Karenia brevis red tides,brevetoxins in the food web,and impacts on natural resources: Decadal advancements

As recently as a decade ago, Karenia brevis red tides and their effects on animal resources in the Gulf of Mexico were principally perceived as acute blooms that caused massive fish kills. Although occasional mortalities of higher vertebrates were documented, it has only been in the past decade that conclusive evidence has unequivocally demonstrated that red tides and their brevetoxins are lethal to these organisms. Brevetoxins can be transferred through the food chain and are accumulated in or transferred by biota at many trophic levels. The trophic transfer of brevetoxins in the food web is a complex phenomenon, one that is far more complicated than originally conceived. Unexplained fish kills and other animal mortalities in areas where red tide is endemic are being increasingly linked with post-bloom exposures of biota to brevetoxins. Mass mortality events of endangered Florida manatees (Trichechus manatus latirostris) follow a consistent spatial and temporal pattern, occurring primarily in the spring in southwestern Florida. Persistent blooms can also cause a cascade of environmental changes, affecting the ecosystem and causing widespread die-offs of benthic communities. Ongoing fish kills from sustained blooms can lead to short-term declines in local populations. Although animal populations in areas where red tide is endemic are unquestionably at risk, it remains to be determined to what extent populations can continue to recover from these sustained effects.     

Régulation naturelle des populations deCapitophorus horni Borner [Hom: Aphididae] par hyménoptèresAphidiidae et Entomophthorales

Résumé Le puceronCapitophorus horni peut causer de graves préjudices aux cultures d’artichauts dans l’Ouest de la France. La présente note fait l’inventaire des parasites (Aphidiides et Entomophthorales) de cet aphide et précise leur r?le dans la régulation de ses populations.

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Summary In western France, the aphidCapitophorus horni develops anholocyclic populations during the whole year on the globe-artichoke of which it is the major pest. A study of the parasites of this species was undertaken in 1967 and carried out until 1970. The observations have been done both on a plot of artichokes which never received any insecticidal treatment and on another one where such a treatment was done only once in April 1968. C. horni is attacked by such fungi asEntomophthora planchoniana andE. aphidis and by Hymenopterae, the main species beingAphidius matricariae and the other onePraon flavicorne (tab. 1). A number of hyperparasites have been record of whichCharips tscheki andAsaphes suspensus were the most numerous. Fig. 1 and fig. 2 show, comparatively on untreated and treated plots, the fluctuations of the populations ofC. horni on both young and older leaves of artichoke and the rate of parasitism by fungi and wasps. Fig. 3 gives numerical changes of the two main species of Hymenopterae encountered and shows the incidence of hyperparasitism. From this study it appears that a good regulation of the populations ofC. horni may occur, due to the high percentage and persistence of the parasitism. But in those natural conditions, epizootics occur too late for practical purpose and as a consequence, whole populations collapse, preventing other parasites from multiplying. Exposure of naturally or artificially lowered populations of aphids to constant parasitism seems far better for the prevention of outbreaks.

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Une partie des résultats inclus dans cette note a été exposée lors du ?Second Meeting, I.B.P. — Working Party on Biological control of Aphids? tenu à Paris du 16 au 18 septembre 1970.     

Le complexe des ennemis naturels des Aphides du Pêcher dans la moyenne vallée du Rhone

Résumé Dans le cadre d’une recherche écologique sur les Aphides du Pêcher soutenue par le Comité Lutte Biologique de la DGRST, en vue d’explorer les modalités d’application à cette culture du concept de la Lutte intégrée, une première étude a présenté l’analyse de la dynamique des populations des pucerons les plus nuisibles et montré l’importance de certains ennemis naturels (Leclant & Remaudière, 1970). L’objet du présent article est de dégager les principaux éléments du complexe biologique intervenant dans la régulation des populations et de préciser les rapports existant entre quelques-uns d’entre eux. Les recherches ont été accomplies de 1968 à 1970 dans plusieurs vergers vergers de Pêcher de la région de Valence (Dr?me) et plus particulièrement au domaine de Gotheron (Saint-Marcel-lès-Valence).

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Summary Samples consisting of about 20 infested twigs of 6 to 8 leaves have been collected periodically and kept alive during a period of 10 days. At this time, most of the predators have achieved their growth and the parasites have mummified their hosts; then they are isolated until hatching. Syrphids play a predominant role in the regulation of the aphid population.Epistrophe balteata andSyrphus vitripennis are the most frequent of the six species obtained. 35% of the Syrphids is destroyed by parasites; however their action at the highth of the orchards is not affected because their number appears to depend on the immigrants. Spiders have a significative impact at the end of the winter when fundatrix larvae hatch. Coccinellids (Col.), Chamaemyiids (Dipt.) and other predators and parasites are not very important factors of regulation. Aphids parasitized by fungi (Entomophthora aphidis andplanchoniana) are found more or less frequently according to the climatic conditions of the year. An epizootic situation occurred in June 1968 which determined a quicker decline of the populations ofMyzus persicae. Cultural practices affect the relative importance of the various natural enemies: in orchards with high weeds the action of Coccinellids may be preponderant.

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Communication présentée à la 2^e réunion du groupe de travail PBI ?Biological Control of Aphids?, Paris, 16–18 septembre 1970.     

Optimal growth strategies under divergent predation pressure

The conditions leading to gigantism in nine‐spined sticklebacks Pungitius pungitius were analysed by modelling fish growth with the von Bertalanffy model searching for the optimal strategy when the model's growth constant and asymptotic fish size parameters are negatively related to each other. Predator‐related mortality was modelled through the increased risk of death during active foraging. The model was parameterized with empirical growth data of fish from four different populations and analysed for optimal growth strategy at different mortality levels. The growth constant and asymptotic fish size were negatively related in most populations. Optimal fish size, fitness and life span decreased with predator‐induced mortality. At low mortality, the fitness of pond populations was higher than that of sea populations. The differences disappeared at intermediate mortalities, and sea populations had slightly higher fitness at extremely high mortalities. In the scenario where all populations mature at the same age, the pond populations perform better at low mortalities and the sea populations at high mortalities. It is concluded that a trade‐off between growth constant and asymptotic fish size, together with different mortality rates, can explain a significant proportion of body size differentiation between populations. In the present case, it is a sufficient explanation of gigantism in pond P. pungitius.     

Les symbioses entre plantes et fourmis arboricoles

In communities of tree-nesting ants in tropical rain forests, energy-rich resources from plants enable ants to achieve high densities and permit the evolution of energy-intensive strategies of prédation. Ants are more abundant, and can maintain populations of phytophagous insects at lower densities than if ant populations were limited simply by insect prey. A large proportion of woody plants in tropical forests are involved in such loose protection mutualisms. Many epiphytes are also involved in loose associations with ants, in which benefits to plants may include nutrition and seed dispersal as well as protection. From such opportunistic interactions numerous symbiotic mutualisms have evolved, in which ants and plants are more intimately and often more specifically associated. The diversity of these symbiotic interactions between ants and plants make these associations good models for examining many general questions in ecology. Plants that have evolved specialised structures (‘ant-domatia’) to house ants are termed ‘myrmécophytes’ or ‘ant-plants’, and their specialised associates are termed ‘plant-ants’. In these symbioses, plants and ants have coevolved. Ant colonies that provide increased benefits to the host plant enhance its growth and survival, thus receiving more benefits from it, and vice versa. Selection favours mutualistic traits, and interests of the two partners tend to converge. However, because these associations are horizontally transmitted, neither partner obtains benefits from reproduction of the other. Because reproduction draws away resources from growth (from which the partner benefits), it introduces conflicts of interest between ants and plants, and several examples show the importance of such conflicts in the dynamics of coevolution. Antplant coevolution has produced parasites as well as mutualists. Much is still unknown on the evolutionary ecology of these symbioses. Mechanisms of interaction at the chemical level (chemical ecology) are little explored. The functioning of ant-plant associations at the level of populations and communities is poorly understood, and information in this domain is crucial for the conservation of these intricate symbioses in forests increasingly subjected to disturbance and fragmentation.     

Monogenea (platyhelminthes) as hazards for fish in confinement

Occurrences of large-scale, parasite-related fish diseases have increased as the number and size of hatcheries, mass culture facilities, and public and private desplay aquaria have grown. This increase is due to concentration of hosts and parasites in confined quarters resulting in enhanced opportunity for infestations and epizootics. Monogeneans, usually well accommodated to their fish hosts in nature and causing few easily detectable effects, frequently cause severe epizootics in cultured or aquarium-held populations. In such cases, the short, direct, one-host life cycles of monogeneans enable them to reach epizootic levels very quickly when hosts and parasites are confined closely together. Also, despite their high host-specificity in the wild some monogeneans may infest numbers of other hosts when several or many fish species are confined together. Further, debilitating attacks by these helminths may be accompanied by invasions of viruses, bacteria, or fungi. Such invasion is usually via portals of entry caused by trauma of feeding or attachment by the worms. Blood-feeding monogeneans may transmit disease directly.When single or mixed-species host populations are under stress, monogeneans, like other normally benign parasites, may become pathogenic. Stress in hosts results from any single factor, or combinations of factors including: poor sanitation, poor nutrition, poor water quality, overcrowding, behaviorally incompatible species, or improperly designed culture systems or aquaria.Whatever the cause or route of infestation by monogeneans or associated flora and fauna, health and growth of hosts is usually impaired by heavy parasite attack. Deaths are not uncommon. Culturists and aquarists alike must take precautions against monogenean-related disease. Foremost among the precautions is design and thoughtful utilization of adequate holding and display facilities, and adherence to adequate acquisition and introduction procedures, such as the ICES (International Council for the Exploration of the Seas) methods. Careful quarantine and introduction procedures, with thorough monitoring of water quality are paramount. Chemical prophylaxis is possible and should be practiced as needed. When infestations occur, as they often do despite precautions, chemical treatments must be introduced. When epizootics persist, stocks must be harvested quickly or discarded. In aquaria, isolation and treatment of infested individuals or systems may be useful. If this fails, destruction is recommended. The best measure against parasitic monogeneans is avoidance or prevention. Effective sanitation is critical and must be rigorously applied. These and other aspects of monogenean infestation, associated pathology, and prevention or treatment are described and discussed.     

The impact of parasites on the life history evolution of guppies (Poecilia reticulata): the effects of host size on parasite virulence

There is large spatial and temporal variation in the Gyrodactylus parasite fauna across natural guppy (Poecilia reticulata) populations in Trinidad. The life history evolution of these fish could be affected differently in the various habitats depending on the local parasite selection pressure. Here, we experimentally infected three guppy populations with three gyrodactylid strains in the laboratory and monitored the infection by recording the number of parasites and host mortality in a full factorial design. The origin of the guppy population and parasite strain, and the size of the hosts explained significant variation in the survival of hosts. Larger fish carried the highest parasite loads and experienced the highest mortality rates, which suggests that parasite-mediated selection may favour smaller phenotypes, possibly counter-balancing selection pressures by gape-limited predators, mate choice and female fecundity. We observed significant variation in virulence between parasite strains with the captive-bred experimental strain (Gt3) causing the highest mortality of hosts whilst reaching only relatively low maximum burdens. This suggests that adaptations to the captive environment and/or inbreeding depression may alter the virulence of such captive-bred parasites. There were significant differences in survival rate between guppy populations, with infected guppies from the large population of the Lower Aripo River showing a higher survival rate than the fish from the small and genetically less diverse Upper Aripo River population.     

Interactions of pelagic cnidarians and ctenophores with fish: a review

Medusae, siphonophores and ctenophores (here grouped as `pelagic coelenterates') interact with fish in several ways. Some interactions are detrimental to fish populations, such as predation by gelatinous species on pelagic eggs and larvae of fish, the potential competition for prey among pelagic coelenterates and fish larvae and zooplanktivorous fish species, and pelagic coelenterates serving as intermediate hosts for fish parasites. Other interactions are positive for fish, such as predation by fish on gelatinous species and commensal associations among fish and pelagic coelenterates. The interactions range from beneficial for the gelatinous species (food, parasite removal), to negative (predation on them). We review existing information and present new data on these topics. Although such interactions have been documented frequently, the significance to either fish or pelagic coelenterate populations is poorly understood. The effects of pelagic coelenterates on fish populations are of particular interest because of the great importance of fisheries to the global economy. As fishing pressures mount, it becomes increasingly important to understand how they may influence the balance between pelagic coelenterates and fish.     

Biology of the sand-smelt (Atherina presbyter Valenciennes) around Fawley power station

Past studies of fish impingement at Fawley power station have shown that sand-smelts, Atherina presbyter Valenciennes, are susceptible to impingement and may be a useful species for examining effects of impingement mortalities on population size and structure. A 21 month study of populations in the vicinity of Fawley power station was carried out to obtain data on the biology of the species for future population dynamics modelling studies. Aspects included in the study were growth, sex ratio, reproduction, survival and mortality rates and diet. Differences in apparent and true rates of growth indicated some size-selective mortality towards younger age groups, possibly caused by the power station. A seasonallyoscillating von Bertalanffy growth equation is given to predict mean length-at-age. Most female sand-smelts (73%) matured at the end of their first year of life, fecundities ranging on average from 1394 eggs per female for 1+ group fish to 6872 for III+ fish. The sex ratio was even (1 : 1). An annual survival rate of 10–8% was estimated for mature fish, and the consequent age structure of the population indicates that the bulk of egg production lies with the smaller (1+ group) fish. Since smaller fish are generally more vulnerable to impingement this factor may render populations sensitive to impingement mortalities. Gut content analysis indicated that sand-smelts probably actively select zooplankters from the general plankton when young and, once larger than 104 mm in length, increasingly feed upon natant macrofaunal species.     

Female blue tits with brighter yellow chests transfer more carotenoids to their eggs after an immune challenge

Predicting the consequences of predator biodiversity loss on prey requires an understanding of multiple predator interactions. Predators are often assumed to have independent and additive effects on shared prey survival; however, multiple predator effects can be non-additive if predators foraging together reduce prey survival (risk enhancement) or increase prey survival through interference (risk reduction). In marine communities, juvenile reef fish experience very high mortality from two predator guilds with very different hunting modes and foraging domains—benthic and pelagic predator guilds. The few previous predator manipulation studies have found or assumed that mortality is independent and additive. We tested whether interacting predator guilds result in non-additive prey mortality and whether the detection of such effects change over time as prey are depleted. To do so, we examined the roles of benthic and pelagic predators on the survival of a juvenile shoaling zooplanktivorous temperate reef fish, Trachinops caudimaculatus, on artificial patch reefs over 2 months in Port Phillip Bay, Australia. We observed risk enhancement in the first 7 days, as shoaling behaviour placed prey between predator foraging domains with no effective refuge. At day 14 we observed additive mortality, and risk enhancement was no longer detectable. By days 28 and 62, pelagic predators were no longer significant sources of mortality and additivity was trivial. We hypothesize that declines in prey density led to reduced shoaling behaviour that brought prey more often into the domain of benthic predators, resulting in limited mortality from pelagic predators. Furthermore, pelagic predators may have spent less time patrolling reefs in response to declines in prey numbers. Our observation of the changing interaction between predators and prey has important implications for assessing the role of predation in regulating populations in complex communities.     

Diseases caused by parasites in the aquaculture of warm water fish

Parasitic infections of fish cultured in fresh and marine waters of subtropical and tropical geographic regions are reviewed. The following parasites are discussed: Ectoparasitic protozoa, Coccidia, Myxosporea, Monogenea, Metacercariae of trematodes, the Asian tapeworm, nematodes, ergasilids, lernaeids, and argulids. Criteria for selection were their impact on farmed fish and the availability of data beyond case reports and surveys. Epizootic ectoparasitic infections are usually the outcome of adverse growth conditions, either climatic or resulting from management practices. Epizootiology of infections by internal parasites, heteroxenous in particular, is more complex, particularly in man-made systems, as it is determined by a wider range of interacting ecological parameters. Exotic species, particularly cyprinids, comprise the bulk of freshwater fish farmed in warm water systems. Most important diseases affecting fishes in such systems are caused by introduced pathogens. Only a few of the autochthonous parasites become involved in epizootic infections, also where indigenous species are farmed. Marine farming is usually based on indigenous species. The relatively short history of piscine mariculture has already recorded epizootic infections and mortalities both by parasites contracted from the local environment and by specific parasites associated with the cultured species. As in freshwater systems, culture practices often create an environment favorable to epizootic infection.     

How parasitism and pollution affect the physiological homeostasis of aquatic hosts

Parasitism poses a serious threat to hosts under certain circumstances, while the well-being of organisms is also negatively affected by environmental pollution. Little information is available on the simultaneous effects of parasites and pollutants on the physiological homeostasis of organisms. The present paper demonstrates that parasites: (i) may influence the metabolism of pollutants in infected hosts, (ii) interact with pollution in synergistic or antagonistic ways, and (iii) may induce physiological reactions in hosts which were thought to be pollutant-induced. Experimental studies on the uptake and accumulation of metals by fish reveal that fish infected with acanthocephalans have lower metal levels than uninfected hosts; e.g. Pomphorhynchus laevis reduces lead levels in fish bile, thereby diminishing or impeding the hepatic intestinal cycling of lead, which may reduce the quantity of metals available for fish. Alterations in pollutant uptake and accumulation in different intermediate and final hosts due to parasites are thus very important in the field of ecotoxicology. In addition to such alterations, there is a close interaction between the effects of pollutants and parasites which seems to be mediated at least partly by the endocrine system, which itself is closely related to the immune system in fish. Laboratory studies on eels experimentally infected with the swimbladder nematode Anguillicola crassus reveal that toxic chemicals such as polychlorinated biphenyls produce immunosuppressive effects which facilitate parasite infection. Similarly, an increase in serum cortisol concentration in eels due to chemical exposure and infection is correlated with decreasing levels of anti-A. crassus antibodies. Furthermore, parasites are able to elicit physiological changes which are attributed to chemicals with endocrine disrupting activity, e.g. the cestode Ligula intestinalis is known to suppress gonad development in roach. The most thoroughly documented examples of endocrine disruption in wild fish are in roach, and it is conceivable that this disruption is not only due to chemical activity but also to parasites such as L. intestinalis or species of the phylum Microspora.     

Immune response of fish to parasitic protozoa

Epizootic outbreaks of fish diseases are increasingly common as a result of intensive aquaculture, fish farming and sea ranching. Very few drugs are available for treatment or prophylaxis against fish diseases, and development of such compounds is inhibited by different national regulations governing the use of chemicals in fish for human or animal consumption. Alternative approaches are urgently needed. But although the taxonomy and biology of fish parasites have been extensively studied, relatively little is known about protective immunity in fish and the effects of parasites on the piscine immune system. In this article, Patrick Woo discusses the immune responses of fish to parasitic protozoa, showing that vaccination is a viable control strategy, and stressing the need for a coordinated global research programme on fish diseases.     

Alternative predation tactics of a tropical naticid gastropod

Aquarium observations of naticid gastropods from Hong Kong show that different species attack their bivalve prey in different ways. Natica gualteriana and Glossaulax didyma appeared always to use conventional modes of boring, i.e., through one shell valve, before consuming the prey, but some larger prey of C. didyma with incomplete borings were consumed after having apparently suffocated before boring was complete. In contrast, Polinices tumidus prey may be side-bored, edge-bored (i.e., through the commisure of the valves) or suffocated and consumed without boring. The frequency of each of these modes of attack vary with different prey species. Non-boring prédation, in aquarium experiments, accounted for 14.7–54.9% of attacks with different species of prey. Suffocated prey were found to be enwrapped in a thick, viscous coat of mucus, which in partially consumed prey showed a round hole overlying the ventral shell gape marking the entrance hole made by the proboscis. The observations reveal considerable flexibility in predation behaviour in this tropical naticid and have important implications in the interpretation of naticid prédation rates in recent and fossil dead shell assemblages.     

Influence des facteurs abiotiques sur la physiologie alimentaire des larves de la coccinelle aphidiphage,Semiadalia undecimnotata [Col.: Coccinellidae] — I. Action de la température

Résumé L'élevage de la coccinelleSemiadalia undecimnotata Schn. dans différentes conditions thermiques nous a permis de préciser l'action de la température sur la physiologie des stades préimaginaux. Les relations linéaires entre la consommation alimentaire exprimée sous forme cumulée et le poids des individus, qui avaient été mises en évidence dans un travail antérieur ne sont pas modifiées par les variations de ce facteur physique du milieu. I1 en résulte qu'à l'égard de la température, la méthode d'estimation de la consommation alimentaire qui a été proposée dans une publication antérieure peut être utilisée pour estimer la prédation dans le milieu naturel.

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Summary The larvae of the lady beetleSemiadalia undecimnotata Schneider have been reared under various thermic conditions (constant and alternate temperatures). We showed for every instar or only for the last one, that the length of the stage, the length of the feeding period, the daily or the total food consumption (in mg.), the gain of weight, and the top weight are depending on temperature. But the food consumption expressed in a cumulative form and the corresponding weight of the larvae show a constant ratio which is equal to the slope of the linear relations set up in a previous work. With respect to temperature, the method of estimation of larval food consumption which have been described in a former work, can be used to estimate the predatory efficiency of this species in natural conditions.

     

Life in the fast lane: Temperature,density and host species impact survival and growth of the fish ectoparasite Argulus foliaceus

With expanding human populations, the food sector has faced constant pressure to sustainably expand and meet global production demands. In aquaculture this frequently manifests in an animal welfare crisis, with fish increasingly farmed under high production, high stress conditions. These intense environments can result in fish stocks having a high susceptibility to infection, with parasites and associated disease one of the main factors limiting industry growth. Prediction of infection dynamics is key to preventative treatment and mitigation. Considering the climatic and technology driven changes facing aquaculture, an understanding of how parasites react across a spectrum of conditions is required. Here we assessed the impact of temperature, infection density and host species on the life history traits of Argulus foliaceus, a common palearctic fish louse, representative of a parasite group problematic in freshwater aquaculture and fisheries worldwide. Temperature significantly affected development, growth and survival; parasites hatched and developed faster at higher temperatures, but also experienced shorter lifespans when maintained off the host. At high temperatures, these parasites will likely experience a short generation time as their life history traits are completed more rapidly. A. foliaceus additionally grew faster on natural hosts and at lower infection densities. Ultimately such results contribute to prediction of population dynamics, aiding development of effective control to improve animal welfare and reduce industry loss.     

Diseases of echinoderms

Diseases of echinoderms are poorly documented. Most reports concern biotic diseases caused by animal agents. While parasites on echinoderms have been described in increasing numbers of papers for more than one century, the host-parasite relationship and the effects of parasitism on echinoderm life-cycles were rarely considered. The parasitic fauna differs markedly according to the echinoderm group concerned, depending on various factors such as feeding-habits or symbiogenesis. Microorganismic diseases undoubtedly occur in echinoderms but they were not investigated until recently. Microorganisms have frequently been assumed to act as agents causing mass mortalities. As for stress-caused diseases, the only — and very preliminary — data available concern almost exclusively those induced by pollution. Since echinoderms are major components of benthic ecosystems, echinoderm diseases may be expected to exert prominent ecological effects.     

Migration load in males and females

Many theories have been proposed for interpreting the relative sexual mortalities in natural populations, but the effects of maladaptive genes from migrants on sexual mortality have not been examined. In this study, I systematically explore the properties of migration load in dioecious plants where gender dimorphism is determined by sex chromosomes (the XX-XY or similar system). Two stages of gene flow (pollen and seed) and two stages of selection (gametophyte and sporophyte) are jointly examined in the one- and two-locus cases. The general results show that several factors, including the maladaptive allele frequencies in migrants, migration rate, selection, and linkage disequilibria, can individually or jointly alter the relative sexual mortalities at either the gametophyte or the sporophyte stage, or both. When there are the same values between sexes in the parameters relevant to the migrant gene frequencies and two-stage selection coefficients under the additive viability model (without Y-chromosome and linkage disequilibrium effects), the average mortality is higher in ovules than in pollen since the males do not inherit the maladaptive genes from migrating pollen. The average mortality per individual in the homogametic sex is almost twice as large as that in heterogametic sex at the sporophyte stage. Under the additive viability model, the relative sexual mortalities at the gametophyte stage are negatively related to the relative sexual mortalities at the sporophyte stage.     

Micro-organismes de type mycoplasme chez les Coléoptères

Résumé Des micro-organismes présentant les caractères des mycoplasmes ont été observés dans les tissus adipeux et cardiaques de larves du Coléoptère ScarabeideMelolontha melolontha, soit sur des tissus prélevés directement sur larves, soit maintenus en culture d'organes pendant 15 jours à 3 semaines.

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Summary Microorganisms with characteristics usually observed in mycoplasms when examined under the electron microscope have been seen in fat and cardiac cells from larvae ofMelolontha melolontha, [Scarabaeidae Coleoptera]. These observations have been made either on tissues taken directly from larvae or kept in organotypic culture for 15 days to 3 weeks.

     

Evolution of aging: Theoretical and practical implications from rattlesnakes

Many reptiles live relatively long lives wherein senescence is postponed to an advanced age. Altering nutrition, reproduction, temperature, and other physiological parameters may favorably contribute to increased life spans. But life spans are also evolved characteristics of populations, and the distinctive longevities also result from selective regimes arising within particular environments. Aging is not favored directly by evolution as a way to clear a population of senescent individuals. Instead, aging is probably an indirect byproduct of selection for early physical vitality. Senescence may result from delayed appearance of deleterious genes later in life (mutation accumulation) or from multiple effects of single genes with overriding favorable effects early but coupled deleterious effects later in life (antagonistic pleiotropy). Both physiological and evolutionary causes contribute to species or even population-specific aging characteristics. Separating environmentally imposed mortality from that attributable to senescence has been aided by compiling maximum life spans of captive reptiles. Further understanding the underlying aging biology of reptiles would be aided by following mortalities of age cohorts, identifying differences in aging between populations, documenting the effects, favorable or not, of husbandry practices, and by characterizing senescence not just by mortality, but also by changes in age-related performance. Theoretical issues, inspired by experimental results in rattlesnakes, suggest conditions under which the chance mortalities of young rattlesnakes together with continued growth of adults might favor late appearance of beneficial genes and thereby account for postponed senescence in some reptiles. © 1996 Wiley-Liss, Inc.