend port; pairwise F ST .02-.04; p < .01). Remarkably, no population structure was detected between geographically distant regions in the North Sea (the Southern part of the North Sea vs. the Kattegat/Skagerrak region), which indicates substantial gene flow at this geographical scale and recent population expansion of nonindigenous M. leidyi. Additionally, seven specimens collected at one location in the indigenous range (Chesapeake Bay, USA) were highly differentiated from the North Sea populations (pairwise F ST .36-.39; p < .01). This study demonstrates the utility of GBS to investigate fine-scale population structure of gelatinous zooplankton species and shows high population connectivity among nonindigenous populations of this recently introduced species in the North Sea.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at: The DNA sequences generated for this study are deposited in the NCBI sequence read archive under SRA accession numbers SRR6950721-SRR6950884, and will be made publically available upon publication of this manuscript.

Patterns of invasive ctenophore Mnemiopsis leidyi distribution and variability in different recipient environments of the Eurasian seas: A review

Harmful invader ctenophore Mnemiopsis leidyi's expansions in the Eurasian Seas, its spatio-temporal population dynamics depending on environmental conditions in recipient habitats have been synthesized. M. leidyi found suitable temperature, salinity and productivity conditions in the temperate and subtropical environments of the semi-enclosed seas, in the coastal areas of open basins and in closed water bodies, where it created autonomous populations. M. leidyi changes its phenology depending on seasonal temperature regime in different environments. We assessed ranges of sea surface temperature, sea surface salinity and sea surface chlorophyll values, sufficient for M. leidyi general occurrence and reproduction based on comprehensive long-term datasets, contributed by co-authors. This assessment revealed that there are at least two eco-types (Southern and Northern) in the recipient seas of Eurasia with features specific for their donor areas. The range of thresholds for M. leidyi establishment, occurrence and life cycle in both eco-types depends on variability of environmental parameters in their native habitats.

Regeneration in the ctenophore Mnemiopsis leidyi occurs in the absence of a blastema, requires cell division, and is temporally separable from wound healing

BACKGROUND

The ability to regenerate is a widely distributed but highly variable trait among metazoans. A variety of modes of regeneration has been described for different organisms; however, many questions regarding the origin and evolution of these strategies remain unanswered. Most species of ctenophore (or "comb jellies"), a clade of marine animals that branch off at the base of the animal tree of life, possess an outstanding capacity to regenerate. However, the cellular and molecular mechanisms underlying this ability are unknown. We have used the ctenophore Mnemiopsis leidyi as a system to study wound healing and adult regeneration and provide some first-time insights of the cellular mechanisms involved in the regeneration of one of the most ancient extant group of multicellular animals.

RESULTS

We show that cell proliferation is activated at the wound site and is indispensable for whole-body regeneration. Wound healing occurs normally in the absence of cell proliferation forming a scar-less wound epithelium. No blastema-like structure is generated at the cut site, and pulse-chase experiments and surgical intervention show that cells originating in the main regions of cell proliferation (the tentacle bulbs) do not seem to contribute to the formation of new structures after surgical challenge, suggesting a local source of cells during regeneration. While exposure to cell-proliferation blocking treatment inhibits regeneration, the ability to regenerate is recovered when the treatment ends (days after the original cut), suggesting that ctenophore regenerative capabilities are constantly ready to be triggered and they are somehow separable of the wound healing process.

CONCLUSIONS

Ctenophore regeneration takes place through a process of cell proliferation-dependent non-blastemal-like regeneration and is temporally separable of the wound healing process. We propose that undifferentiated cells assume the correct location of missing structures and differentiate in place. The remarkable ability to replace missing tissue, the many favorable experimental features (e.g., optical clarity, high fecundity, rapid regenerative performance, stereotyped cell lineage, sequenced genome), and the early branching phylogenetic position in the animal tree, all point to the emergence of ctenophores as a new model system to study the evolution of animal regeneration.

The ctenophore Mnemiopsis leidyi regulates egg production via conspecific communication

Background

Communication between individuals of the same species is an important aspect of mating and reproduction in most animals. In simultaneously hermaphroditic species with the ability to self-fertilize, communication with conspecifics can be essential to avoid inbreeding depression. One such behavioral adaptation observed in some simultaneous hermaphrodites is gamete trading. This behavior involves individual hermaphrodites in pairs alternating between reproducing as the male and female, and, as such, necessarily requires communication and coordination between mates. Little is known about communication in ctenophores and conspecific communication has not been described in this group; however, our previous work suggested that the ctenophore Mnemiopsis leidyi might engage in gamete trading. We tested for this possibility by constructing divided arenas (both sealed and permeable) that allowed us to measure individual egg output for paired M. leidyi.

Results

We found that, when not allowed to interact, size-matched individuals produced similar numbers of eggs on each side of the arena. However, if allowed to interact and exchange water, size-matched pairs produce significantly different numbers of eggs on each side of the arena, suggesting that these pairs use chemical communication to modulate reproduction in the presence of conspecifics as would be expected in gamete trading.

Conclusion

This finding presents exciting new possibilities for future investigations into the nature of signaling in M. leidyi. Furthermore, this first evidence of conspecific communication in Ctenophora, a group that branched off from the rest of animals more than 600 million years ago, has significant implications for the signaling ability of the last common ancestor of all animals.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0169-9) contains supplementary material, which is available to authorized users.

Identifying the drivers of abundance and size of the invasive ctenophore Mnemiopsis leidyi in Northwestern Mediterranean lagoons

Acknowledged as among the worst invasive marine species, Mnemiopsis leidyi has spread through European Seas since the mid-1980's. Here we report a bimonthly survey conducted in 2010-11 in three lagoons (Bages-Sigean, Thau and Berre) and at two adjacent coastal stations (Sète and SOMLIT-Marseille) along the French Mediterranean coast. M. leidyi was present only in Berre and Bages-Sigean with maximum abundances observed in late summer. M. leidyi adults were present year round in Berre with the largest organisms (∼6 cm) observed in April. In Bages-Sigean, they occurred in sufficient abundance to be recorded by fishermen between August and November. Multiple linear regressions highlighted that abundance in both lagoons was mainly influenced by direct effects of salinity and chlorophyll-a, and temperature to a lesser extent. While M. leidyi has not yet been recorded in Thau, the lagoon is continually monitored to detect the potential establishment of M. leidyi.

The sex lives of ctenophores: the influence of light, body size, and self-fertilization on the reproductive output of the sea walnut, Mnemiopsis leidyi

Ctenophores (comb jellies) are emerging as important animals for investigating fundamental questions across numerous branches of biology (e.g., evodevo, neuroscience and biogeography). A few ctenophore species including, most notably, Mnemiopsis leidyi, are considered as invasive species, adding to the significance of studying ctenophore ecology. Despite the growing interest in ctenophore biology, relatively little is known about their reproduction. Like most ctenophores, M. leidyi is a simultaneous hermaphrodite capable of self-fertilization. In this study, we assess the influence of light on spawning, the effect of body size on spawning likelihood and reproductive output, and the cost of self-fertilization on egg viability in M. leidyi. Our results suggest that M. leidyi spawning is more strongly influenced by circadian rhythms than specific light cues and that body size significantly impacts spawning and reproductive output. Mnemiopsis leidyi adults that spawned alone produced a lower percentage of viable embryos versus those that spawned in pairs, suggesting that self-fertilization may be costly in this species. These results provide insight into the reproductive ecology of M. leidyi and provide a fundamental resource for researchers working with them in the laboratory.

On the distribution and population dynamics of the ctenophore Mnemiopsis leidyi in the Belgian part of the North Sea and Westerschelde estuary

The spatio-temporal distribution and population dynamics of the non-indigenous ctenophore Mnemiopsis leidyi A. Agassiz 1865 were investigated through monthly and quarterly surveys in 2011-2012 at several locations in the Belgian part of the North Sea, the main coastal ports and the adjacent Westerschelde estuary. M. leidyi occurred from August to December, but was never found more than 30 km offshore. Densities were generally low (average 0.8 ± SD 2.8 ind m(-3)) compared to other invaded European systems. Highest densities of M. leidyi were found in the semi-enclosed basin (port of Oostende; 18.4 ind m(-3)) and Westerschelde estuary (1.9 ind m(-3)). The presence of larvae and sudden appearance of high numbers across the size distribution in August indicated that ports and estuaries may act as sources, populating the adjacent coastal area. The zero-inflated logistic regression model showed that there is a higher chance of finding M. leidyi (presence) when temperature declines from late summer onwards. Combined with a negative binomial regression, our model suggests that increasing M. leidyi densities are associated with decreasing autumn temperatures, low wave height (low energetic systems) and low dissolved oxygen concentrations Although densities remained relatively low since its first appearance in 2007, a permanent population seems to be established in Belgian waters. As population outbursts may occur with only a small change in environmental parameters, further monitoring of this notorious invasive species is recommended.

Expression of multiple Sox genes through embryonic development in the ctenophore Mnemiopsis leidyi is spatially restricted to zones of cell proliferation

Background

The Sox genes, a family of transcription factors characterized by the presence of a high mobility group (HMG) box domain, are among the central groups of developmental regulators in the animal kingdom. They are indispensable in progenitor cell fate determination, and various Sox family members are involved in managing the critical balance between stem cells and differentiating cells. There are 20 mammalian Sox genes that are divided into five major groups (B, C, D, E, and F). True Sox genes have been identified in all animal lineages but not outside Metazoa, indicating that this gene family arose at the origin of the animals. Whole-genome sequencing of the lobate ctenophore Mnemiopsis leidyi allowed us to examine the full complement and expression of the Sox gene family in this early-branching animal lineage.

Results

Our phylogenetic analyses of the Sox gene family were generally in agreement with previous studies and placed five of the six Mnemiopsis Sox genes into one of the major Sox groups: SoxB (MleSox1), SoxC (MleSox2), SoxE (MleSox3, MleSox4), and SoxF (MleSox5), with one unclassified gene (MleSox6). We investigated the expression of five out of six Mnemiopsis Sox genes during early development. Expression patterns determined through in situ hybridization generally revealed spatially restricted Sox expression patterns in somatic cells within zones of cell proliferation, as determined by EdU staining. These zones were located in the apical sense organ, upper tentacle bulbs, and developing comb rows in Mnemiopsis, and coincide with similar zones identified in the cydippid ctenophore Pleurobrachia.

Conclusions

Our results are consistent with the established role of multiple Sox genes in the maintenance of stem cell pools. Both similarities and differences in juvenile cydippid stage expression patterns between Mnemiopsis Sox genes and their orthologs from Pleurobrachia highlight the importance of using multiple species to characterize the evolution of development within a given phylum. In light of recent phylogenetic evidence that Ctenophora is the earliest-branching animal lineage, our results are consistent with the hypothesis that the ancient primary function of Sox family genes was to regulate the maintenance of stem cells and function in cell fate determination.

Stimulated bacterioplankton growth and selection for certain bacterial taxa in the vicinity of the ctenophore Mnemiopsis leidyi

Episodic blooms of voracious gelatinous zooplankton, such as the ctenophore Mnemiopsis leidyi, affect pools of inorganic nutrients and dissolved organic carbon by intensive grazing activities and mucus release. This will potentially influence bacterioplankton activity and community composition, at least at local scales; however, available studies on this are scarce. In the present study we examined effects of M. leidyi on bacterioplankton growth and composition in incubation experiments. Moreover, we examined community composition of bacteria associated with the surface and gut of M. leidyi. High release of ammonium and high bacterial growth was observed in the treatments with M. leidyi relative to controls. Deep 454 pyrosequencing of 16 S rRNA genes showed specific bacterial communities in treatments with M. leidyi as well as specific communities associated with M. leidyi tissue and gut. In particular, members of Flavobacteriaceae were associated with M. leidyi. Our study shows that M. leidyi influences bacterioplankton activity and community composition in the vicinity of the jellyfish. In particular during temporary aggregations of jellyfish, these local zones of high bacterial growth may contribute significantly to the spatial heterogeneity of bacterioplankton activity and community composition in the sea.