A mechanism for self-inhibition of population growth in the flatworm Mesostoma ehrenbergii (Focke)

Summary By producing subitaneous eggs the rhabdocoel Mesostoma ehrenbergii may quickly increase the population size. The production of this kind of eggs by young worms is inhibited by the presence of adults in the culture medium. The inhibition is specific, and it is observed also if young and adult worms are not in direct contact, or if young worms are added to the water where adults had been previously kept: there is therefore evidence for a chemical mediator. The sensitivity to the inhibition occurs during the first five days after birth, with a maximum on the 2nd to 3rd day.     

Behavior of the rhabdocoel flatworm Mesostoma ehrenbergii in prey capture and feeding

The prey-capture and feeding behavior of the rhabdocoel flatworm Mesostoma ehrenbergii (Focke, 1836) was analyzed using a variety of live and dead prey, including Daphnia, mosquito larvae, and tubifex annelids. Prey-capture behavior was broken down into its individual components. Mesostoma could accommodate to and change its behavior depending on the size and type of prey. Mechanical rather than chemical cues were effective in inducing prey-capture behavior. No evidence for a special chemical paralysis as suggested by other workers was found. The apparent paralysis observed in cladocera such as Daphnia and mosquito larvae was, in part a behavioral response of the prey in ‘playing possum’ and also in part due to immobilization of the prey by the flatworm with mucous threads. This revised version was published online in July 2006 with corrections to the Cover Date.     

Predation on ciliates by freshwater calanoid copepods: rates of predation and relative vulnerabilities of prey

• 1 The susceptibility of ciliates in a mesotrophic lake to predation by Epischura lacustris, Diaptomus minutus and D. pygmaeus was assessed during summer. Oligotrichs, particularly Strobilidium velox (c. 43 μm), were removed efficiently by adult copepods and contributed substantially to the diet of female D. minutus. The presence of approximately 1,6 adult Epischura 1-^?1, or sixteen adult female Diaptomus 1-^?1, could halt the growth of S. vechx populations in summer.
• 2 Factors affecting the rate at which copepods prey on ciliates were examined in experiments with D. pygmaeus and cultured ciliates. Rate of predation on S. velox, the preferred species, became saturated at 5 S. velox ml^?1 (45ngCml^?1) and did not change with a 10-fold increase in alternative algal food.
• 3 Behavioural differences among ciliates, and the presence of other ciliates, contributed to differences in ciliate susceptibility to predation and suggest reasons why rates of removal of ciliates are not related to ciliate size.
• 4 By feeding selectively, at high rates, calanoids may suppress populations of some ciliates and thereby influence microzooplankton community structure.

     

The embryonic development of the rhabdocoel flatworm Mesostoma lingua (Abildgaard, 1789)

The embryonic development of the flatworm Mesostoma lingua was studied using a combination of life observation and histological analysis of wholemount preparations and sections (viewed by both light and electron microscopy.) We introduce a series of stages defined by easily recognizable morphological criteria. These stages are also applicable to other platyhelminth taxa that are currently under investigation in our laboratory. During cleavage (stages 1 and 2), the embryo is located in the center of the egg, surrounded by a layer of yolk cells. After cleavage, the embryo forms a solid, disc-shaped cell cluster. During stage 3, the embryo migrates to the periphery of the egg and acquires bilateral symmetry. The side where it contacts the egg surface corresponds to the future ventral surface of the embryo. Stage 4 is the emergence of the first organ primordia, the brain and pharynx. Gastrulation, as usually defined by the appearance of germ layers, does not exist in Mesos-toma; instead, organ primordia emerge ”in situ” from a mesenchymal mass of cells. Organogenesis takes place during stages 5 and 6. Cells at the ventral surface form the epidermal epithelium; inner cells differentiate into neurons, somatic and pharyngeal muscle cells, as well as the pharyngeal and protonephridial (excretory) epithelium. A junctional complex, consisting initially of small septate junctions, followed later by a more apically located zonula adherens, is formed in all epithelial tissues at stage 6. Beginning towards the end of stage 6 and continuing throughout stages 7 and 8, cytodifferentiation of the different organ systems takes place. Stage 7 is characterized by the appearance of eye pigmentation, brain condensation and spindle-shaped myocytes. Stage 8 describes the fully dorsally closed and differentiated embryo. Muscular contraction moves the body in the egg shell. We discuss Mesostoma embryogenesis in comparison to other animal phyla. Particular attention is given to the apparent absence of gastrulation and the formation of the epithelial junctional complex. Received: 10 February 2000 / Accepted: 10 April 2000     

On the ecology of marine cyclopoid copepods (Crustacea, Copepoda)

The goal of these studies was to improve our understanding ofhow Oithonidae and Oncaeidae can exist in nearly every partof the ocean. In particular, it was intended to obtain quantitativeinformation on reproduction rates and the longevity of adultfemales, and on feeding and growth rates of Oncaeidae. Whenfeeding at relatively high food levels, early copepodids ofOncaea mediterranea ingested     

Colonization of the pelagic realm by calanoid copepods

The evolution of calanoid copepods probably extends back into the mid-Paleozoic. Environmental change from the Paleozoic through to the Tertiary is reviewed. Turbidity, water clarity, oxygen, pelagic primary production, and tectonically induced changes in the morphology of the oceans are probably all important drivers of calanoid evolution and their invasion of the pelagic realm. Current views of the phylogeny of the Calanoida are presented as well as a review of some recent work on metabolic potential, female genital system, and nervous system. It is hypothesized that ancestors of the Arietelloidea and Diaptomoidea invaded the water column in the Devonian at a similar time to the Ostracoda and that the ancestors of the Calanoidea–Clausocalanoidea, with their myelinated axons, arose in the Permian during the major deep ventilation of the ocean. Present day distributions of some Diaptomidae, Centropagidae, and Calanidae suggest that these families successfully came through the Jurassic/Cretaceous expansion of the oxygen minimum zone and the K-T boundary event. Some Arietelloidea and Clausocalanoidea became secondarily benthic and may have survived the K-T boundary event in this environment. It is postulated that some Clausocalanoidea reinvaded the water column (e.g. Clausocalanidae, Aetideidae, Phaennidae, Scolecitrichidae) in the Tertiary after finding refuge in deep, low oxygen water away from the sea surface. It is possible that these hypotheses may be testable using genetic information in the near future.     

Fine structure and functional role of the coverings of the eggs inMesostoma ehrenbergii (Focke) (Turbellaria,Neorhabdocoela)

Summary European populations of the turbellarianMesostoma ehrenbergii can produce two types of eggs according to a seasonal cycle. Subitaneous eggs (S eggs) are produced throughout the warm season and develop rapidly inside the parental uteri. They are meant to ensure a fast increase in the number of worms. Dormant eggs (D eggs) are produced during the warm season and are usually deposited in water at the death of the parent. They can hatch at the beginning of the next warm season and are, therefore, meant to ensure the survival of the population throughout the cold season, when adult worms are no longer present.S eggs are bounded by a thin, translucent covering which has a loose structure and is mainly proteinaceous in composition. These characteristics account for the functional role played by the S egg covering during embryonic development. The covering has, in fact, to be permeable to parental nutrients, and to keep pace, by stretching, with the enlargement of the embryo. By contrast, D eggs are bounded by a thick shell, presumably composed of a sclerotin, the precursors of which (polyphenols) are synthesized together with yolk in the vitelline cells. The shell has a smooth surface devoid of pores and has the function of isolating the developing D egg from the external environment.Supported by a grant from the Consiglio Nazionale della RicercheThe authors wish to thank Dr. P. Roach for his help in translating the paper     

Population dynamics of calanoid copepods and the implications of their predation by clupeid fish in the Central Baltic Sea

Population dynamics of major Baltic calanoid copepod speciesin the Gotland Basin during the last two decades were characterizedby a decline of Pseudocalanus elongatus associated with decliningsalinities, and an increase of Temora longicornis and Acartiaspp. potentially due to warmer conditions. Additionally thisstudy investigated the effect of predation by the major planktivorousfish species herring (Clupea harengus) and sprat (Sprattus sprattus)for the period 1977–1996 in the Gotland Basin (CentralBaltic Sea). Examination of consumption by these fish speciesin relation to copepod production estimates showed a switchby herring from consuming mainly CV/VI of P. elongatus and T.longicornis, to preying on CII of the latter copepod. This switchwas potentially due to increased competition with the drasticallyincreased sprat stock since the late 1980s. Further, an increasedpredation pressure by sprat on CV/CVI of both copepod speciesin spring resulted in higher copepod mortality rates. In consequence,based on these results we suggest that the increase in the spratstock since the late 1980s contributed to a decline of P. elongatus,and additionally prevented an even more pronounced temperature-drivenincrease in the T. longicornis stock, as was observed for Acartiaspp., which was not significantly consumed.     

Origin and evolution of the parasitic cyclopoid copepods

Six of the 10 recognised families of the order Cyclopoida are parasitic, with 4 of them occurring on marine invertebrates and the remaining 2 on freshwater gastropods and fishes, respectively. A cladistic analysis of the 10 families indicates that evolution of parasitism occurred twice in the history of the cyclopoids. The first attempt was made by the marine epibenthic ancestors seeking food and shelter in sessile tunicates — the ascidians. This event led to the evolution of 2 ascidicolous families: Archinotodelphyidae and Notodelphyidae. The descendant of this lineage had also invaded the mantle cavity of marine bivalve molluscs, eventually leading to the evolution of the Mantridae. The second attempt for the parasitic mode of life was launched by the ancestor which was the sister group of the ancestral cyclopoids — the most successful family of freshwater copepods. This ancestral stock, while living in the coastal zone, split into 2 groups: one group stayed behind in the ocean and colonised again the ascidians; the other groups invaded freshwater and evolved into the fish-parasitising Lernaeidae and the gastropod-parasitising Ozmanidae.     

Genome sizes of cyclopoid copepods (Crustacea): evidence of evolutionary constraint

Genome sizes for 36 species of cyclopoid copepods were determined by DNA-Feulgen cytophotometry of nuclei from adults collected from diverse habitats and locales in North America, South America, Europe, and Asia. Genome sizes are small, show a 20-fold range ( C  = 0.10–2.02 pg DNA), and vary in a discontinuous fashion. The genomes of cyclopoid copepods are remarkably small and constant within each species, unlike the large and variable genomes of marine calanoid species. These differences may reflect the evolutionary antiquity of marine copepods in relation to marine, brackish, and freshwater copepods, as well as differences in mechanisms used to modulate genome size. The small genome sizes of contemporary cyclopoids provide substantive evidence of evolutionary constraint, possibly favouring small genomes, rapid replication rates and accelerated development as adaptive strategies for survival in often fragmented, stressful, and changing habitats. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 625–635.     

Achiasmatic meiosis and complex heterozygosity in female cyclopoid copepods (Copepoda,Crustacea)

In Mesocyclops edax S.A. Forbes, 2n=14, a North American copepod, the females are heterozygous for several interchanges leading to the formation of large rings of chromosomes (rings of 14, or of 12 plus 1 bivalent) at meiotic metaphase, comparable to those of the plant Oenothera, although no chiasmata are present. The chromosomes are more or less metacentric and have large terminal H-segments. In the rings homologous arms are held together by connecting fibers which insert close to the euchromatin-heterochromatin junctions. Coordinated orientation of the zigzag type seems to be the role.     

The Eyes of Mesostoma ehrenbergi (Focke, 1836) (Platyhelminthes,Rhabdocoela). Fine Structure and Photoreceptor Membrane Turnover

Abstract Each pigment-cup eye of Mesostoma ehrenbergi consists of two photoreceptor cells, the anterior cell being bilobate. the posterior almost linear, and of a multicellular pigment cup. The nuclei of the photoreceptor cells are located inside the medial region of the brain. Thin cytoplasmic photoreceptor projections provided with neurosecretory-like granules are interposed between the inner surface of the eye cup and the distal extremity of the microvilli. The breakdown and renewal of microvillar membranes was analysed. Membrane turnover is a continuous process. At dusk and during the night abscission of photoreceptive membranes occurs. At dawn the membrane fragments are degraded to granular material, which is then endocytosed into the submicrovillar cytoplasm as coated vesicles. These vesicles form multivesicular bodies. The degradation of multivesicular body content occurs during the following light hours. The dark period is correlated with membrane synthesis for elongation of reticular membranes, which are converted into ellipsoid bodies. The formation of new microvillar membranes occurs at the base of the microvillar border, and involves the fusion with the old microvillar membranes of small vesicles detached from the tubular endoplasmic membranes and from the flattened concentric cisternae of ellipsoid bodies. The correlations with daily cycles of other invertebrates are discussed.     

Feeding behavior of newly settled winter flounder (Pseudopleuronectes americanus) on calanoid copepods

Field and laboratory investigations were conducted to examine feeding by newly settled winter flounder (Pseudopleuronectes americanus) on two co-occurring calanoid copepods, Eurytemora affinis and Acartia hudsonica. During the spring, these prey are present when winter flounder initiate their demersal lifestyle in estuaries of the northeastern United States. Epibenthic zooplankton were collected concurrently with winter flounder in the Navesink River estuary, NJ, in May 1998 and 1999. Although both calanoid species were in the estuary during the 2-year survey, E. affinis was consumed nearly to the exclusion of A. hudsonica by newly settled winter flounder. Annually, E. affinis and A. hudsonica had similar size distributions in field collections, indicating that species choice was not size selective. However, when preying on E. affinis, winter flounder preferred the larger sized organisms. In single species laboratory experiments, E. affinis and A. hudsonica were consumed equally by newly settled winter flounder (19-23 mm TL), but there were more strikes made toward E. affinis. Despite the lower catch efficiency, E. affinis was selected over A. hudsonica when the prey species were offered together in equal numbers. The selection for E. affinis over A. hudsonica by newly settled winter flounder may be the result of behavioral and/or morphological differences in the prey species.     

Filtering and feeding rates of cyclopoid copepods feeding on phytoplankton

The algal biomass ingested by omnivorous cyclopoid copepods (Cyclops kolensis and C. vicinus) was measured by two methods in the hypertrophic Heiligensee in Berlin (West Germany). The clearance and ingestion rates inferred from measurements of natural populations of ¹⁴C labelled phytoplankton were compared with those obtained from chlorophyll a determinations using the presence/absence method (observed chlorophyll a content of natural lake phytoplankton with and without addition of cyclopoids). Both methods gave similar results. Nevertheless, the radio tracer method is preferred, mainly because the short feeding duration excludes high variations in both the food composition and food concentration that limit the presence/absence method.     

Post-embryonic development of cyclopoid copepods in various seasons at Lake Balaton (Hungary)

The development, generation number and diapausing state of threeCyclops species dominant in the open water of Lake Balaton werestudied by means of vials placed back into the lake, in naturalfeeding conditions. The males of all three species turned intoadults earlier than the females. The copepodite V stage wasof the longest duration. Three generations of Cyclops vicinusdeveloped yearly; in spring, autumn and winter. About 50% ofthe copepodite stage IV of the spring generation entered activediapause at the end of May, beginning of June, then entereddormancy between the middle of June and middle of July. Consideringthe literary data as well, it seems that the generation numberof the species is the function of temperature and of the water'strophic state. The Acanthocydops robustus f. limnetica is fondof warm water (> 14°C). Seven generations of this speciesdeveloped completely yearly, during an average of 2-week intervalsbetween May and September. The eightli generation died in thenaupliar stage. The generation number of A. robustus f. limneticadepends on the average temperature of the lake. Inactive diapausecould not be observed for this species. Only one generationof Mesocyclops latckarti developed in autumn. The copepoditeV of the winter generation entered active diapause below a watertemperature of 10°C, and inactive diapause from the endof January, which state ended in March.     

Expression of serotonin and enkephalins in calanoid copepods (Crustacea): an immunohistochemical study

Copepods are the main metazoan component of zooplankton, interms of both the number of species and biomass. Thus, theyhave a key role in pelagic food webs; but curiously, nothingis known of the neuroendocrine substances involved in theirphysiological processes. In higher crustaceans, especially theDecapoda, the role of such molecules in different physiologicalprocesses (motility, feeding, reproduction, day-night migrationsand so on) has been well explored; so, we have mapped expressionsites of some of these substances to provide a better understandingof copepod physiology. Serotonin and, for the first time, leucine-and methionine-enkephalin were detected in copepods using immunofluorescencetechniques. Serotonin has mainly been identified in the centralnervous system acting probably as a neurotransmitter, notablyin the control of the escape reflex. In contrast, enkephalinsare only present in peripheral organs such as the naupliar eye,gut and shell glands. This localization suggests that opioidsare involved in visual function and reproductive and digestiveprocesses. In memory of Professor Jacques Mazza, an eminent copepodologist.     

Laboratory observations on the vertical distribution of a tropical pelagic flatworm (Mesostoma sp.) in relation to satiation

Mesostoma sp., isolated from different lakes in Brazil, was taken to the laboratory and fed Moina sp. The vertical position of satiated and non-satiated individuals was monitored in 25 cm high columns exposed to natural sunlight. Phototactic responses were studied under standardised experimental conditions.Satiated individuals resided nearer to the bottom than non-satiated Mesostoma. Non-satiated individuals showed an irregular vertical distribution due to swimming behaviour, whereas satiated animals remained stationary.     

Food consumption and utilization in two freshwater cyclopoid copepods (mesocyclops edax and cyclops vicinus)

The predatory feeding of Mesocyclops edax and Cyclops vicinus was studied using the ¹⁴C-labelled Ceriodaphnia and some other kinds of the crustacean zooplankters. When given a choice, both species exhibited clear selection of smaller specimens of prey. The feeding rates depend on the kind of prey. The daily ration for the prey readily accepted by Mesocyclops was roughly equal to the predator's biomass, while it was only one half or one third of the body biomass in Cyclops vicinus. Only about one third of the killed biomass was actually ingested and 21–32% was assimilated when the copepods preyed upon the cladocerans or the copepodid larvae. When fed with naupliar larvae the cyclopoid copepods assimilated over 70% of the killed biomass. Almost one half of the freshly assimilated carbon passes through the short term cycle and is quickly respired and excreted.