Invasion Pathway of the Ctenophore Mnemiopsis leidyi in the Mediterranean Sea

Gelatinous zooplankton outbreaks have increased globally owing to a number of human-mediated factors, including food web alterations and species introductions. The invasive ctenophore Mnemiopsis leidyi entered the Black Sea in the early 1980s. The invasion was followed by the Azov, Caspian, Baltic and North Seas, and, most recently, the Mediterranean Sea. Previous studies identified two distinct invasion pathways of M. leidyi from its native range in the western Atlantic Ocean to Eurasia. However, the source of newly established populations in the Mediterranean Sea remains unclear. Here we build upon our previous study and investigate sequence variation in both mitochondrial (Cytochrome c Oxidase subunit I) and nuclear (Internal Transcribed Spacer) markers in M. leidyi, encompassing five native and 11 introduced populations, including four from the Mediterranean Sea. Extant genetic diversity in Mediterranean populations (n = 8, N _a = 10) preclude the occurrence of a severe genetic bottleneck or founder effects in the initial colonizing population. Our mitochondrial and nuclear marker surveys revealed two possible pathways of introduction into Mediterranean Sea. In total, 17 haplotypes and 18 alleles were recovered from all surveyed populations. Haplotype and allelic diversity of Mediterranean populations were comparable to populations from which they were likely drawn. The distribution of genetic diversity and pattern of genetic differentiation suggest initial colonization of the Mediterranean from the Black-Azov Seas (pairwise F _ST = 0.001–0.028). However, some haplotypes and alleles from the Mediterranean Sea were not detected from the well-sampled Black Sea, although they were found in Gulf of Mexico populations that were also genetically similar to those in the Mediterranean Sea (pairwise F _ST = 0.010–0.032), raising the possibility of multiple invasion sources. Multiple introductions from a combination of Black Sea and native region sources could be facilitated by intense local and transcontinental shipping activity, respectively.     

The development of bioluminescence in the ctenophore Mnemiopsis leidyi

The photocytes of the ctenophore Mnemiopsis have a discontinuous distribution along the radial canal between the sites where the comb plate cilia cells are located on the side of the canal which contains the testes. They are separated from the lumen of the canal by a population of gastric cells. Cytologically these cells are characterized by a condensed nucleus and cytoplasm which stains lightly with basophilic dyes.The ability of the ctenophore embryo to produce light appears at the developmental stage when the comb plate cilia first begin to grow out. At this stage four light-producing areas are present; each area corresponds to one quadrant of the adult animal. At the sites of light production, a population of cells can be identified that have some of the cytological properties of the photocytes of the adult animal. Within 8–10 hr after light production begins there is a 10-fold increase in the amount of light produced by an embryo and a cytological maturation of its photocytes; during this time period there is no increase in photocyte number. At about the time the embryo begins to feed, each light-producing region splits into two regions, each of which corresponds to a radial canal.During the process of embryogenesis the photocyte cell lineage is first segregated from non-photocytes at the differential division which gives the 8-cell stage embryo. The M macromere lineage goes on to form photocytes, but the E macromere lineage does not. The M macromeres form a micromere at the aboral pole of the embryo at each of the next two cleavages; during these cleavages the potential for photocyte differentiation continues to segregate with the M macromeres. During the division which gives the 64-cell stage the M macromeres divide equally; the potential for photocyte differentiation segregates with the M macromeres nearest the oral-aboral axis. M macromeres which are isolated from the embryo at the 8-, 16-, or 32-cell stage of development will continue to cleave as though they were part of a normal embryo and differentiate to form photocytes.The events that are responsible for the differential division during the formation of the 8-cell stage embryo have been studied by centrifuging eggs to produce fragments of different cytoplasmic composition. Egg fragments which contain only cortical cytoplasm differentiate comb plate cilia cells, but do not produce photocytes. Cortical fragments with a small amount of yolk differentiate comb plate cilia cells and photocytes. Both the M and E macromeres from cortical fragments with no yolk produce comb plate cilia. Only M macromeres containing yolk form photocytes; if an M macromere forms photocytes it does not form comb plate cilia.     

Regulation and regeneration in the ctenophore Mnemiopsis leidyi

Lobate ctenophores (tentaculates) generally exhibit a remarkable ability to regenerate missing structures as adults. On the other hand, their embryos exhibit a highly mosaic behavior when cut into halves or when specific cells are ablated. These deficient embryos do not exhibit embryonic regulation, and generate incomplete adult body plans. Under certain conditions, however, these deficient animals are subsequently able to replace the missing structures during the adult phase in a process referred to as "post-regeneration." We have determined that successful post-regeneration can be predicted on the basis of a modified polar coordinate model, and the rules of intercalary regeneration, as defined by French et al. (V. French, P. J. Bryant, and S. V. Bryant, 1976, Science 193, 969-981.) The model makes certain assumptions about the organization of the ctenophore body plan that fit well with what we have determined on the basis of cell lineage fates maps, and their twofold rotational ("biradial") symmetry. The results suggest that cells composing the ctenophore adult body plan possess positional information, which is utilized to reconstruct the adult body plan. More specifically, we have found that the progeny of three specific cell lineages are required to support post-regeneration of the comb rows (the e(1), e(2), and m(1) micromeres). Furthermore, post-regeneration of the comb rows involves a suite of cell-cell inductive interactions, which are similar to those that take place during their embryonic formation. The significance of these findings is discussed in terms of the organization of the ctenophore body plan, and the mechanisms involved in cell fate specification. This situation is also contrasted with that of the atentaculate ctenophores, which are unable to undergo post-regeneration.     

Calcium-sensitive action potential of long duration in the fertilized egg of the ctenophore Mnemiopsis leidyi

The membrane properties of fertilized eggs of the ctenophore Mnemiopsis leidyi were studied using standard microelectrode techniques. The resting potential was approximately -80 mV, and was dependent on the extracellular K concentration. Depolarizing current injections elicited an action potential with an initial peak amplitude of +20 to +40 mV (duration about 5 sec) and a long lasting (duration 3 to 10 min) plateau phase. The depolarizing phase and the plateau phase appeared to have different ionic mechanisms. The entire action potential could be prevented by removal of extracellular Ca, but only the amplitude of the depolarizing phase, not the plateau phase, was dependent on the extracellular Ca concentration. The plateau phase was not observed in the absence of Ca, but in the presence of Ca its duration was dependent on the external Ca concentration. The data suggest that the plateau phase is activated as a consequence of Ca influx during the initial depolarizing phase. Removal of external Na resulted in only minor changes in the waveform of repolarization. The action potential was resistant to low concentrations of Mn and Cd in the presence of Ca. The role of this action potential in ctenophore development is not known, but in its waveform and duration it resembles the sperm-gated potentials that have been seen in eggs of other phyla. These experiments show ctenophore embryos to be excitable at very early stages, and suggest their utility in the study of the differentiation of cellular electrical properties.     

Specific immune priming in the invasive ctenophore Mnemiopsis leidyi

Specific immune priming enables an induced immune response upon repeated pathogen encounter. As a functional analogue to vertebrate immune memory, such adaptive plasticity has been described, for instance, in insects and crustaceans. However, towards the base of the metazoan tree our knowledge about the existence of specific immune priming becomes scattered. Here, we exposed the invasive ctenophore Mnemiopsis leidyi repeatedly to two different bacterial epitopes (Gram-positive or -negative) and measured gene expression. Ctenophores experienced either the same bacterial epitope twice (homologous treatments) or different bacterial epitopes (heterologous treatments). Our results demonstrate that immune gene expression depends on earlier bacterial exposure. We detected significantly different expression upon heterologous compared with homologous bacterial treatment at three immune activator and effector genes. This is the first experimental evidence for specific immune priming in Ctenophora and generally in non-bilaterian animals, hereby adding to our growing notion of plasticity in innate immune systems across all animal phyla.     

leidyi Is a New Group of DD41D Transposons in Mnemiopsis leidyi Genome

Russian Journal of Genetics - The IS630/Tc1/mariner superfamily of transposable elements (TE) is one of the most numerous and widespread among DNA transposons. The IS630/Tc1/mariner TE are divided...     

Bacterial communities associated with the ctenophores Mnemiopsis leidyi and Beroe ovata

Residing in a phylum of their own, ctenophores are gelatinous zooplankton that drift through the ocean's water column. Although ctenophores are known to be parasitized by a variety of eukaryotes, no studies have examined their bacterial associates. This study describes the bacterial communities associated with the lobate ctenophore Mnemiopsis leidyi and its natural predator Beroe ovata in Tampa Bay, Florida, USA. Investigations using terminal restriction fragment length polymorphism (T-RFLP) and cloning and sequencing of 16S rRNA genes demonstrated that ctenophore bacterial communities were distinct from the surrounding water. In addition, each ctenophore genus contained a unique microbiota. Ctenophore samples contained fewer bacterial operational taxonomic units (OTUs) by T-RFLP and lower diversity communities by 16S rRNA gene sequencing than the water column. Both ctenophore genera contained sequences related to bacteria previously described in marine invertebrates, and sequences similar to a sea anemone pathogen were abundant in B.?ovata. Temporal sampling revealed that the ctenophore-associated bacterial communities varied over time, with no single OTU detected at all time points. This is the first report of distinct and dynamic bacterial communities associated with ctenophores, suggesting that these microbial consortia may play important roles in ctenophore ecology. Future work needs to elucidate the functional roles and mode of acquisition of these bacteria.     

Multiple inductive signals are involved in the development of the ctenophore Mnemiopsis leidyi.

Ctenophores possess eight longitudinally arrayed rows of comb plate cilia. Previous intracellular cell lineage analysis has shown that these comb rows are derived from two embryonic lineages, both daughters of the four e(1) micromeres (e(11) and e(12)) and a single daughter of the four m(1) micromeres (the m(12) micromeres). Although isolated e(1) micromeres will spontaneously generate comb plates, cell deletion experiments have shown that no comb plates appear during embryogenesis following the removal of e(1) descendents. Thus, the m(1) lineage requires the inductive interaction of the e(1) lineage to contribute to comb plate formation. Here we show that, although m(12) cells are normally the only m(1) derivatives to contribute to comb plate formation, m(11) cells are capable of generating comb plates in the absence m(12) cells. The reason that m(11) cells do not normally make comb rows may be attributable either to their more remote location relative to critical signaling centers (e.g., e(1) descendants) or to inhibitory signals that may be provided by other nearby cells such as sister cells m(12). In addition, we show that the signals provided by the e(1) lineage are not sufficient for m(1)-derived comb plate formation. Signals provided by endomesodermal progeny of either the E or the M lineages (the 3E or 2M macromeres) are also required.     

Multiple introductions and invasion pathways for the invasive ctenophore Mnemiopsis leidyi in Eurasia

The introduction and spread of non-indigenous species (NIS) in marine ecosystems accelerated during the twentieth century owing to human activities, notably international shipping. Genetic analysis has proven useful in understanding the invasion history and dynamics of colonizing NIS and identifying their source population(s). Here we investigated sequence variation in the nuclear ribosomal Internal Transcribed Spacer region of the ctenophore Mnemiopsis leidyi, a species considered one of the most invasive globally. We surveyed four populations from the native distribution range along the Atlantic coasts of the United States and South America, as well as six populations in the introduced range from the Black, Azov, Caspian and Baltic seas. Allelic and nucleotide diversity of introduced populations were comparable to those of native populations from which they were likely drawn. Introduced populations typically exhibited lower genetic differentiation (F _ST = ?0.014?C0.421) than native populations (F _ST = 0.324?C0.688). Population genetic analyses supported the invasion of Eurasia from at least two different pathways, the first from the Gulf of Mexico (e.g., Tampa Bay) to the Black Sea and thence to the Caspian Sea, the second from the northern part of the native distribution range (e.g., Narragansett Bay) to the Baltic Sea. The relatively high genetic diversity observed in introduced populations is consistent with large inocula and/or multiple invasions, both of which are possible given ballast water transport and the extensive native distribution of the ctenophore in the Atlantic Ocean.     

Regeneration of ciliary comb plates in the ctenophore Mnemiopsis leidyi. i. morphology

Regeneration of missing body parts in model organisms provides information on the mechanisms underlying the regeneration process. The aim here is to use ctenophores to investigate regeneration of their giant ciliary swimming plates. When part of a row of comb plates on Mnemiopsis is excised, the wound closes and heals, greatly increasing the distance between comb plates near the former cut edges. Video differential interference contrast (DIC) microscopy of the regeneration of new comb plates between widely separated plates shows localized widenings of the interplate ciliated groove (ICG) first, followed by growth of two opposing groups of comb plate cilia on either side. The split parts of a new plate elongate as their bases extend laterally away from the ICG widening and continue ciliogenesis at both ends. The split parts of a new plate grow longer and move closer together into the ICG widening until they merge into a single plate that interrupts the ICG in a normal manner. Video DIC snapshots of dissected gap preparations 1.5–3‐day postoperation show that ICG widenings and/or new plates do not all appear at the same time or with uniform spacing within a gap: the lengths and distances between young plates in a gap are quite variable. Video stereo microscopy of intact animals 3–4 days after the operation show that all the new plates that will form in a gap are present, fairly evenly spaced and similar in length, but smaller and closer together than normal. Normal development of comb plates in embryos and growing animals is compared to the pattern of comb plate regeneration in adults. Comb plate regeneration differs in the cydippid Pleurobrachia that lacks ICGs and has a firmer mesoglea than Mnemiopsis. This study provides a morphological foundation for histological, cellular, and molecular analysis of ciliary regeneration in ctenophores. J. Morphol. 2011. © 2011 Wiley Periodicals, Inc.     

The effects of altering the position of cleavage planes on the process of localization of developmental potential in ctenophores.

During the transition from the four- to the eight-cell stage in ctenophore embryos, each blastomere produces one daughter cell with the potential to form comb plate cilia and one daughter cell that does not have this potential. If the second cleavage in a two-cell embryo is blocked, at the next cleavage these embryos frequently form four blastomeres which have the configuration of the blastomeres in a normal eight-cell embryo. At this division there is also a segregation of comb plate-forming potential. By compressing a two-cell embryo in a plane perpendicular to the first plane of cleavage it is possible to produce a four-cell blastomere configuration that is identical to that produced following the inhibition of the second cleavage. However, under these circumstances the segregation of comb plate potential does not occur. These results suggest that the appropriate plane of cleavage must take place for a given cleavage cycle, in order for localizations of developmental potential to be properly positioned within blastomeres.     

Use of environmental DNA in early detection of Mnemiopsis leidyi in UK coastal waters

Biological Invasions - Since 2005, the ctenophore Mnemiopsis leidyi has been reported as present in northern Europe. Although the impacts of this voracious predator on North Sea fisheries remain...     

Life-stage dependent, in situ dietary patterns of the lobate ctenophore Mnemiopsis leidyi Agassiz 1865

Analysis of in situ gut contents of the ctenophore Mnemiopsisleidyi collected during summer months of 1998 at Woods Hole,MA, USA indicates that dietary changes accompanied developmentof this zooplankter. The transition from cydippid to lobatemorphologies involved a shift from a microplanktonic diet dominatedby protists to a metazoan-based diet. Dietary diversity increasedrapidly during the transition from cydippid to lobate stagebut remained essentially similar through most (>3.0 cm totallength) of the lobate stage. These stage-dependent dietary differencesmay have important implications for understanding populationdynamics of M. leidyi.     

Impact of hypoxia conditions on the Mnemiopsis leidyi A. Agassiz, 1865 bioluminescence

The findings of the study on the impact of hypoxia on the glow of the Black Sea ctenophore Mnemiopsis leidyi A. Agassiz, 1865 of three size groups (20–30, 30–45, and 45–60 mm) were obtained under experimental conditions. Peculiarities of ctenophore bioluminescence were studied during mechanical and chemical stimulation under the conditions of normoxia (at an oxygen concentration of 5.6–6.7 mg O₂ L⁻¹), moderate hypoxia (2.5–2.8 mg O₂ L⁻¹), and acute hypoxia (1.2–1.5 mg O₂ L⁻¹). An increase in the amplitude and energy of luminescence of the ctenophores mechanically and chemically stimulated was observed at an oxygen concentration of 1.2–1.5 mg O₂ L⁻¹ (acute hypoxia) in two size groups in the lobate form (30–45 and 45–60 mm). The inhibition of amplitude, energy, and duration of the signal was registered in M. leidyi ctenophores at the transitional stage from larva to the lobate form under conditions of acute hypoxia. It was noted that in normoxia, the values of the amplitude and energy of the bioluminescent signal of M. leidyi increase along with a size growth of an individual. This phenomenon was observed both during mechanical and chemical stimulations. Under conditions of acute hypoxia, this trend was mainly preserved. The universality of the relation between the bioluminescence of the organisms and their bioenergetics is obvious. The bioluminescent system of ctenophores has the role of an antioxidant system and is engaged in the neutralization of reactive oxygen species (ROS), that is the process during which photons are emitted. The response of the bioluminescent system to a decrease in oxygen concentration can be associated with an increase in the production of ROS that provides high values of the ctenophore luminescence under hypoxic conditions.     

The role of unequal cleavage and the polar lobe in the segregation of developmental potential during first cleavage in the embryo of Chætopterus variopedatus

Summary The inequality of the first cleavage division of the Chætopterus embryo is caused by the production of a small polar lobe and the internal shifting of the first cleavage spindle. This division produces a two-celled embryo containing a small AB and a large CD blastomere. These blastomeres have different morphogenetic potentials. Only the larvae resulting from isolated CD blastomeres are able to form bioluminescent photocytes, eyes and lateral hooked bristles. The removal of the polar lobe during first cleavage does not have a great effect on development. These lobeless embryos display a normal pattern of cleavages through the time of mesentoblast formation. The resulting larvae are essentially normal, however they do not form functional photocytes. If the CD cell is isolated after the removal of the first polar lobe, the resulting larva is virtually identical to those formed by the intact CD cell except it lacks the photocyte cells. These results indicate that two separate pathways are involved in the segregation of developmental or morphogenetic potential which takes place during first cleavage. One set of factors, which are necessary for photocyte formation, are associated with the first polar lobe. Other factors that are necessary for the formation of the eyes and lateral hooked bristles are segregated by the unequal cleavage which results from an internal shifting of the cleavage spindle. The removal of a large portion of the vegetal region of the embryo during first cleavage leads to the production of larvae which display a decreased ability to form eyes and lateral hooked bristles. These embryos frequently display an abnormal pattern of cleavages. They do not form the primary somatoblast or the mesentoblast. These results indicate that the vegetal region of the CD cell of Chætopterus is analogous to polar lobes which have been studied in other species, and is therefore important in the specification of the D quadrant. These features of the first cleavage of Chætopterus are a combination of those displayed by forms with direct unequal cleavage and other forms which cleave unequally through the production of large polar lobes. The significance of these findings is discussed relative to the origins of these different types of unequal cleavage.     

Feeding interactions between the introduced ctenophore Mnemiopsis leidyi and juvenile herring Clupea harengus in the Wadden Sea

We analysed feeding interactions between Mnemiopsis leidyi and juvenile Clupea harengus in the Wadden Sea. Biomass, diet overlap, prey selectivity, predation impact and stable isotope composition (C, N) of both species were assessed from June to September 2010. High biomass of C. harengus was found in June and July (wet weight 3.0 ± 1.8 g m^?3) followed by a steep decline from August to September (wet weight 0.01 ± 0.01 g m^?3), coinciding with a dramatic increase in M. leidyi biomass (wet weight 18.3 ± 16.1 g m^?3 during August). These two species showed a high overlap in their respective diets (copepods, meroplankton) during the study period. Predation impact of C. harengus on calanoid copepods was highest in June and July where 84 and 41 % of the standing stock were eaten per day in June and July, respectively. Predation impact of M. leidyi on calanoid copepods was highest in September (16 %). Based on stable isotope analysis C. harengus and M. leidyi were assigned to a trophic level of 3.08 and 2.47, respectively. Furthermore, we assessed the potential of competition between M. leidyi and C. harengus in a mesocosm experiment. Results indicated that at present zooplankton densities intraspecific competition in C. harengus seemed to be greater than interspecies competition with M. leidyi. Due to the low predation impact of M. leidyi and the reduced temporal overlap, competition between M. leidyi and C. harengus during the study period was estimated as low. Nevertheless, considering the high dietary overlap and the inter-annual variation in biomass and occurrence of both species and their zooplankton prey, competition in the Wadden Sea area cannot be excluded.