Life cycles of marine nematodes

Summary Monhystera denticulata Timm, a free-living nematode present in the aufwuchs assemblages of several marine macrophytes located in North Sea Harbor, Southampton, New York, was isolated from Zostera marina and established in laboratory culture in order to study the influences of temperature and salinity on its life history. Under experimental conditions, M. denticulata has a generation time (Measured as the time elapsing between the first egg depositions of consecutive generations) of 10–12 days at 25° C and 26 S, which represent optimal growth conditions in the laboratory. The organism has a generation time of 20 days at 25° C and 13, 17 days at 25° C and 39, 18 days at 15° C and 26, 36 days at 15° C and 13 and 34 days at 15° C and 39. As conditions vary from the optimum of 25° C and 26 S, a decrease in temperature of 10° C and an increase or decrease in salinity of 13 results in a doubling of the generation time. At 5° C the generation time is about 180–197 days.Assuming optimum conditions and average generation time, about 15 generations of M. denticulata could occur in North Sea Harbor during the year. The number of generations occurring in reality is probably less, however, due to the fact that the females deposit their eggs over a period of several days.This work was supported by National Science Foundation Grant GB-19245.Contribution No. 04 from the Institute of Oceanography, City University of New York.     

Life cycles of successful genes

By exploring time-series data from MEDLINE abstracts, we observe that only a few genes have been quoted with increasing frequency during the past 25 years. This is probably the result of selective pressure by the scientific community. Over the years, this selection has produced an extreme power law distribution of the information available for individual genes. Interestingly, those genes that are successfully selected are not necessarily the most important genes to the cell. To stress the implication of this finding we show that there is no correlation between a gene's impact in the scientific literature and its centrality in protein-interaction networks.     

Life cycles,plasticity and palaeoecology in temnospondyl amphibians

In the largest early tetrapod clade, the temnospondyls, ontogenies were diverse and quite distinct from the life cycles of extant amphibians. Three well‐studied clades exemplify the diversity of these long‐extinct ontogenies, here analysed with respect to their bearing on developmental plasticity, reaction norms and evolution. Sclerocephalus readily adjusted by means of developmental evolution to different lake environments. In addition, plasticity (reaction norm) played a significant role, apparent both morphologically and by altered developmental traits. Size increase and extension of the ontogenetic trajectory gave larger predators, a phenomenon also found in the dissorophoid Micromelerpeton. Whereas Sclerocephalus was throughout preying on the same fishes, Micromelerpeton was able to fit into different trophic levels. In the branchiosaurid Apateon, a biphasic life cycle was established, with metamorphosis producing a terrestrial morph in some species; truncation of the ontogenetic trajectory gave a sexually mature larva as an alternative morph (neoteny). Plasticity was high in the larval morphs, permitting neotenes to live as filter feeders or small carnivores. Fine‐tuning of development permitted Apateon populations to adjust to specific lake properties and readily change from a filter‐feeding to carnivorous mode of life. In the nonmetamorphosing Triassic Gerrothorax, morphology was extremely conserved, but histology reveals much plasticity at the microscopical level, correlating with fluctuating salinity and water energy. In responding to environmental fluctuations by enhanced plasticity, the studied temnospondyls managed to populate lakes inhabitable to other tetrapods and fishes.     

Life cycles and habitats of wisconsin heptageniidae (ephemeroptera)

Detailed studies were made of the life cycles of Heptageniidae known to occur in Wisconsin. 19 species had univoltine cycles while two and possibly a third had bivoltine cycles. Three univoltine species developed in late spring and early summer while the other univoltine species developed in fall, winter and early spring. For three of the univoltine species, eggs hatched both in fall and the following spring. Diagrams of the life cycles of 15 Wisconsin heptageniids are presented, illustrating the different types of life cycles. Also presented are observation on the food and habitats of the nymphs and flight periods of the adults.     

Life cycles of Echinococcus multilocularis in relation to human infection

The cycle of Echinococcus multilocularus in natural and synanthropic hosts was investigated during 10 yr in an endemic focus of alveolar hydatid disease in the Massif Central of France. The natural cycle, involving red foxes, Vulpes vulpes, and voles, Arvicola terrestris, existed immediately surrounding a village in which human cases of alveolar hydatid disease occurred. Both foxes and free-ranging dogs could serve as the source of infection for the human population.     

Life cycles shape parasite evolution: comparative population genetics of salmon trematodes

Little is known about what controls effective sizes and migration rates among parasite populations. Such data are important given the medical, veterinary, and economic (e.g., fisheries) impacts of many parasites. The autogenic-allogenic hypothesis, which describes ecological patterns of parasite distribution, provided the foundation on which we studied the effects of life cycles on the distribution of genetic variation within and among parasite populations. The hypothesis states that parasites cycling only in freshwater hosts (autogenic life cycle) will be more limited in their dispersal ability among aquatic habitats than parasites cycling through freshwater and terrestrial hosts (allogenic life cycle). By extending this hypothesis to the level of intraspecific genetic variation, we examined the effects of host dispersal on parasite gene flow. Our a priori prediction was that for a given geographic range, autogenic parasites would have lower gene flow among subpopulations. We compared intraspecific mitochondrial DNA variation for three described species of trematodes that infect salmonid fishes. As predicted, autogenic species had much more highly structured populations and much lower gene flow among subpopulations than an allogenic species sampled from the same locations. In addition, a cryptic species was identified for one of the autogenic trematodes. These results show how variation in life cycles can shape parasite evolution by predisposing them to vastly different genetic structures. Thus, we propose that knowledge of parasite life cycles will help predict important evolutionary processes such as speciation, coevolution, and the spread of drug resistance.     

Life cycles of two isopora species in the canary, Serinus canarius Linnaeus

The endogenous stages of Isospora serini Arog?o and Isospora canaria Box are described from experimentally infected canaries, Serinus canarius Linnaeus. Unlike other Coccidia, the first part of the I. serini life cycle takes place in mononuclear phagocytes. Five asexual generations are described from this cell type; 2 additional asexual generations and the sexual stages take place in the intestinal epithelium. Isospora canaria, on the other hand, has a conventional coccidian life cycle in that all of the endogenous stages are in the epithelium of the small intestine, with 3 asexual generations and the sexual generation described in the duodenal epithelium. The 2 species differ in their position relative to the nucleus of the intestinal epithelial cell. Isospora serini is usually on the lumenal side of the nucleus while I. canaria is below the nucleus, toward the basement membrane. The prepatent period is 4-5 days for I. canaria and 9-10 days for I. serini. Patency lasts for 11-13 days in I. canaria infections, but duration of oocyst output is more chronic in I. serini infections, persisting for as long as 231 days. Both species have a diurnal periodicity of oocyst discharge which occurs in late afternoon and evening.     

Life cycles and challenges for fish species that breed in South African estuaries

The literature currently recognizes four guilds of estuarine resident fish species, namely solely estuarine, estuarine and marine, estuarine and freshwater, and estuarine migrant. In this review the life cycles of actual representatives from these four guilds are assessed to determine whether the current definitions, which have never been formally tested, are appropriate to fish species resident in South African estuaries. Detailed information and diagrammatic life cycles are provided for the selected species covered by this review. A potential new estuarine resident guild category is also identified, namely, those taxa that are primarily estuarine but also have subpopulations recorded in both adjacent marine and freshwater habitats. The full range of reproductive characteristics employed by estuary resident species is examined, ranging from live bearers, pouch and nest brooders, to a suite of oviparous taxa that attach their ova to estuarine rocks, shells, and submerged vegetation, all of which assists with larval retention within the estuarine environment. The small size and early reproductive maturity of most estuarine resident species is highlighted, with reduced vulnerability to predation in shallow, sheltered, often turbid estuary waters offering considerable protection during spawning events when compared to the open ocean. In addition, these small fish would not have to move considerable distances at any stage of their life cycle, since egg, larval, juvenile, and adult stages all occur in the same place. The existence of contingent subpopulations within many estuarine resident species is noted, physico-chemical stresses on these species are highlighted, and the eurytopic nature of these small fish taxa emphasized.     

Life history constraints on the evolution of abbreviated life cycles in parasitic trematodes

Abbreviations of the complex life cycle of trematodes, from three to two hosts, have occurred repeatedly and independently among trematode lineages. This is usually facultative and achieved via progenesis: following encystment in the second intermediate host, the metacercaria develops precociously into an egg-producing adult, bypassing the need to reach a definitive host. Given that it provides relatively cheap insurance against a shortage of definitive hosts, it is not clear why facultative progenesis has only evolved in a few taxa. Here a comparative approach is used to test whether progenetic trematodes are characterized by larger body size and egg volumes, two traits that correlate with other key life history features, than other trematodes. These traits may constrain the evolution of progenesis, because precocious maturation might be impossible when the size difference between the metacercaria and a reproductive adult is too large. First, trematode species belonging to genera in which progenesis has been documented were found not to differ significantly from other trematode species. Second, using within-genus paired comparisons across 19 genera in which progenesis has been reported, progenetic species did not differ, with respect to body size or egg size, from their non-progenetic congeners. Third, using intraspecific paired comparisons in species where progenesis is facultative, no difference was observed in the sizes of eggs produced by worms in both the intermediate and definitive host, suggesting that opting for progenesis does not influence the size of a worm's eggs. Overall, the lack of obvious differences in body or egg size between trematodes with truncated life cycles and those with the normal three-host cycle indicates that basic life history characteristics are not acting as constraints on the evolution of progenesis; trematodes of all sizes can do it. Why facultative progenesis is not more widespread remains a mystery.     

Life cycles and food habits of mayflies and stoneflies from temporary streams in western Oregon

1. Field data and results from laboratory rearing are combined to describe life cycles and food habits of the mayflies Paraleptophlebia gregalis and Ameletus n. sp., and the stoneflies Soyedina interrupts, Ostrocerca foersteri, Sweltsa fidelis and Calliperla luctuosa. 2. P. gregalis, A. n. sp., S. interrupts and O. foersteri have univoltine life cycles which are characterized by a high degree of plasticity. S. fidelis and C. luctuosa have semivoltine life cycles which are more tightly synchronized. 3. Laboratory feeding trials and field observations characterize P. gregalis as a collector, A. n. sp. as a scraper, O. foersteri and S. interrupta as shredders and C. luctuosa as a predator mainly of midge larvae. Late-instar larvae of S. fidelis are believed to be scavengers. 4. Laboratory rearing yielded a negative correlation between growth rates (Y) and larval size in autumn (X) for S. interrupta. This indicates compensatory growth by small larvae in order to achieve synchronized emergence. The correlation can be described by the equation: Y = 0.0053–0.0036X (R²= 0.82; P < 0.01; n = 22) 5. The field and laboratory data indicate that photoperiod mainly determines the rate of development and size of emerging subimagos in P. gregalis.     

Life cycles and distribution of atherinids in the marine and estuarine waters of southern Australia

New and published data have been collated for the biology and distribution of atherinid species abundant in the coastal saline waters of Australia below 30°S. This information has been used to determine whether these species typically spawn at sea or pass through the whole of their life cycle in estuaries, and in one case, also lagoons and saline lakes. Length-frequency data, gonadosomatic indices and distribution records indicate that in south-eastern AustraliaCraterocephalus honoriae andAtherinosoma microstoma typically reach total lengths less than 90 mm, have a one-year life cycle and breed within estuaries. This parallels the situation recently described forAtherinosoma elongata, Atherinosoma wallacei andAllanetta mugiloides in south-western Australia (Princeet al., 1982a; Prince & Potter, 1983). The marine speciesAtherinosoma presbyteroides, which reaches a similar size and has a one year life cycle in both south-western and south-eastern mainland Australia, only enters estuaries in large numbers in the former region. WhileAtherinomorus ogilbyi is also found in estuaries and typically breeds at sea, it reaches total lengths as great as 189 mm and has a longer life thanA. presbyteroides. The limited data forAtherinason esox andAtherinason hepsetoides demonstrate that both these marine atherinids can attain total lengths of 139 and 108 mm respectively and live for longer than a year but do not enter estuaries in large numbers. The latter species is unique amongst southern Australian atherinids in having a distribution which extends into deeper water. It is suggested that landlocking may have played a role in the evolution and success of the estuarine mode of lifesensu stricto ofA. wallacei, A. elongata, A. microstoma, A. honoriae andA. mugiloides in southern Australian waters.     

Life cycles of yeast spindle pole bodies: getting microtubules into a closed nucleus.

The spindle pole body (SPB) is the principal microtubule organizing center of budding and fission yeast. We have examined SPBs and their associated microtubules from both organisms, using electron microscopy and three-dimensional reconstruction techniques, to identify the structural changes that accompany progression through the cell cycle. In this report, we compare these changes in the two kinds of yeasts and present a model for how microtubules get into a closed nucleus.     

Life cycles of Gammarus oceanicus and G. salinus (Amphipoda) in the Oslofjord, Norway

In the Oslofjord the amphipods Gammarus oceanicus Segerstråle, 1947 and G. salinus Spooner, 1947 were estimated to live for a maximum of 15 months. All specimens which survived during winter died in spring, mainly in May. A G. oceanicus population living on a freshwater influenced shore contained smaller specimens, had a lower proportion of the female population in breeding condition and probably produced fewer broods than a population living on a fully marine shore. The differences are discussed in relation to environmental factors and the distribution of G. salinus. Gammarus oceanicus possibly was breeding from December to May, some females even in June, resulting in an estimated maximum of three broods from December to May. Gammarus salinus showed two breeding periods, the first from December to May, a few of these females even bred in June, while new females bred from June to October, giving estimated maxima of respectively three (December to May) and five broods. Gammarus salinus females entered a reproductive resting stage in September-October. The sex ratio was mostly female dominated. A shift to male dominance was noted in one population and was related to possible infections. Information from the literature on longevity and breeding periods was compiled and compared to the Oslofjord data.     

Moulting cycles

Moulting in mammals is a cyclic phenomenon which often occurs in a wave–like pattern. The moult cycle depends upon an inherent rhythm of activity in the hair follicles, which may be modified by systemic factors. In laboratory rodents a number of hormones affect the timing of the moult, as well as affecting the amount of hair produced and the loss of club hairs.
In Microtus agrestis a seasonal moult results in a sparse coat with coarse hairs in summer and a dense coat with fine hairs in winter. The moult appears to be adjusted to the environment by way of the endocrine system, with adrenal and thyroid hormones, as well as sex hormones, involved in the regulation. The importance of such adaptive coat changes are discussed.     

Astronomical cycles

Abstract

Talorchestia quoyana, a sand beach amphipod, shows a rhythm of locomotor activity controlled by a circadian clock and an inhibitory circatidal clock. This article reports on an investigation of the entrainment of the circadian dock to skeleton photoperiods. Four important mathematical models for circadian rhythms are examined with respect to the results of the entrainment experiments and to predictions from the phase response curve for Talorchestia. Significant differences between the models are described, and properties of circadian rhythms not accounted for by present models are outlined.