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, Na = 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 FST = 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 FST = 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

Modelling assessment of interactions in the Black Sea of the invasive ctenophores Mnemiopsis leidyi and Beroe ovata

We analyzed the main factors that controlled the prey-predator dynamics of two invasive ctenophores, Mnemiopsis leidyi and Beroe ovata in the Black Sea using a demographic model. We assessed the bottom up cascading effect from edible zooplankton to its consumer M. leidyi and its predator B. ovata. For these purposes, we used life cycles of both ctenophores (ova, larva, juvenile, transitional and adult stages), variability of annual phenology and physiological features obtained from our field observations and experiments made in the northeastern Black Sea over 27 years, combined with a long-term change in temperature and food availability for both ctenophores. Model outputs were compared with field observations. Then, model scenarios were tested to understand which environmental conditions control M. leidyi and B. ovate development. Using our model, we found that the maximum annual abundances of M. leidyi and B. ovata increased with mean springtime temperature and development of spring-early summer zooplankton which is important for creation of M. leidyi abundance and consequently development of B. ovate. An assessment with changing concentration of the M. leidyi food (i.e. zooplankton) at the time of its annual development changed the maximum annual values reached by M. leidyi, and consequently B. ovate. It was found that time of appearance of B. ovata had changed to May since 2012 with increasing temperature, and as a result M. leidyi did not have time to reach high abundance, being grazed by B. ovate already since May. Model results were qualitatively the same as those from long-term field observations. As a result, we obtained a model of two ctenophores interacting with total life structure: duration and scale of reproduction, growing from stage to stage, mortality, seasonal disappearance from water depending on temperature and prey availability. Similar analyses have never been done in the Black Sea and can be used for other seas where M. leidyi or both ctenophores invaded

Interactions between invasive ctenophores in the Black Sea: assessment of control mechanisms based on long-term observations

International audienceInvasion of the carnivorous ctenophore Mnemiopsis leidyi in the Black Sea in the 1980s disrupted the ecosystem, which started to recover with the arrival of the predatory ctenophore Beroe ovata in 1997. We used the results of 25 yr of field observations and experiments in the northeastern Black Sea to assess 3 hypotheses that should explain most of the population dynamics of M. leidyi and B. ovata. The first hypothesis is that since its arrival, B. ovata has controlled the period of the year during which M. leidyi was present in sizable concentrations. This hypothesis is supported by the observation that M. leidyi abundance was sizable almost year-round (spring, summer, autumn) before the arrival of B. ovata but was sizable only for a period of 1.3 to 3.1 mo (mostly summer) after its arrival. The second hypothesis is that the same sequence of predator prey mechanisms that led B. ovata to shorten the duration of a sizable M. leidyi population occurred every year irrespective of interannual environmental variability. This is supported by the repetition of the same reproductive sequences of the 2 ctenophores yearly since 1999 despite differences in environmental factors. The third hypothesis (i.e. environmental conditions influenced the joint abundances of the 2 species) is supported by the observed covariability between the 2 species every year. Experimental and field results identified temperature, food and wind as the key factors influencing M. leidyi, which suggested that the interannual environmental variations that affect M. leidyi abundance cause proportional changes in B. ovata abundance. Some aspects of these hypotheses have been previously examined in the literature, but this is the first study in which they are assessed using a consistent set of data

Molecular insights into the ctenophore genus Beroe in Europe: new species, spreading invaders

The genus Beroe Browne, 1756 (Ctenophora, Beroidae) occurs worldwide, with 25 currentlydescribed species. Because the genus is poorly studied, the definitive number of species is uncertain. Recently, a possible new Beroe species was suggested based on internal transcribed spacer 1 (ITS1) sequences from samples collected in Svalbard, Norway. Another species, Beroe ovata, was introduced to Europe from North America, initially in the Black Sea and subsequently (and possibly secondarily) into the Mediterranean and Baltic Seas. In areas where ctenophores have been introduced, they have often had significant detrimental ecological effects. The potential for other cryptic and/or undescribed Beroe species and history of spread of some species in the genus give reason for additional study. When alive, morphological hallmarks may be challenging to spot and photograph owing to the animals’ transparency and near-constant motion. We sampled and analyzed 109 putative Beroe specimens from Europe, using morphological and molecular approaches. DNA analyses were conducted using cytochrome oxidase 1 and internal transcribed spacer sequences and, together with published sequences from GenBank, phylogenetic relationships of the genus were explored. Our study suggests the presence of at least 5 genetic lineages of Beroe in Europe, of which 3 could be assigned to known species: Beroe gracilis Künne 1939; Beroe cucumis Fabricius, 1780; and Beroe ovata sensu Mayer, 1912. The other 2 lineages (here provisionally named Beroe “norvegica” and Beroe “anatoliensis”) did not clearly coincide with any known species and might therefore reflect new species, but confirmation of this requires further study

Venn diagram illustrating shared haplotypes/alleles between regions.

<p>Venn diagram showing COI haplotypes (A) and ITS alleles (B) sharing between Mediterranean and possible source populations from North America and Black-Azov Seas. Note that haplotype Ml01 and allele G from South America are excluded.</p