Ecological and developmental dynamics of a host-parasite system involving a sea anemone and two ctenophores

The lined sea anemone Edwardsiella lineata has evolved a derived parasitic life history that includes a novel body plan adapted for life inside its ctenophore hosts. Reputedly its sole host is the sea walnut, Mnemiopsis leidyi, a voracious planktivore and a seasonally abundant member of many pelagic ecosystems. However, we have observed substantially higher E. lineata prevalence in a second ctenophore species, the ctenophore predator Bero? ovata. The interplay among these 3 species has important conservation consequences as M. leidyi introductions are thought to be responsible for the severe depletion of numerous commercial fisheries in the Mediterranean basin, and both E. lineata and B. ovata have been proposed as biological controls for invasive M. leidyi. Over a 3-yr period (2004-2006), we collected 8,253 ctenophores from Woods Hole, Massachusetts, including M. leidyi, B. ovata, and a third ctenophore, Pleurobrachia pileus, and we recorded E. lineata infection frequencies, parasite load, and parasite location. We also conducted laboratory experiments to determine the likely mechanisms for parasite introduction and the effect of each host on parasite development. We observed peak E. lineata infection frequencies of 0% in P. pileus, 59% in M. leidyi, and 100% in B. ovata, suggesting that B. ovata could be an important natural host for E. lineata. However, in laboratory experiments, E. lineata larvae proved far more successful at infecting M. leidyi than B. ovata, and E. lineata parasites excised from M. leidyi exhibited greater developmental competence than parasites excised from B. ovata. Although we show that E. lineata is efficiently transferred from M. leidyi to B. ovata when the latter preys upon the former, we conclude that E. lineata larvae are not well adapted for parasitizing the latter species and that the E. lineata parasite is not well adapted for feeding in B. ovata; these developmental and ecological factors underlie the host specificity of this recently evolved parasite.     

Native and non-native ctenophores in the Gulf of Trieste, Northern Adriatic Sea

Observations of ctenophore species were made in the Gulf ofTrieste between 2003 and 2006. We examined native ctenophorespecies with special attention to representatives of the ordersLobata and Beroida, and we recorded among them two non-nativectenophores: Mnemiopsis leidyi A. Agassiz 1865 and Beroe ovatasensu Mayer, 1912. The validity of the Mediterranean speciesBeroe ovata is discussed. We determined that among the nativespecies, it is not Beroe ovata but rather Beroe cucumis sensuMayer, 1912 that occurs in the Mediterranean Sea.     

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.     

In vitro fertilization in ctenophores: sperm entry, mitosis, and the establishment of bilateral symmetry in Beroe ovata.

We have found ways to control in vitro fertilization in a ctenophore (Beroe ovata) for the first time. This is based on the existence of a partial block to self-fertilization at the time of gamete release which can be overcome by removal of the egg envelope. It has allowed us to exploit the excellent optical properties of Beroe eggs to make detailed observations on all events from sperm penetration or penetrations in these physiologically polyspermic eggs to first cleavage, and to extend our initial observations (Carré and Sardet, 1984). Sperm entry is characterized by local modifications of the egg cortex in a 70-microns zone around the penetration site or sites. Upon sperm entry, the egg surface contracts and relaxes locally, then a fertilization cone forms and disappears. These events are accompanied by localized exocytosis, growth of a ring of microvilli, thickening of the egg cortex, and gathering of mitochondria around the sperm pronuclei. The female pronucleus then migrates beneath the egg surface toward one or successive sperm pronuclei. The fusion of pronuclei, sperm and egg chromatin intermixing, and mitosis were also observed with exceptional clarity. Furthermore, we have noticed that the direction of the last trajectory of the female pronucleus tends to define the orientation of the mitotic spindle, and as a consequence the position of first unipolar cleavage furrow. This in turn determines the future sagittal plane of the embryo and of the adult B. ovata.     

Laboratory studies of predation by the ctenophore Mnemiopsis leidyi on the early stages in the life history of the bay anchovy, Anchoa mitchilli

The lobate ctenophore, Mnemiopsis leidyi, consumed eggs andlarvae of the bay anchovy, Anchoa mitchilli, in laboratory experiments.This ctenophore exhibited a type I functional response to increasesin egg densities without reaching saturation at high prey densities.Clearance rate increased with increasing experimental containervolume. There was a 3-fold increase in the volume of water clearedby 2.0–2.5-cm ctenophores and a >5-fold increase for4.5–5.0-cm ctenophores in 15–1 versus 100–200–1containers. Clearance rate was dependent on tenophore lengthbut was probably underestimated for the larger animals due tocontainer effects. The presence of various densities of alternateprey, Acartia hudsonica or Anemia sp. nauplii, in addition toAnchoa mitchilli eggs did not affect the clearance rates onthe eggs alone. Comparison of clearance rates of 2.0–2.5-cmctenophores on various ages of starved and fed bay anchovy larvaeindicated that predation may be higher on yolk-sac larvae thanon eggs but decreases as the larvae grow. After 3 days posthatch starved anchovy larvae become more vulnerable to predationthan fed larvae. The ctenophore, M.leidyi has the potentialto inflict substantial predation pressure on early stages inthe life history of bay anchovy. ¹ Present address: University of Maryland, Center for Environmentaland Estuarine Studies, Chesapeake Biological Laboratory, POBox 38, Solomons, MD 20688-0038, USA     

Growth dynamics of a ctenophore (Mnemiopsis) in relation to variable food supply. II. Carbon budgets and growth model

Our goal was to test our understanding of ingestion, assimilationefficiency and metabolism for Mnemiopsis mccradyi by formulatingand validating a simulation model of growth under differentconditions of food availability. The model was based on a carbonbudget approach using formulations derived from empirical results,including how each process was affected by food availabilityand ctenophore size. An experimentally measured carbon budgetfor pulsed food availability indicated that, relative to totalingestion, growth was high (17–48%), respiration plusorganic release was relatively low (24–48%) and little(<10%) of the ingested carbon was unaccounted for. New laboratoryinvestigations of feeding and assimilation efficiency were necessaryto refine the formulations so that model predictions comparedfavorably with a variety of laboratory measurements of growth,and growth efficiency, as well as the complete experimentallymeasured carbon budget. The refined model predicted a high ratioof growth to metabolism (>2) and a high gross growth efficiency(>30%) for smaller ctenophores at high food concentrations(>20 prey l^–1). Both growth rates and growth efficiencieswere predicted to decrease for larger ctenophores. Model predictionswere generally consistent with experimental results, includinginvestigations using pulsed food availability to simulate environmentalpatchiness. Although the model underpredicted ctenophore growthin some experiments at low food densities, the model predictionof a minimum prey concentration of about 8 l^–1 (24 µgC l^–1) for sustaining a ctenophore population of reproductivesize agreed with field observations.     

Salinity gradient of the Baltic Sea limits the reproduction and population expansion of the newly invaded comb jelly Mnemiopsis leidyi

The recent invasion of the comb jelly Mnemiopsis leidyi into northern European waters is of major public and scientific concern. One of the key features making M. leidyi a successful invader is its high fecundity combined with fast growth rates. However, little is known about physiological limitations to its reproduction and consequent possible abiotic restrictions to its dispersal. To evaluate the invasion potential of M. leidyi into the brackish Baltic Sea we studied in situ egg production rates in different regions and at different salinities in the laboratory, representing the salinity gradient of the Baltic Sea. During October 2009 M. leidyi actively reproduced over large areas of the Baltic Sea. Egg production rates scaled with animal size but decreased significantly with decreasing salinity, both in the field (7-29) and in laboratory experiments (6-33). Temperature and zooplankton, i.e. food abundance, could not explain the observed differences. Reproduction rates at conditions representing the Kattegat, south western and central Baltic Sea, respectively, were 2.8 fold higher at the highest salinities (33 and 25) than at intermediate salinities (10 and 15) and 21 times higher compared from intermediate to the lowest salinity tested (6). Higher salinity areas such as the Kattegat, and to a lower extent the south western Baltic, seem to act as source regions for the M. leidyi population in the central Baltic Sea where a self-sustaining population, due to the low salinity, cannot be maintained.     

Zooplankton biomass gradient off south-western Nova Scotia: nearshore ctenophore predation or hydrographic separation?

A persistent large-scale cross-shelf gradient in zooplanktonbiomass >1050 µm was evident off south-western NovaScotia during annual spring surveys between 1985 and 1987, withrelatively low levels inshore and higher levels offshore. Conversely,the abundance of the tentaculate ctenophore Pleurobrachia pileuswas the greatest inshore, and distributed reciprocally to zooplankton>1050 µm. The principle prey of both adult ctenophoresand post-larval cod is zooplankton >1050 µm (primarilycalanoid copepods), and cod growth rates are strongly influencedby prey biomass. Ctenophore predation appears to have been responsiblefor the low nearshore zooplankton biomass, whereas the influenceof hydrographic factors on the zooplankton gradient was minimal.On a smaller scale, persistent, abrupt changes in zooplanktonbiomass >1050 µm and ctenophore density existed 3–30km from shore, in contrast to linear gradients in water density(₁) during a 5 week sampling period in spring 1987. Ctenophoreswere confined to depths <55 m and zooplankton >1050 µmpredominantly occurred at depths >55 m. High concentrationsof chlorophyll and phaeopigment were evident at depths <55m also suggesting intense predation by ctenophores on largeherbivores. The relatively high proportion of smaller zooplankton(153–308 –m) in the nearshore is also consistentwith the predation hypothesis. The reduced growth experiencedby post-larval cod inshore appears generated by ctenophore predationof a common prey resource.     

The nutritional ecology of the ctenophore Bolinopsis vitrea: comparisons with Mnemiopsis mccradyi from the same region

Bolinopsis vitrea is a warm water lobate ctenophore which doesnot overlap in its distribution with Mnemiopsis mccradyi incontiguous waters. We examined its feeding ecology on a seriesof cruises. B. virrea ingested increasingly more prey at higherfood concentrations (2–100 prey l^–1) but feedingeffort (clearance rate) decreased with increasing food availability.On a dry weight basis, smaller tentaculate Bolinopsis ingestedseveral times more than larger lobates, but based on carbonweight, specific ingestion was fairly uniform over the entiresize range investigated (6–60 mm total length). Bolinopsiscollected during the daytime in the Bahamas rarely had morethan three prey items in their guts. These results and laboratorymeasurements of digestion times (av. = 1.9 h) allowed computationof daily rations, which could not account for the metabolicrequirement as measured on the same cruises. Results of feedingexperiments, however, implied that prey densities in excessof 11^–1 were sufficient to sustain a growing populationof Bolinopsis. Prey concentrations about an order of magnitudehigher were required for M. mccradyi based on similar experiments.These results were in general agreement with observed densitiesand distributions of ctenophores and their zooplankton preyin the Bahamas and coastal South Florida.     

Spatial and temporal distribution of mesozooplankton in the Gulf of Aqaba and the northern Red Sea in February/March 1999

The distribution pattern, taxonomic composition and communitystructure of mesozooplankton was studied along a transect with10 positions between the Gulf of Aqaba and the northern RedSea. Five positions were resampled two or three times duringa cruise of RV ‘Meteor’ in February/March 1999.In spite of clear differences in the density stratificationbetween the Gulf of Aqaba and the northern Red Sea, the mesozooplanktoncomposition was very similar: Copepods were by far the mostabundant taxon, contributing 76–95% to the total community.The remainder was composed largely of ostracods, chaetognaths,appendicularians and molluscs. The mesozooplankton of the deeplymixed stations was homogeneously distributed, at all other stationsthe bulk of the mesozooplankton (>70%) was concentrated inthe mixed surface layer with peaks of calanoids, cyclopoidsand appendicularians in the vicinity of the chlorophyll a (Chla) maximum layer. Ostracods and poecilostomatoids dominatedthe layers below. Standing stocks within the total water column(550–1200 m) varied between 93 and 431 x 10³ individualsm^–2 for copepods and 5–76 x 10³ individuals m^–2for other mesozooplankton with highest numbers in the northernGulf of Aqaba, where vertical mixing was deep (400–500m) and Chl a and mesozooplankton distributions homogeneous throughoutthe water column. Towards the south, the mixed depth decreasedfrom 300 m in the central Gulf of Aqaba to 50 m in the Red Sea.Cluster analysis separated three distinct groups of stations,compounding the observed differences between the northern Gulfof Aqaba (Position I) and the other positions. The analysisalso revealed temporal differences between the February andMarch sections of the cruise, indicating the winter–springtransition. The stations sampled in March are characterisedby a higher total abundance and by a higher percentage of appendiculariansand ostracods than the stations sampled in February     

Growth and feeding rates of the newly hatched larval ctenophore Mnemiopsis leidyi A. Agassiz (Ctenophora, Lobata)

Mnemiopsis leidyi: larvae depend on microplankton (<200 µm) prey duringthe first few days following hatching until larvae are >0.5mm in length and can successfully capture and consume mesozooplanktonprey. Feeding and growth rates of newly hatched M. leidyi larvaewere measured in controlled laboratory experiments. When fednatural microplankton assemblages, newly hatched larvae consumedsignificant quantities of both autotrophic and heterotrophicprey, including diatoms, phototrophic, heterotrophic and mixotrophicdinoflagellates, euglenoid flagellates, aloricate and tintinnidciliates, and rotifers. Average per capita clearance rates were1.99–7.59 mL individual^–1 h^–1 ( = 4.01 mL individual^–1 h^–1; SD = 1.95)and total per capita ingestion was 0.01–4.70 µgC individual^–1 day^–1 x 10² ( = 0.83 µg C individual^–1 day^–1 x 10²; SD =1.89). Larval growth rates were –0.13 to 0.56 mm individual^–1day^–1 (equivalent to –1.72 to 4.33 µg C individual^–1day^–1) over a range of larval sizes from 0.5 (<0.5µg C) to 5 mm (85 µg C). A diet consisting entirelyof microplankton prey supported larval growth for >2 weeks,and growth rate decreased when larvae reached 4–5 mm inlength, corresponding to the beginning of their morphologicaltransition from tentaculate to lobate feeding mode. The grossgrowth efficiency of larvae fed natural microplankton assemblageswas 3%.     

Microzooplankton, vertical mixing and advection in a larval fish patch

A large ({small tilde}30 ? 75 km) patch of larval walleye pollock,Theragra chalcogramma, was located south of the Alaska Peninsuladuring May 1986. A drifter deployed in this patch followed ananticyclonic path consistent with dynamic topography. Changesin community composition and vertical distribution of microzooplankton>40 µm were sampled for 4 days alongside this drifterto examine feeding conditions for larvae. Biological and physicalchanges during the first 2 calm days revealed substantial small-scalevariability within the larger circulation pattern. Changes duringthe last 2 days were dominated by vertical mixing due to strongwinds. Despite mixing, prey concentrations remained adequatefor feeding by larval pollock as determined by laboratory studies.A satellite-tracked drifter replaced the first drifter and wasstill located within the patch 6 days later. Overall distributionsof larvae and movements of the drifters show a net translationof 7.8 km day^–1 south-westward, but details of the studyreveal complex interactions between coastal waters and a coastalcurrent. During the 10-day period there was an increase in standardlength of the larval fish population of 0.13 mm day^–1and a decline in abundance of {small tilde}7.6% day^–1.Both calculated rates must be underestimates due to continuingrecruitment of small larvae from hatching eggs.     

Spatial patterns of surface blooms and recruitment dynamics of Calanus finmarchicus in the NE Norwegian Sea

Spatial dynamics of phytoplankton blooms and Calanus finmarchicuswere analysed in a large-scale oceanic area (10274 km) in theNE Norwegian Sea (69–71°N, 12–16°E). Dataon hydrography, chlorophyll a (Chl a) and mesozooplankton (netsamples and Optical Plankton Counter) were collected in surfacewaters (0–30 m) during spring and early summer 2003. Spatialpatterns of copepodites in relation to hydrography and Chl aconcentration were analysed by constrained correspondence analysis.Distribution of phytoplankton and C. finmarchicus was highlypatchy. The overwintering generation (CV and adult females)had highest abundances in areas with phytoplankton blooms, whereasthe recruiting cohort (CII + CIII copepodites) was found inwater parcels with low Chl a concentration. Differences in recruitmentdynamics between the southern and northern Norwegian Sea arediscussed. Our data confirm the importance of phytoplanktonspring blooms in initiating the recruitment of C. finmarchicus.This opens up for a future mapping of sea surface chlorophyllto depict the large-scale variability in the demography of C.finmarchicus during the reproductive season in the NorwegianSea.     

Relationships between microzooplankton and mesozooplankton: competition versus predation on natural assemblages of the Gulf of Trieste (northern Adriatic Sea)

We performed, on a seasonal basis, 16 dilution experiments and,simultaneously, copepod or cladoceran grazing experiments onnatural assemblages from Gulf of Trieste (northern AdriaticSea). The autotrophic fraction was almost entirely composedof diatoms in late winter. As the seasons progressed, relativeabundance of nanoplankton and cyanobacteria increased. Microzooplanktonwas always present in the diet of mesozooplankton, even if inpercentages usually not exceeding 6% of diet intake on carbonbasis. Microzooplankton took advantage of ephemeral increasesof autotrophic biomass when prey were in the optimal size rangebut did not consume diatoms when these were large. When autotrophicresources were scarce, micrograzers used heterotrophic biomasswhich, in turn, fuelled the upper trophic levels through predationby mesozooplankton on microzooplankton. Microzooplankton grazingwas the most important loss term of primary production in theGulf of Trieste (on average, microzooplankton consumed 100%of primary production, while mesozooplankton only 76%), whichcan be considered a mesotrophic coastal system. This paper is one of six on the subject of the role of zooplanktonpredator–prey interactions in structuring plankton communities.     

Phenological shifts of three interacting zooplankton groups in relation to climate change

Over the past several decades, global warming has been linked to shifts in the distributions and abundances of species. In the southern North Sea, temperatures have increased in the last three decades and this will likely have consequences on the seasonality of marine organisms living in the area. Ctenophores such as Beroe gracilis and Pleurobrachia pileus could be particularly affected by changes in their own phenology and that of their prey, thus causing shifts in ecosystem function. Despite their global relevance and their potentially deleterious effect on the fishing industry, only a few long‐term records of ctenophore abundance exist, and most of these records are semiquantitative in nature. Therefore, our knowledge of the influence of environmental factors on their population development is presently very limited. In this study, the long‐term abundance dynamics of B. gracilis, P. pileus and their food calanoid copepods were analysed for a highly temporally resolved time series in the German Bight at Helgoland Roads. Special attention was focused on the response of these organisms to climate warming. Bayesian statistics showed that the phenology of the two ctenophores shifted in a step‐like mode in the year 1987/1988 to permanent earlier appearances. The seasonal change in the population blooms of P. pileus and B. gracilis correlated remarkably well with a step‐like increase in winter and spring sea surface temperatures of the southern North Sea. Possible explanations for the changes observed in these organisms include higher reproductive rates, increased winter survival rates or both. Interannual variations in ctenophore abundances correlated best with the interannual changes in spring temperatures, although the impact of temperature on B. gracilis appeared less pronounced. The changes in copepods abundance were not consistent with changes in P. pileus and B. gracilis. P. pileus showed longer periods of high abundance after the permanent seasonal advancement. These longer periods were correlated with a decline in the average autumn abundance of copepods. Changes in the phenology of these organisms raise the concerns on the declining state of fish stocks, which could potentially be exacerbated by gelatinous zooplankton outbreaks. These conditions may ultimately lead to trophic dead ends by channelling the flow of energy away from higher trophic levels.     

Feeding and metabolism of the siphonophore Sphaeronectes gracilis

The in situ predation rate of the siphonophore Sphaeronectesgracilis was estimated from gut content analysis of hand-collectedsiphonophores and from laboratory data on digestion rates ofprey organisms. At daytime prey densities of 0.25 copepods 1^–1,S. gracilis was estimated to consume 8.1 – 15.4 prey day^–1siphonophore^–1. From data on abundances of siphonophoresand copepods, S. gracilis was estimated to consume 2–4%of the copepods daily. In laboratory experiments, ingestionrates averaged 13.8 prey day^–1 siphonophore^–1 atprey densities of 5 copepods 1^–1 and 36.9 at 20 copeods1^–1. This was equivalent to a specific ingestion rate(for both carbon and nitrogen) of –17% day^–1 and45% day^–1, respectively, while specific ingestion in situwas only 2% day^–1. Ammonium excretion averaged 0.095 µg-atsiphonophore^–1 day^–1 at 5 prey 1^–1, and 0.162at 20 prey 1^–1. The specific respiration (carbon) andspecific excretion (nitrogen as ammonium) were calculated tobe 3% day^–1 at the lower experimental food level, and5% day^–1 at the higher food level. ¹Contribution from the Catalina Marine Science Center No. 66. ²Present address: Dept. of Biology, University of Victoria,Victoria, B.C., Canada V8W 2Y2.     

Capelin larvae (Mallotus villosus) and community structure of zooplankton off the coast of northern Norway

Capelin larvae (Mallotus villosus) and mesozooplankton werecollected in May 2001 in the southern part of the Barents Sea.During the period of hatching and first feeding, capelin larvaeexperience high mortality rates. In order to understand theunderlying mechanisms influencing larval survival, we monitoreda selected site for 10 days using a variety of different samplingapproaches. Hydrological parameters conductivity–temperature–depth–fluorescence(CTDF) were recorded and zooplankton samples obtained simultaneouslyusing MOCNESS at three time periods. The mesozooplankton consistedessentially of copepods (85), dominated by Calanus finmarchicus.The zooplankton community consisted of different assemblagesderived from different areas (i.e. different water masses; Atlanticand coastal). Capelin larvae were found in surficial coastalwaters. Copepod nauplii and C. finmarchicus were less abundantin coastal waters where small copepodites (such as Acartia spp.and Temora longicornis) were found together with early stagesof coastal invertebrates that are possible prey for capelinlarvae. We suggest that capelin larvae base their foraging onsmall prey organisms frequently found among the coastal zooplanktonand that these larvae are less dependent on the propagationof the recruiting generation of C. finmarchicus present fromApril to June.     

Axis establishment and microtubule-mediated waves prior to first cleavage in Beroe ovata

The single axis (oral-aboral) and two planes of symmetry of the ctenophore Beroe ovata become established with respect to the position of zygote nucleus formation and the orientation of first cleavage. Bisection of Beroe eggs at different times revealed that differences in egg organisation are established in relation to the presumptive oral-aboral axis before first cleavage. Lateral fragments produced after but not before the time of first mitosis developed into larvae lacking comb-plates on one side. Time-lapse video demonstrated that waves of cytoplasmic reorganisation spread through the layer of peripheral cytoplasm (ectoplasm) of the egg during the 80 minute period between pronuclear fusion and first cleavage, along the future oral-aboral axis. These waves are manifest as the progressive displacement and dispersal of plaques of accumulated organelles around supernumerary sperm nuclei, and a series of surface movements. Their timing and direction of propagation suggest they may be involved in establishing cytoplasmic differences with respect to the embryonic axis.Inhibitor experiments suggested that the observed cytoplasmic reorganisation involves microtubules. Nocodazole and taxol, which prevent microtubule turnover,blocked plaque dispersal and reduced surface movements.The microfilament-disrupting drug cytochalasin B did not prevent plaque dispersal but induced abnormal surface contractions. We examined changes in microtubule organisation using immunofluorescence on eggs fixed at different times and in live eggs following injection of rhodamine-tubulin. Giant microtubule asters become associated with each male pronucleus after the end of meiosis. Following pronuclear fusion they disappear successively, those nearest the zygote nucleus shrinking first, to establish gradients of aster size within single eggs. Regional differences in microtubule behaviour around the time of mitosis were revealed by brief taxol treatment, which induced the formation of small microtubule asters in the region of the nucleus or spindle during both first and second cell cycles. The observed wave of change may thus reflect the local appearance and spreading of mitotic activity as the zygote nucleus approaches mitosis.     

Decadal change in zooplankton seasonal succession in the Bahia Blanca estuary, Argentina, following introduction of two zooplankton species

Seasonal zooplankton succession (mainly of the mesozooplankton)in the inner zone of Bahía Blanca estuary during theperiod 1990–1991 was compared to that from the period1979–1980 in order to detect changes having taken placein 10 years. Taxonomic composition, zooplankton abundance, speciesoccurrence and temperature–salinity conditions were studiedwithin the two annual cycles. This estuarine ecosystem has beendeeply affected by a population increase and by the rapid growthof industrial activity and maritime traffic over the last twodecades. Several changes in the zooplankton assemblage compositionand structure were detected, probably due to temperature–salinitygradient patterns, which showed discernible differences. Dominance,during winter–spring, of two new invading species, namely,the cirriped Balanus glandula and the copepod Eurytemora americana,causing a decrease in the autochthonous key copepod Acartiatonsa, was one of the principal features of the period 1990–1991.Seasonal changes in diversity and species richness, as wellas differences in the multivariate general pattern of the zooplanktoncommunity structure with a consequent decrease in the communitystability during the second period, were also observed.     

Spatio-temporal overlap of the alien invasive ctenophore <Emphasis Type="Italic">Mnemiopsis leidyi</Emphasis> and ichthyoplankton in the Bornholm Basin (Baltic Sea)

In 2007 the alien invasive ctenophore Mnemiopsis leidyi A. Agassiz 1865 was recorded for the first time in the Bornholm Basin, an area which serves as important spawning ground for Baltic fish stocks. Since M. leidyi is capable of preying upon early life stages of fish and further might act as food competitor for fish larvae, it is of major concern to investigate the potential threat that this non-indigenous species poses to the pelagic ecosystem of the Baltic Sea. The present study investigates the temporal and spatial overlap of M. leidyi with eggs and larvae of Baltic cod (Gadus morhua L.) and sprat (Sprattus sprattus L.) in order to assess the potential impact of this new invader on two of the most important Baltic fish stocks. Results show variable inter-seasonal distribution and overlap dynamics and thus different seasonal threat-scenarios for the early life stages of cod and sprat. The spatial overlap between M. leidyi and ichthyoplankton was low for most of the period observed, and we conclude that M. leidyi presently does not have a strong impact. However, we detected situations with high overlaps, e.g. for sprat larvae and cod eggs in spring. As the population dynamics of M. leidyi in the central Baltic are not yet fully understood, a future population explosion of the alien ctenophore with possible effects on fish recruitment cannot be ruled out. Furthermore, a possible shift in peak spawning of cod to the early season, when ctenophore abundances were relatively high, might increase the impact of M. leidyi on cod.