Zur fortpflanzungsbiologie von mesostoma ehrenbergii (Focke, 1836) (Turbellaria)

1. At temperature levels from 10 to 25°C animals from resting eggs produce subitaneous eggs independent on temperature. In contrast animals from subitaneous eggs produce subitaneous eggs dependent on temperature. At a high rate subitaneous eggs are only formed at temperature levels above 20°C.
2. Below 10°C no development occurs in the juveniles. At temperatures of 30/22°C (24.7°C) the first subitaneous eggs are formed after 6–9 days, at 14/9°C (10.7°C) they are formed after 34 days. At different temperature levels the developmental rate of the young is from 10.5 to 42 days. One generation extends over 16.5 (30/22°C) to 75 days (14/9°C). The average egg production is 10–20 subitaneous eggs or 30–60 resting eggs. The maximum egg production of one individual is 50 subitaneous eggs or 84 resting eggs. 50% of the animals have just formed resting eggs, before the juveniles are hatched. Resting eggs in the first egg-batch are formed 6–20 days later than subitaneous eggs. The duration of life is between 65 (30/22°C) and 140 days (19/13°C).
3. Young worms in resting eggs have a dormance period of at least 15–30 days.

At room temperatures (20°C) no juvenile in resting eggs hatches from water. By combining room and refrigerator (3.5°C) temperatures the hatching rate increases to a maximum of 85%. To reach a hatching rate of 50–65% the influence of low temperatures must be at least 30 days. At room temperatures 60% of the young in resting eggs hatch from mud covered with water. Combining high and low temperatures the hatching success is between 67 and 81%, where the highest percentage of the young may hatch at room temperature. Up to 90 days low temperatures cause a maximum hatching rate of 79%. It decreases to approximately 30% after 180 days. At high temperatures resting eggs preserved in 100% moist mud, survive for two months. By adding a period of low temperatures the hatching rate increases to a maximum of 52%. Low temperatures are survived for more than 6 months. Up to 30 days preservation at 3.5°C causes a maximum hatching rate of 61%, up to 12o days it decreases to 30%. At room temperature the young in resting eggs are not resistant against air-dried mud (30–40% rel. air moisture). Combining high and low temperatures air-dried mud is endured 1 month (hatching rate 5–14%). Preservation of 30–120 days at 3.5°C and 70% rel. air moisture result in a hatching rate of 43–61%. li]4. In the open air in Middle-Europe there occur 5–6 generations of M. ehrenbergii per life-cycle. The first generation hatches from resting eggs in May, where the production of subitaneous eggs is independent on temperature. All other generations up to October hatch from subitaneous eggs. The egg-production of those worms is dependent on environmental factors. In summer subitaneous egg production prevails, in autumn resting egg production. The abundance during the life-cycle is dependent on the number of animals which produce subitaneous eggs. Resting eggs are predestinated to endure periods of dryness and cold. The life-cycles of the species M. lingua and M. productum are different from those of M. ehrenbergii in length and in the number of generations. In both species 7 generations occur over 8 to 8.5 respectively 5.5 months. M. nigrirostrum only forms resting eggs. The life-cycle consists of one generation from February/March to May/June.     

Seasonal and individual variability of lipid reserves in <Emphasis Type="Italic">Oithona similis</Emphasis> (Cyclopoida) in an Arctic fjord

The lipid storage of the cyclopoid copepod Oithona similis was investigated from spring to late summer 2007 in Kongsfjorden (Svalbard, Norway). The volume of lipid droplets in each individual reflected the amount of stored wax esters. Seasonal changes of lipid storage coupled with informative inter-individual variability were thus obtained. The seasonal pattern showed an increase in lipid store during the spring bloom, starting before the chlorophyll a maximum for both copepodids stage V and females. Those reserves were used during the main reproductive event in June. Individual variability was very high, with a significant proportion of copepods having no droplet, while others were lipid rich. Because of the overlap of generation, females could have different age and feeding history, particularly in September. Consideration of intra-population variability in lipid storage using an optical approach has been shown to be important to understand O. similis’s ecology and life cycle.     

Seasonal Variation in Diel Production Pattern of Centropages tenuiremis in Xiamen Waters, China

The seasonal variations in diel production pattern, and egg hatching time of the copepod Centropages tenuiremis in Xiamen waters, China, were investigated between January and June of 2003. The results show that in the winter–spring (January to early May) the adult females tended to lay subitaneous eggs at night, while in the beginning of summer (late May–June) they tended to lay diapause eggs as well as subitaneous eggs during the daytime. The egg hatching time negatively correlated with water temperature. These results, combined with the fact that C. tenuiremis migrates vertically in Xiamen waters, may explain the reproductive strategy of this dominant species in winter–spring.     

Population differences in the timing of diapause: adaptation in a spatially heterogeneous environment

Summary Populations of the planktonic copepod, Diaptomus sanguineus, live in permanent and temporary freshwater ponds in Rhode Island. All ponds in which they occur become uninhabitable at some time during the year, but the nature and timing of the harsh period varies both spatially and temporally. Females produce discrete clutches either of subitaneous eggs which hatch immediately or of diapausing eggs which hatch the following season. The two egg types show distinct chorion morphologies under transmission electron microscopy. In permanent ponds the copepods start making diapausing eggs in March, one month before rising water temperatures induce planktivorous sunfish to become active. In temporary ponds diapausing eggs are produced, in a complex pattern from May to July, before the water disappears in late summer or early fall.We investigated the spatial scale at which D. sanguineus is adapted to this complex environment. In a reciprocal transfer experiment between temporary and permanent bodies of water, female copepods placed in new ponds made subies of water, female copepods placed in new ponds made subitaneous and diapausing eggs in the same sequence as control females retained in their home ponds. The copepod populations enter diapause at times appropriate for the local habitat conditions they experience, but inappropriate for other, nearby ponds. Transplanted females were unable to sense a change in pond type or to adjust egg production accordingly. We conclude that D. sanguineus populations are adapted to the specific conditions of isolated ponds rather than to a broader geographical region containing several pond types.     

Population differences in the timing of diapause: a test of hypotheses

Summary The reproductive phenology of the freshwater copepod Diaptomus sanguineus differs markedly between populations residing in two Rhode Island ponds. In a permanent pond the population switches abruptly from making subitaneous (immediately hatching) eggs to diapausing eggs at the end of March each year. In contrast, a temporary pond population switches egg types in May, returns to production of subitaneous eggs in June, and concludes the reproductive season by making diapausing eggs in July. An ESS model suggests that the pattern of diapause expected of a copepod population is a function of annual variation in the onset of harsh conditions (catastrophe date). When variation is relatively low, the superior strategy is for diapause to begin a constant period before the mean catastrophe date. When variation is high, females should make first subitaneous eggs and then diapausing eggs irrespective of the expected catastrophe date. With discrete generations, such a population would alternate between egg types. In the permanent pond, variation of catastrophe date the spring onset of planktivory by sunfish is low, whereas in the temporary pond variation of the catastrophe (pond drying) is high. The model predicts well the phenology of the two copepod populations.In the research reported here, we tested the hypothesis that copepods from the permanent pond, which switch to diapause at the same time every year, are cued by the environment to begin diapause (i.e. by photoperiod, temperature, or both), whereas those from the temporary pond make both egg types regardless of environmental conditions. In opposition to our hypothesis, experimental results indicate that diapause in both populations is cued by the environment. The distinct reproductive phenologies documented in the two populations apparently result from the copepods responding to different environmental cues, rather than one being responsive to the environment while the other is not.     

Fine structure of the subitaneous eggshell of the sessile rotifer Stephanoceros millsii (Monogononta) with observations on vesicle trafficking in the integument during ontogeny

Rotifers that engage in cyclical parthenogenesis produce two types of eggs: subitaneous eggs that hatch as clonal females and meiotic eggs that hatch as haploid males, or if fertilized, as females after a period of diapause (resting eggs). The ultrastructure of resting eggshells is known for some motile species, but there are limited data on subitaneous eggshells, and no data on any eggshells of sessile rotifers. Here, we investigated the ultrastructure of the subitaneous eggshell of the sessile rotifer Stephanoceros millsii and its potential origins of secretion, the maternal vitellarium and embryonic integument. We also explored secretory activity in the larval and adult integuments to determine whether activity changes during ontogeny. The eggshell consists of a single layer with two sublayers: an external granular sublayer apparently derived from the maternal vitellarium, and an internal flocculent sublayer secreted by the embryonic integument that may form a hatching membrane or glycocalyx. Secretory activity remains high in both the larva and adult and appears to be the source of the thickening glycocalyx. Altogether, the subitaneous eggshell of S. millsii is the thinnest among monogonont rotifers. Thin eggshells may have evolved in response to the added protection provided by the mother’s extracorporeal tube.     

Females from resting eggs and parthenogenetic eggs in the rotifer Brachionus calyciflorus: lipid droplets, starvation resistance and reproduction

1. In the heterogonic life cycle of monogonont rotifers, amictic (female‐producing) females develop from two types of eggs: fertilised resting (diapausing) eggs and parthenogenetic subitaneous eggs. Females hatched from resting eggs initiate clonal populations by female parthenogenesis and are called stem females. This study compares females from resting and parthenogenetic eggs that were produced under identical culture conditions and were of similar birth order. 2. Newborn stem females had many more lipid droplets in their tissues than similar‐sized, newborn females from parthenogenetic eggs. When neonates were stained with Nile Red and viewed under epifluorescent illumination, these droplets were shown to be sites of neutral‐lipid storage products. 3. Stem females had no posterolateral spines and short anterior spines, while their mothers and offspring in subsequent, parthenogenetic generations typically had long posterolateral spines and elongated anterior spines. 4. Newborn stem females survived starvation significantly longer than newborn females from parthenogenetic eggs. 5. When females from resting and parthenogenetic eggs were cultured from birth to death at a high food concentration, the reproductive potential (r day^?1) of the stem females was significantly higher (0.82–0.88 versus 0.70), primarily because of egg production at an earlier age. The mean lifetime fecundity (R_o) of stem females was significantly greater than that of females from parthenogenetic eggs. 6. Extensive lipid reserves should increase the ability of stem females to colonise new habitats. Firstly, compared with females from parthenogenetic eggs, stem females are more likely to experience starvation or food limitation. Resting eggs hatch in response to physical and chemical factors that are not directly related to food availability, and from sediments that may be far from food‐rich surface waters. Secondly, when food is abundant, stem females have a greater reproductive potential.     

Ecological and evolutionary significance of resting eggs in marine copepods: past, present, and future studies

The occurrence of a resting egg phase in the life cycle of marine and freshwater planktonic copepods is well documented and receiving increasing attention by investigators. The species generally occur in coastal marine waters, freshwater ponds and lakes in areas that undergo strong seasonal fluctuations, though examples have been reported for tropical and sub-tropical areas not subject to such extreme fluctuations. Typically such species disappear from the water column for portions of the year, but remain in the region as benthic resting eggs. Studies to date have focused on the conditions that promote the occurrence of resting eggs, the factors that affect their survival and hatching from sediments, the existence of egg banks in sediments, and the impact of resting eggs on plankton community structure. Benthic resting eggs of copepods include diapause eggs as well as subitaneous (non-diapause) eggs that are quiescent due to conditions in the sediments. As with other groups of organisms the resting egg phase is viewed as being critical for the perpetuation of species year after year, especially those that disappear from the water column for portions of the year. Some data indicate that eggs can survive for many years in sediments which would expand their influence to evolutionary time scales. This paper summarizes our understanding of embryonic dormancy in marine copepods.     

Dormancy in the Copepoda — an overview

Dormancy affects copepods in their anatomy, physiology, genetics, population biology, community ecology, evolution and local and geographic distribution. It is known from freeliving representatives of three copepod taxa, namely the Harpacticoida, Cyclopoida and Calanoida. Species showing dormancy occur in various realms and habitats, both freshwater and marine, being benthic, planktic or ice-dwelling. Depending on the taxon, dormancy occurs at various times of the year, prevailing in higher and temperate latitudes. Copepod dormancy is expressed in various ontogenetic stages, such as resting eggs, arrested larval development, juvenile and adult encystment, or arrested development of nonencysted copepodids or adults. Ecologically, dormancy is an energy saving trait, allowing the individual to bridge periods of environmental harshness. Adverse environmental conditions could be abiotic (e.g. desiccation, temperature, oxygen availability) or biotic in nature (e.g. food availability, predation). Diapause s. str. is initiated, maintained and terminated by triggering factors (e.g. photoperiod, temperature, chemical cues, population density/physiological factors). The dormant state and emergence patterns directly affect reproduction, population dynamics, community composition, coexistence and distribution of copepods, as well as the phenology of their predators and living food items. Populations having dormancy, in most cases belong to and affect communities of two realms: the water column and the bottom. Dormant stages may provide means for dispersal as well as for staying in special localities. The variability of dormancy permits flexible and complex life histories. Dormancy is subjected to and on the other hand affects copepod evolution.     

Morphological differences between subitaneous and diapause eggs of Boeckella triarticulata (Copepoda: Calanoida)

1. Many calanoid copepods produce subitaneous eggs that hatch immediately and diapause eggs that remain dormant for long periods of time. Boeckella triarticulata (Copepoda: Calanoida) produces diapause eggs that hatch asynchronously over an extended period of time and appear identical under light microscopy to those that hatch immediately.
2. We compared the internal morphology of subitaneous and diapause eggs of B. triarticulata that had been prepared for transmission electron microscopy by four methods.
3. Subitaneous eggs had a thin, single-layered shell whereas diapause eggs had a thick, three-layered shell that appeared consistent over 3 months of diapause. Boeckella triarticulata appears to enter diapause at the multicellular stage.
4. Consistent morphological differences between subitaneous and diapause eggs of B. triarticulata suggest that these two egg types also have distinct differences in physiology, and are not merely extremes of a continuum from subitaneous to diapause eggs.     

Food quality triggers the reproductive mode in the cyclical parthenogen <Emphasis Type="Italic">Daphnia</Emphasis> (Cladocera)

Cyclic parthenogenesis (heterogony) is a widespread reproductive mode found in diverse taxa such as digenean trematodes, gall wasps, gall midges, aphids, cladocerans and rotifers. It is of particular interest as it combines the advantages of asexual reproduction (rapid population growth) and sexual reproduction (recombination). Usually sexual reproduction is initiated when, or slightly before, environmental conditions deteriorate, and often results in the production of resting stages. The optimal timing of diapause induction must thus be under strong natural selection. Using the cladoceran Daphnia as a model system, we show here for the first time that the switch from parthenogenetic to sexual reproduction in a cyclical parthenogenetic organism can be influenced by the chemical composition of food. Under crowding conditions Daphnia reproduced parthenogenetically with subitaneous eggs when fed the algal species Cryptomonas sp., but started the production of resting eggs when fed with the green algal species Scenedesmus obliquus. Supplementation experiments with lipids and especially proteins showed that the induction of resting egg production in two clones of different Daphnia species was due to a dietary deficiency in the green alga. Hence, the low food quality induced a switch in the reproductive mode that may contribute to optimal timing of the sexual reproduction of Daphnia in nature. Furthermore, our results have two other major implications: first, they suggest that protein compounds should be added to the list of diet constituents potentially limiting or influencing Daphnia reproduction. Second, we show that the role of food quality goes far beyond the up to now documented effects of food quality on somatic growth and trophic transfer efficiency of herbivores: due to its effects on sexual reproduction and the production of resting eggs, food quality might influence genetic diversity and long-term persistence of Daphnia in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Patterns of energy storage in Pseudoboeckella poppei (Crustacea,copepoda) from two contrasting lakes on Signy Island,Antarctica

The copepod Pseudoboeckella poppei (Daday) (Calanoida, Centropagidae) was sampled from Sombre and Heywood Lakes on Signy Island, Antarctica (60° S, 45° W) between January 1984 and March 1985. Sombre Lake is clear and oligotrophic with little phytoplankton and a bottom sediment low in organic content. By contrast Heywood Lake is turbid and mesotrophic; a substantial phytoplankton develops in summer and the bottom sediments are comparatively rich in organics. Both lakes freeze over for much of the year, forcing the copepods to adopt a benthic feeding strategy over winter. Adult Pseudoboeckella feed on phytoplankton when this is available, but also on detritus, diatoms and short algal filaments stirred up from the sediment. In Heywood Lake, male copepods show a smooth seasonal trend in lipid content with lipid being synthesised in early summer and utilised in late summer and winter. The summer increase in lipid content is associated with an increase in dry weight. Female lipid contents show evidence of two peaks of egg production. In Sombre Lake both male and female copepods increase in size during summer and show a wider range of lipid contents than in Heywood Lake; it is likely that this is due to the poorer winter feeding conditions which necessitate the synthesis of a much larger store of reserves during the summer. In contrast to marine calanoid copepods, lipid stores are exclusively triacylglycerol with no trace of wax ester.     

Reproduction strategies of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis> population in a high mountain lake of Southern Spain

For three consecutive years, a population study of Daphnia pulicaria was undertaken in Río Seco Lake, a small high-mountain lake, in order to elucidate the reproductive strategies adopted by Daphnia in this system. Daphnia appears to colonize this lake every spring by hatching from ephippia and reproduce by means of subitaneous (non-diapausing) and ephippial (diapausing) eggs. D. pulicaria in this lake is an obligate parthenogenetic population. There is a short time period for subitaneous egg production and a much longer period for ephippial egg production. The contribution of subitaneous eggs to Daphnia population density and structure appears to be low. Diapause onset showed a high temporal synchronization in the three studied years in Río Seco Lake, and day-length emerged as the main cue triggering diapause onset and the main explanatory factor for the proportion of ephippial females observed. The development and reproduction of D. pulicaria in Río Seco Lake involves taking a gamble on resting forms to guarantee inter-annual Daphnia persistence in the lake, giving priority to investment in future generations. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Saving for the future: Pre‐winter uptake of algal lipids supports copepod egg production in spring

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Coexistence with similar life cycles in two species of freshwater copepods (Crustacea)

Two species of freshwater cyclopoid copepods, Cyclops scutifer Sars and Mesocyclops leuckarti (Claus), with different ecological requirements, coexist in three humic ponds near the timberline in boreal forests in south-central Norway. The life cycle in both species is basically regulated by complete depletion of oxygen during the long winter. There is a short period of reproduction in spring followed by a rapid development during summer and autumn to advanced instars, capable of diapause when oxygen depletion starts in early winter. Both species have an exclusive 1-yr cycle.The long winter under anoxic conditions is passed in diapause as large copepodids (late developmental instars) with usually no individuals left in the plankton.Even if co-occurring in the same habitats with similar life cycles, there is a temporal and spatial segregation.     

Benthic resting periods of pelagic cyclopoids in an oligotrophic lake

The benthic resting stages of pelagic cyclopoids were studied in the oligotrophic lake Pääjärvi (maximum depth 87 m), southern Finland. Stage 5 copepodids of Thermocyclops oithonoides were found in the bottom from September to April, with highest abundances in the littoral at 1–2 m. Dormant Mesocyclops leuckarti (mainly stage 5 copepodids) were found from mid-August so April, with a strong concentration at the depth of 1.5 m, and resting stage 4 copepodids of Cyclops kolensis from mid-summer to late winter at the depths of 1–13 m. Single stage 4 and 5 copepodids of C. strenuus and C. lacustris were also found in the bottom during autumn and winter. Available information on the periodicity of these species conforms well with the hypothesis of a temperature-adjusted photoperiodic control of diapause.
In the sediment, the resting stages of pelagic cyclopoids were confined to the uppermost 2–3 cm, C. kolensis penetrating slightly deeper than the smaller Thermocyclops and Mesocyclops species.
Losses during the resting period were highest in T. oithonoides , in which the seasonal mortality rates were significantly correlated with temperature. Its overall mortality was slightly lower at 2 m than at 13 or 40 m. C. kolensis had the lowest mortality, with no clear relation to temperature.
The mean winter biomass of the benthic resting stages of pelagic cyclopoids exceeded that of the true benthic copepods, and the winter losses of the former were equal to about one fifth of the total annual production of the true benthic copepods.     

Morphological aspects of subitaneous and resting eggs from Acartia josephinae (Calanoida)

A. josephinae collected along the Salento Peninsula (Ionian and Adriatic Seas) laid two types of eggs, under laboratory conditions, and they are described for the first time. Under a light microscope subitaneous eggs looked smooth on the surface, but scanning electron microscopy revealed that the surface was not perfectly smooth but tubercular with many small protuberances. Resting eggs had long and thick spines whose apical part was multi-branched. The use of egg morphology as a taxonomic character, in the systematic of copepods, is proposed.     

The production of subitaneous and diapause eggs: a reproductive strategy for Acartia bifilosa (Copepods: Calanoida) in Southampton Water, UK

Acartia bifilosa from Southampton Water lays two morphologically distinct types of egg which are described for the first time. Eggs with a smooth surface are considered subitaneous, while eggs covered with thick 'spines' are diapause. During the seasonal occurrence of A.bifilosa in Southampton Water, from November/December to June, subitaneous eggs are laid during the first months of this period. The production of diapause eggs is restricted to a 2 month period before A.bifilosa disappears from the water column. There are significant differences between the response of the eggs produced at seasonal field temperatures (5-12C) and those reported for other Acartia species. In particular, the numbers of eggs female^-1 day^-1, both subitaneous and diapause, are lower and typically <4; and the hatching time of subitaneous eggs is longer, at up to 10 days, at optimum temperatures between 15 and 20°C and optimum salinity >20 PSU. Females acclimated to higher laboratory temperature regimes show higher egg production rates at field salinity. A delayed-hatch subitaneous egg is also reported. Results suggest that fecundity in A.bifilosa from Southampton Water might be limited to some degree by temperature and the responses of the eggs produced appear to offer A.bifilosa a reproductive repertoire to ensure its sustained presence in this seasonally influenced environment.      

Growth and fecundity of Daphnia after diapause and their impact on the development of a population

Laboratory and field investigations revealed that the life history traits of exephippial and parthenogenetic generations of Daphnia differ substantially. Daphniids hatching from resting eggs grow faster and their definitive body sizes are bigger than of hatchlings from subitaneous eggs. Size at maturity for exephippial animals is significantly larger. In spite of this, they mature a few days earlier than parthenogenetic females. In this study, the difference was 3–4 days for the laboratory experiments and 1–3 days for the field. Fecundity of the exephippial generation is markedly higher. Here, the clutch size for this generation was up to 3.5–4.0 times as large as for the parthenogenetic generation. Moreover, obtained results suggest that the relationship between clutch size and body length for both generations differ significantly.Estimates of the intrinsic rate of increase for field Daphnia populations demonstrated that life history traits of exephippial animals lead to a two or threefold higher rate of increase in the conditions of invertebrate predation pressure. Under moderate fish pressure, obtained r values for the daphniids hatching from resting eggs were larger than those from subitaneous. High growth rate of exephippial females is disadvantageous only under the conditions of severe pressure by fish. Obtained results suggest that hatchlings from diapausing eggs an acceleration of population increase by several times during the beginning of the development of a population with periodical re-establishment from resting eggs.     

Single dietary amino acids control resting egg production and affect population growth of a key freshwater herbivore

The enormous success of the genus Daphnia in freshwater ecosystems is at least partially due to their cyclical parthenogenetic life cycle, in which asexual and sexual reproduction alternate periodically. This temporal change between reproductive strategies allows for (1) rapid population growth via subitaneously developing eggs when environmental conditions are appropriate and (2) the maintenance of genetic diversity via sexual reproduction and the production of resting eggs when environmental conditions deteriorate. We show here that dietary amino acids are involved in triggering the switch between reproductive modes in Daphnia pulex. Supplementation experiments demonstrate that specific dietary amino acids, in particular arginine and histidine, avert crowding-induced resting egg production, enhance subitaneous reproduction by increasing algal food quality and, as a combined effect of both processes, increase population growth rates. These findings suggest that the availability of single dietary amino acids potentially affects the seasonal dynamics and long-term persistence of Daphnia populations in the field, which may have consequences for the efficiency of carbon transfer and thus the trophic structure of freshwater food webs.