Ovoviviparity in platyhelminth life-cycles

Parasitology ◽  
1983 ◽  
Vol 86 (4) ◽  
pp. 161-196 ◽  
Author(s):  
R. C. Tinsley
Keyword(s):  
Time Delay ◽  
Population Growth ◽  
Schistosoma Mansoni ◽  
Larval Development ◽  
External Environment ◽  
Reproductive Potential ◽  
Life Cycles ◽  
Environmental Instability ◽  
Host Behaviour ◽  
Exogenous Nutrients

SUMMARYThe encapsulated embryos of platyhelminths may be retained and complete their developmentin uteroin a range of circumstances. However, hatching within the parent (the criterion of ovoviviparity) is relatively rare and larvae generally emerge only after deposition. Viviparity is characterized by the nutritional dependency of the unencapsulated larva upon the parent, but in several cases larvae retained within a shell also receive parental nutrients during intra-uterine development. Uptake of exogenous nutrients via shell pores occurs inSchistosoma mansonibut the eggs, which gain all the advantages of intra-uterine retention, are supported by host nutrients.Intra-uterine larval development avoids the hazards of development in the external environment and eliminates the time delay between oviposition and infection. Deposition of immediately infective offspring may be concentrated in time and space to exploit periods of host vulnerability. The control and precision of transmission is illustrated by examples in which the opportunity for invasion is restricted because of either host behaviour or environmental instability. This strategy has been an important factor in the evolution of polystomatid monogeneans, and its effectiveness is demonstrated by comparison of the life-cycles ofPolystoma integerrimumandPseudodiplorchis americanus. Ovoviviparity also increases reproductive potential in some polystomatids by extending the period of multiplication and by increasing established populations through internal re-infection. InEupolystoma alluaudi, the capacity for ovoviviparity is programmed into larval development and this regulates population growth within individual hosts.

Metabolism, homeostasis and growth

2017 ◽  
Author(s):  
Derek Burton ◽  
Margaret Burton
Keyword(s):  
Energy Storage ◽  
White Muscle ◽  
External Environment ◽  
Life Cycles ◽  
Fish Growth ◽  
Chloride Cells ◽  
Internal Environment ◽  
Ion Regulation ◽  
Yolk Proteins ◽  
Active Substances

Metabolism consists of the sum of anabolism (construction) and catabolism (destruction) with the release of energy, and achieving a fairly constant internal environment (homeostasis). The aquatic external environment favours differences from mammalian pathways of excretion and requires osmoregulatory adjustments for fresh water and seawater though some taxa, notably marine elasmobranchs, avoid osmoregulatory problems by retaining osmotically active substances such as urea, and molecules protecting tissues from urea damage. Ion regulation may occur through chloride cells of the gills. Most fish are not temperature regulators but a few are regional heterotherms, conserving heat internally. The liver has many roles in metabolism, including in some fish the synthesis of antifreeze seasonally. Maturing females synthesize yolk proteins in the liver. Energy storage may include the liver and, surprisingly, white muscle. Fish growth can be indeterminate and highly variable, with very short (annual) life cycles or extremely long cycles with late and/or intermittent reproduction.

I Feel That! Fluid Dynamics and Sensory Aspects of Larval Settlement across Scales

2018 ◽  
Keyword(s):  
Fluid Dynamics ◽  
Marine Invertebrates ◽  
Larval Settlement ◽  
External Environment ◽  
Life Cycles ◽  
Decision Making Process ◽  
Larval Biology ◽  
Sea Floor ◽  
External Cues ◽  
Juvenile Habitat

A commonality among oceanic life cycles is a process known as settlement, where dispersing propagules transition to the sea floor. For many marine invertebrates, this transition is irreversible, and therefore involves a crucial decision-making process through which larvae evaluate their juvenile habitat-to-be. In this chapter, we consider aspects of the external environment that could influence successful settlement. Specifically, we discuss water flow across scales, and how larvae can engage behaviors to influence where ocean currents take them, and enhance the likelihood of their being carried toward suitable settlement locations. Next, we consider what senses larvae utilize to evaluate their external environment and properly time such behavioral modifications, and settlement generally. We hypothesize that larvae integrate these various external cues in a hierarchical fashion, with differing arrangements being employed across ontogeny and among species. We conclude with a brief discussion of the future promises of larval biology, ecology, and evolution.

scholarly journals Emergent multicellular life cycles in filamentous bacteria owing to density-dependent population dynamics

2011 ◽  
Vol 8 (65) ◽  
pp. 1772-1784 ◽  
Author(s):  
Valentina Rossetti ◽  
Manuela Filippini ◽  
Miroslav Svercel ◽  
A. D. Barbour ◽  
Homayoun C. Bagheri
Keyword(s):  
Cell Division ◽  
Population Growth ◽  
Generation Time ◽  
Single Cells ◽  
Bacterial Species ◽  
Life Forms ◽  
Life Cycles ◽  
Morphological Properties ◽  
Filamentous Bacteria ◽  
Growth Phases

Filamentous bacteria are the oldest and simplest known multicellular life forms. By using computer simulations and experiments that address cell division in a filamentous context, we investigate some of the ecological factors that can lead to the emergence of a multicellular life cycle in filamentous life forms. The model predicts that if cell division and death rates are dependent on the density of cells in a population, a predictable cycle between short and long filament lengths is produced. During exponential growth, there will be a predominance of multicellular filaments, while at carrying capacity, the population converges to a predominance of short filaments and single cells. Model predictions are experimentally tested and confirmed in cultures of heterotrophic and phototrophic bacterial species. Furthermore, by developing a formulation of generation time in bacterial populations, it is shown that changes in generation time can alter length distributions. The theory predicts that given the same population growth curve and fitness, species with longer generation times have longer filaments during comparable population growth phases. Characterization of the environmental dependence of morphological properties such as length, and the number of cells per filament, helps in understanding the pre-existing conditions for the evolution of developmental cycles in simple multicellular organisms. Moreover, the theoretical prediction that strains with the same fitness can exhibit different lengths at comparable growth phases has important implications. It demonstrates that differences in fitness attributed to morphology are not the sole explanation for the evolution of life cycles dominated by multicellularity.

The regulation of host population growth by parasitic species

Parasitology ◽  
1978 ◽  
Vol 76 (2) ◽  
pp. 119-157 ◽  
Author(s):  
R. M. Anderson
Keyword(s):  
Population Growth ◽  
Reproductive Potential ◽  
Parasite Burden ◽  
Host Population ◽  
Parasitic Species ◽  
Regulatory Influence ◽  
Host Mortality ◽  
Parasite Reproduction ◽  
Over Dispersion ◽  
Host Parasite

SummaryThe nature of parasitism at the population level is defined in terms of the parasite's influence on the natural intrinsic growth rate of its host population. It is suggested that the influence on this rate is related to the average parasite burden/host and hence to the statistical distribution of parasites within the host population.Theoretical models of host–parasite associations are used to assess the regulatory influence of parasitic species on host population growth. Model predictions suggest that three specific groups of population processes are of particular importance: over-dispersion of parasite numbers/host, density dependence in parasite mortality or reproduction and parasite-induced host mortality that increases faster than linearly with the parasite burden. Other population mechanisms are shown to have a destabilizing influence, namely: parasite-induced reduction in host reproductive potential, direct parasite reproduction within the host and time delays in the development of transmission stages of the parasite.These regulatory and destabilizing processes are shown to be commonly observed features of natural host-parasite associations. It is argued that interactions in the real world are characterized by a degree of tension between these regulatory and destabilizing forces and that population rate parameter values in parasite life-cycles are very far from being a haphazard selection of all numerically possible values. It is suggested that evolutionary pressures in observed associations will tend to counteract a strong destabilizing force by an equally strong regulatory influence. Empirical evidence is shown to support this suggestion in, for example, associations between larval digeneans and molluscan hosts (parasite-induced reduction in host reproductive potential counteracted by tight density-dependent constraints on parasite population growth), and interactions between protozoan parasites and mammalian hosts (direct parasite reproduction counteracted by a well-developed immunological response by the host).The type of laboratory and field data required to improve our understanding of the dynamical properties of host–parasite population associations is discussed and it is suggested that quantitative measurement of rates of parasite-induced host mortality, degrees of over-dispersion, transmission rates and reproductive and mortality rates of both host and parasite would provide an important first step. The value of laboratory work in this area is demonstrated by reference to studies which highlight the regulatory influence of parasitic species on host population growth.

Eimeria mitis: a comparison of the endogenous developmental stages of a line selected for early maturation and the parent strain

Parasitology ◽  
1984 ◽  
Vol 88 (1) ◽  
pp. 37-44 ◽  
Author(s):  
V. McDonald ◽  
M. W. Shirley
Keyword(s):  
Life Cycle ◽  
Epithelial Cells ◽  
Parent Strain ◽  
First Generation ◽  
Developmental Stages ◽  
Reproductive Potential ◽  
Life Cycles ◽  
Endogenous Development ◽  
Early Maturation ◽  
Precocious Line

SUMMARYThe endogenous development of the Houghton (H) strain of Eimeria mitis (= mivati) was compared with the life-cycle of a precocious (HP) line derived from the H strain. In both parasites 4 generations of schizonts which developed in epithelial cells were observed: the 1st and 2nd were found in the crypts and the 3rd and 4th in the villi. Gametocytes and zygotes occupied epithelial cells at the tips of the villi. The onset of gametogony normally coincided with the maturation of 4th-generation schizonts. The infection was confined initially to an area of the gut extending from the jejunum to the ileo-caecal junction but 3rd-generation merozoites and subsequent stages were also found in the caeca and rectum. The life-cycle of the precocious line was shorter than that of the parent strain. Gametocytes appeared to develop from 3rd-generation as well as from 4th-generation merozoites. Also, sporozoites of the precocious line transformed to trophozoites before those of the parent strain. First-generation schizonts of the HP line tended to be smaller and to contain fewer merozoites than those of the H strain. The differences between the life-cycles of the two parasites account for the lower reproductive potential of the precocious line.

Organisational Renewal in the Context of Indian Banks

1998 ◽  
Vol 2 (2) ◽  
pp. 34-38
Author(s):  
R. Anuradha
Keyword(s):  
Response Pattern ◽  
External Environment ◽  
Life Cycles ◽  
Top Management ◽  
Indian Banks ◽  
Constant Source ◽  
Term Benefit

Large organisations evolve through different life cycles. Size, age, spread and ownership factors determine their response pattern and potential to sustain over the years. Based on the author’s consultancy experience, this paper describes in detail the conceptual and practical perspectives of organisational renewal in banks as service organisations. As the external environment is becoming turbulent and rapidly changing, many traditional and bureaucratic organisations suffer from the inability to plan and execute necessary internal changes. Transforming large organisations from reactive to proactive enterprises is a constant source of concern to the top management members. This paper emphasises the need for integrating plan and action for the renewal effort to provide long term benefit to a declining organisation.

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