Showing papers on "Population published in 1973"

Analysis of Gene Diversity in Subdivided Populations

Abstract: A method is presented by which the gene diversity (heterozygosity) of a subdivided population can be analyzed into its components, i.e., the gene diversities within and between subpopulations. This method is applicable to any population without regard to the number of alleles per locus, the pattern of evolutionary forces such as mutation, selection, and migration, and the reproductive method of the organism used. Measures of the absolute and relative magnitudes of gene differentiation among subpopulations are also proposed.

Long-lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path.

Abstract: 1. The after-effects of repetitive stimulation of the perforant path fibres to the dentate area of the hippocampal formation have been examined with extracellular micro-electrodes in rabbits anaesthetized with urethane.2. In fifteen out of eighteen rabbits the population response recorded from granule cells in the dentate area to single perforant path volleys was potentiated for periods ranging from 30 min to 10 hr after one or more conditioning trains at 10-20/sec for 10-15 sec, or 100/sec for 3-4 sec.3. The population response was analysed in terms of three parameters: the amplitude of the population excitatory post-synaptic potential (e.p.s.p.), signalling the depolarization of the granule cells, and the amplitude and latency of the population spike, signalling the discharge of the granule cells.4. All three parameters were potentiated in 29% of the experiments; in other experiments in which long term changes occurred, potentiation was confined to one or two of the three parameters. A reduction in the latency of the population spike was the commonest sign of potentiation, occurring in 57% of all experiments. The amplitude of the population e.p.s.p. was increased in 43%, and of the population spike in 40%, of all experiments.5. During conditioning at 10-20/sec there was massive potentiation of the population spike (;frequency potentiation'). The spike was suppressed during stimulation at 100/sec. Both frequencies produced long-term potentiation.6. The results suggest that two independent mechanisms are responsible for long-lasting potentiation: (a) an increase in the efficiency of synaptic transmission at the perforant path synapses; (b) an increase in the excitability of the granule cell population.

Culture of Human Endothelial Cells Derived from Umbilical Veins. IDENTIFICATION BY MORPHOLOGIC AND IMMUNOLOGIC CRITERIA

Abstract: Endothelial cells were isolated from freshly obtained human umbilical cords by collagenase digestion of the interior of the umbilical vein. The cells were grown in tissue culture as a homogeneous population for periods up to 5 mo and some lines were subcultured for 10 serial passages. During the logarithmic phase of cell growth, cell-doubling time was 92 h. Light, phase contrast, and scanning electron microscopy demonstrated that cultured human endothelial cells grew as monolayers of closely opposed, polygonal large cells whereas both cultured human fibroblasts and human smooth muscle cells grew as overlapping layers of parallel arrays of slender, spindle-shaped cells. By transmission electron microscopy, cultured endothelial cells were seen to contain cytoplasmic inclusions (Weibel-Palade bodies) characteristic of in situ endothelial cells. These inclusions were also found in endothelial cells lining umbilical veins but were not seen in smooth muscle cells or fibroblasts in culture or in situ. Cultured endothelial cells contained abundant quantities of smooth muscle actomyosin. Cultured endothelial cells also contained ABH antigens appropriate to the tissue donor's blood type; these antigens were not detectable on cultured smooth muscle cells or fibroblasts. These studies demonstrate that it is possible to culture morphologically and immunologically identifiable human endothelial cells for periods up to 5 mo.

Ecology: The Experimental Analysis of Distribution and Abundance

Abstract: This CD-ROM helps students learn to think like field ecologists, whether estimating the number of mice on an imaginary island or restoring prairie land in Iowa, through 26 interactive field experiments and tutorials. The CD-ROM also includes test questions, quiz questions, weblinks, and a glossary. Included with every student copy of the text.

A rapid method for the isolation of functional thymus‐derived murine lymphocytes

Abstract: A rapid method is described for effectively removing immunoglobulin-bearing cells from either primed or unprimed mouse spleen and lymph node cell suspensions. Incubation of cell suspensions in nylon wool columns for 45 min at 37 °C resulted in a 9 to 100-fold depletion of immunoglobulin-bearing cells and a complementary 1.5 to 2-fold enrichment of T cells in the column effluent populations. The effluent population, derived from passage of spleen cells through these columns, was virtually devoid of B precursor and memory cell activity, but contained all of the helper cell and cytotoxic effector cell precursor activity when compared to unfractionated spleen cells.

Natural selection of parental ability to vary the sex ratio of offspring.

Abstract: Theory and data suggest that a male in good condition at the end of the period of parental investment is expected to outreproduce a sister in similar condition, while she is expected to outreproduce him if both are in poor condition. Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. Data from mammals support the model: As maternal condition declines, the adult female tends to produce a lower ratio of males to females.

The Rise of the Western World: A New Economic History

Abstract: A radically new interpretation, offering a unified explanation for the growth of Western Europe between 900 A. D. and 1700, provides a general theoretical framework for institutional change geared to the general reader. North and Thomas seek to explain the "rise of the Western world" by illuminating the causal importance of an efficient economic organization that guarantees a wide latitude of property rights and both incentives and protection for economic growth. Although they pay homage to both Marxian and neoliberal theory, they take a theoretical middle ground that privileges the sociopolitical backdrop of economic affairs (as opposed to solely private or class-based activity) and in doing so identifies the roots of modernization as far back as the 10th Century. To justify the novelty and originality of this approach, they write that most analysts have misidentified the symptoms of modern economic growth (technological change, human capital, economies of scale) as the causes. In doing so, previous scholars have failed to answer the question "if all that is required for economic growth is investment and innovation, why have some societies missed this desirable outcome?" (2). Their answer is that some societies (England and the Netherlands) were better than others (France and Spain) at providing an efficient economic organization that could guarantee conditions favorable to per capita economic growth among a rapidly growing population.

essay on the principle of population

Abstract: THE GREAT AND UNLOOKED FOR DISCOVERIES that have taken place of late years in natural philosophy, the increasing diffusion of general knowledge from the extension of the art of printing, the ardent and unshackled spirit of inquiry that prevails throughout the lettered and even unlettered world, the new and extraordinary lights that have been thrown on political subjects which dazzle and astonish the understanding, and particularly that tremendous phenomenon in the political horizon, the French Revolution, which, like a blazing comet, seems destined either to inspire with fresh life and vigour, or to scorch up and destroy the shrinking inhabitants of the earth, have all concurred to lead many able men into the opinion that we were touching on a period big with the most important changes, changes that would in some measure be decisive of the future fate of mankind.

On the Interaction between the Quantity and Quality of Children

Abstract: Students of human fertility have been aware for a long time that there may be some special relation between the number (quantity) of children ever born to a family and the "quality" of their children as perceived by others if not by the parents. One need only cite the negative correlation between quantity and quality of children per family so often observed in both cross-section and time-series data. One of us (Becker 1960) more than a decade ago stressed the importance for understanding fertility (quantity) of the interaction between quantity and quality, and we are pleased to note that this interaction is emphasized in most of the papers in this Supplement, especially those by De Tray and Willis. Some economists have argued that the negative relation between quantity and quality often observed is a consequence of a low substitution elasticity in a family's utility function between parents' consumption or level of living and that of their children (see, e.g., Duesenberry 1960; Willis 1969). The approach followed by De Tray in this volume is different, but it makes equally special assumptions about the substitution between quantity and quality in the utility function and in household production.

Small Area Variations in Health Care Delivery

Abstract: Health information about total populations is a prerequisite for sound decision-making and planning in the health care field. Experience with a population-based health data system in Vermont reveals that there are wide variations in resource input, utilization of services, and expenditures among neighboring communities. Results show prima facie inequalities in the input of resources that are associated with income transfer from areas of lower expenditure to areas of higher expenditure. Variations in utilization indicate that there is considerable uncertainty about the effectiveness of different levels of aggregate, as well as specific kinds of, health services. Informed choices in the public regulation of the health care sector require knowledge of the relation between medical care systems and the population groups being served, and they should take into account the effect of regulation on equality and effectiveness. When population-based data on small areas are available, decisions to expand hospitals, currently based on institutional pressures, can take into account a community's regional ranking in regard to bed input and utilization rates. Proposals by hospitals for unit price increases and the regulation of the actuarial rate of insurance programs can be evaluated in terms of per capita expenditures and income transfer between geographically defined populations. The PSRO's can evaluate the wide variations in level of services among residents of different communities. Coordinated exercise of the authority vested in these regulatory programs may lead to explicit strategies to deal directly with inequality and uncertainty concerning the effectiveness of health care delivery. Population-based health information systems, because they can provide information on the performance of health care systems and regulatory agencies, are an important step in the development of rational public policy for health.

Hyperlipidemia in Coronary Heart Disease II. GENETIC ANALYSIS OF LIPID LEVELS IN 176 FAMILIES AND DELINEATION OF A NEW INHERITED DISORDER, COMBINED HYPERLIPIDEMIA

Abstract: To assess the genetics of hyperlipidemia in coronary heart disease, family studies were carried out in 2520 relatives and spouses of 176 survivors of myocardial infarction, including 149 hyperlipidemic and 27 normolipidemic individuals. The distribution of fasting plasma cholesterol and triglyceride values in relatives, together with segregation analyses, suggested the presence of five distinct lipid disorders. Three of these-familial hypercholesterolemia, familial hypertriglyceridemia, and familial combined hyperlipidemia-appeared to represent dominant expression of three different autosomal genes, occurring in about 20% of survivors below 60 yr of age and 7% of all older survivors. Two other disorders-polygenic hypercholesterolemia and sporadic hypertriglyceridemia-each affected about 6% of survivors in both age groups. The most common genetic form of hyperlipidemia identified in this study has hitherto been poorly defined and has been designated as familial combined hyperlipidemia. Affected family members characteristically had elevated levels of both cholesterol and triglyceride. However, increased cholesterol or increased triglyceride levels alone were also frequently observed. The combined disorder was shown to be genetically distinct from familial hypercholesterolemia and familial hypertriglyceridemia for the following reasons: (a) the distribution pattern of cholesterol and triglyceride levels in relatives of probands was unique; (b) children of individuals with combined hyperlipidemia did not express hypercholesterolemia in contrast to the finding of hypercholesterolemic children from families with familial hypercholesterolemia; and (c) analysis of informative matings suggested that the different lipid phenotypes owed their origin to variable expression of a single autosomal dominant gene and not to segregation of two separate genes, such as one elevating the level of cholesterol and the other elevating the level of triglyceride. Heterozygosity for one of the three lipid-elevating genes identified in this study may have a frequency in the general population of about 1%, constituting a major problem in early diagnosis and preventive therapy.

Distribution of gene frequency as a test of the theory of the selective neutrality of polymorphisms

Abstract: The variation in gene frequency among populations or between generations within a population is a result of breeding structure and selection. But breeding structure should affect all loci and alleles in the same way. If there is significant heterogeneity between loci in their apparent inbreeding coefficients F=s(p) (2)/p(1-p), this heterogeneity may be taken as evidence for selection. We have given the statistical properties of F and shown how tests of heterogeneity can be made. Using data from human populations we have shown highly significant heterogeneity in F values for human polymorphic genes over the world, thus demonstrating that a significant fraction of human polymorphisms owe their current gene frequencies to the action of natural selection. We have also applied the method to temporal variation within a population for data on Dacus oleae and have found no significant evidence of selection.

A New Approach to the Economic Theory of Fertility Behavior

Abstract: A static economic theory of lifetime marital fertility is presented within the context of the economic theory of the family. The theoretical model is developed under a set of restrictive assumptions designed to make it analytically tractable and capable of yielding implications which may be tested with individual data on the number of children born to recent cohorts of American women who have completed their fertility. The model also has implications for child "quality" which is defined as a function of the resources parents devote to each child and for the wifes lifetime market earnings capacity and labor supply. Focus is on fertility as a form of economic behavior fertility demand and the demand for child quality the supply of child services and the allocation of time desired fertility and wifes labor force participation and empirical results. On the basis of the evidence presented it appears that the interaction model captures an important empirical regularity in the cross-section relationship between fertility and measures of husbands income and wifes education that has become apparent in the emergence of a U-shaped relationship between fertility and income. This relationship has been observed in many advanced countries in the past 25 years and was an incipient relation at the lower levels of income and education prevailing in earlier periods. This empirical regularity is also consistent with the predictions of the theoretical model of fertility demand developed in this paper and must therefore be counted as evidence in its favor.

A model of mutation appropriate to estimate the number of electrophoretically detectable alleles in a finite population

Abstract: A new model of mutational production of alleles was proposed which may be appropriate to estimate the number of electrophoretically detectable alleles maintained in a finite population. The model assumes that the entire allelic states are expressed by integers (…, A−1, A0, A1, …) and that if an allele changes state by mutation the change occurs in such a way that it moves either one step in the positive direction or one step in the negative direction (see also Fig. 1). It was shown that for this model the ‘effective’ number of selectively neutral alleles maintained in a population of the effective size Ne under mutation rate υ per generation is given byWhen 4Neυ is small, this differs little from the conventional formula by Kimura & Crow, i.e. ne = 1 + 4Neυ, but it gives a much smaller estimate than this when 4Neυ is large.

Slightly deleterious mutant substitutions in evolution.

Abstract: RECENT advances in molecular genetics have had a great deal of influence on evolutionary theory, and in particular, the neutral mutation-random drift hypothesis of molecular evolution1,2 has stimulated much interest. The concept of neutral mutant substitution in the population by random genetic drift can be extended to include random fixation of very slightly deleterious mutations which have more chance of being selected against than of being selected for3,4. If this class of mutant substitution is important, we can predict that the evolution is rapid in small populations or at the time of speciation5. Here I shall organize the observed facts which indicate that this class is in fact important.

Self-regulation of growth in three dimensions

Abstract: Multi-cell spheroids were grown in soft agar. When each spheroid was cultured in a large volume of medium, frequently renewed, all spheroids eventually reached a dormant phase at a diameter of approximately 3-4 mm and a population of approximately 10(6) cells. In the dormant spheroid, newly generated cells at the periphery balanced those lost by necrosis in the center. We propose that this dormant phase is due to a gradual reduction in the ratio of surface area to volume: a size is achieved beyond which there is insufficient surface area for the spheroid to eliminate catabolites and absorb nutrients. Thus, in the face of unlimited space and of new medium, three-dimensional cell populations become self-regulating. This phenomenon contrasts with standard tissue culture in which cell populations, living on a flat plane in two dimensions, will not stop growing in the face of unlimited space and new medium because the ratio of surface area to volume remains constant. These experiments provide a mechanism for our observations in vivo: before vascularization, solid tumors live by simple diffusion as three-dimensional spheroids or ellipsoids. They become dormant at a diameter of only a few millimeters; once vascularized, they are released from this dormant phase and begin exponential growth. Thus, tumor dormancy resulting from absence of angiogenesis in vivo, may operate by the same mechanism responsible for dormancy of spheroids in vitro.

Stability in insect host-parasite models

Abstract: where NS represents the survivors after Pt parasites have searched for Nt hosts resulting in P+ 1 parasite progenyt. All assumptions about parasite searching behaviour are here contained in the functionf[Pt,Nt]. If we consider the simplest case where the parasite population is specific and synchronized temporally with its host population, we can write the following generalized model for a host-parasite interaction:

Gene flow and selection in a cline.

Abstract: A model of the effect of gene flow and natural selection in a continuously distributed, infinite population is developed. Different patterns of spatial variation in selective pressures are considered, including a step change in the environment, a "pocket" in the environment and a periodically varying environment. Also, the problem of the effect of a geographic barrier to dispersal is analyzed. The results are: (1) there is a characteristic length scale of variation of gene frequencies, (see PDF). The population cannot respond to changes in environmental conditions which occur over a distance less than the characteristic length. The result does not depend either on the pattern of variation in selective pressures or on the exact shape of the dispersal function. (2) The reduction in the fitness of the heterozygote causes a cline in gene frequencies to become steeper. (3) A geographic barrier to dispersal causes a drastic change in the gene frequencies at the barrier only when almost all of the individuals trying to cross the barrier are stopped.

Gene flow and population differentiation.

Abstract: There are many possible spatial patterns of selection and gene flow that can produce a given cline structure; the actual geography of natural selection and gene flow must be worked out before an attempt is made to explain a given natural cline in terms of a model. The results of experimental and theoretical models show that it is possible for local differentiation to evolve parapatrically in spite of considerable gene flow if the selection gradients are relatively uniform. Irregularities in environmental gradients increase the sensitivity of clines to the effects of gene flow in proportion to the increase in the differences in gene frequencies between the emigrants and the demes receiving the immigrants. It is not necessary for a sharp spatial environmental change to be present for distinct differentiation to occur. In some cases even a gentle environmental gradient can give rise to marked spatial differentiation along a genetically continuous series of demes; such environmental differences may be below the practical limits of resolution in field studies. Any asymmetry in gene flow does not lead to dedifferentiation if the environmental gradient is smooth; it merely shifts the position of the transition zone between the differentiated areas from that which would be expected if there were no asymmetry. Abrupt geographic differences in gene, genotype, or morph frequencies should not, therefore, be interpreted as evidence for environmental changes in the immediate vicinity of the steepest part of the cline; neither should they be interpreted as evidence for geographic barriers, sharp environmental differences, or sexual isolation among the differentiated groups of populations when there are no other sources of evidence for these phenomena. Gene flow may be unimportant in the differentiation of populations along environmental gradients.

The economics of overexploitation.

Abstract: The general economic analysis of a biological resource presented in this article suggests that overexploitation in the physical sense of reduced productivity may result from not one, but two social conditions: common-property competitive exploitation on the one hand, and private-property maximization of profits on the other. For populations that are economically valuable but possess low reproductive capacities, either condition may lead even to the extinction of the population. In view of the likelihood of private firms adopting high rates of discount, the conservation of renewable resources would appear to require continual public surveillance and control of the physical yield and the condition of the stocks.

Reliability, validity, and clinical application of the Visual Analogue Mood Scale.

Abstract: The reliability and validity of the Visual Analogue Mood Scale (VAMS) has been demonstrated in both a military and a private psychiatric hospital, inpatient population. The repeated and concurrent administration of the VAMS and the Digit-Symbol test identifies a mood-performance correlation which distinguishes patients with affective disorders, of manic or depressed type, from other psychiatric patients.

The Discovery of America The first Americans may have swept the Western Hemisphere and decimated its fauna within 1000 years

Abstract: I propose a new scenario for the discovery of America. By analogy with other successful animal invasions, one may assume that the discovery of the New World triggered a human population explosion. The invading hunters attained their highest population density along a front that swept from Canada to the Gulf of Mexico in 350 years, and on to the tip of South America in roughly 1000 years. A sharp drop in human population soon followed as major prey animals declined to extinction. Possible values for the model include an average frontal depth of 160 kilometers, an average population density of 0.4 person per square kilometer on the front and of 0.04 person per square kilometer behind the front, and an average rate of frontal advance of 16 kilometers per year. For the first two centuries the maximum rate of growth may have equaled the historic maximum of 3.4 percent annually. During the episode of faunal extinctions, the population of North America need not have exceeded 600,000 people at any one time. The model generates a population sufficiently large to overkill a biomass of Pleistocene large animals averaging 9 metric tons per square kilometer (50 animal units per section) or 2.3 x 108 metric tons in the hemisphere. It requires that on the front one person in four destroy one animal unit (450 kilograms) per week, or 26 percent of the biomass of an average section in 1 year in any one region. Extinction would occur within a decade. There was insufficient time for the fauna to learn defensive behaviors, or for more than a few kill sites to be buried and preserved for the archeologist. Should the model survive future findings, it will mean that the extinction chronology of the Pleistocene megafauna can be used to map the spread of Homo sapiens throughout the New World.

Recurrent outbreaks of measles, chickenpox and mumps i. seasonal variation in contact rates

Abstract: London, W. P. (Mathematical Research Branch, National Institute of Arthritis, Metabolism, and Digestive Diseases, Bethesda, Md. 20014) and J. A. Yorke. Recurrent outbreaks of measles, chickenpox and mumps. I. Seasonal variation in contact rates. Am J Epidemiol 98:453-468, 1973. —Recurrent outbreaks of measles, chickenpox and mumps in cities are studied with a mathematical model of ordinary differential delay equations. For each calendar month a mean contact rate (fraction of susceptibles contacted per day by an infective) is estimated from the monthly reported cases over a 30to 35-year period. For each disease the mean monthly contact rate is 1.7 to 2 times higher in the winter months than in the summer months; the seasonal variation is attributed primarily to the gathering of children in school. Computer simulations that use the seasonally varying contact rates reproduce the observed pattern of undamped recurrent outbreaks: annual outbreaks of chickenpox and mumps and biennial outbreaks of measles. The two-year period of measles outbreaks is the signature of an endemic infectious disease that would exhaust itself and become nonendemic if there were a minor increase in infectivity or a decrease in the length of the incubation period. For populations in which most members are vaccinated, simulations show that the persistence of the biennial pattern of measles outbreaks implies that the vaccine is not being used uniformly throughout the population.

Aggregation of preferences with variable electorate

Abstract: IN THIS PAPER we consider procedures for going from several individual preferences among several alternatives, called candidates, to something which may be called a collective preference. The individual preferences take the form of (total) orderings of the alternatives, and the collective preference is to take the form of a (total) weak ordering (i.e., ties allowed). We consider certain properties which seem desirable in such systems and investigate which systems have these properties. The point of view taken here differs from that of other work in this area (e.g., [1, 2, 3, 4]) chiefly in asking that the procedure work for all possible sizes of the voting population, rather than for a fixed population, given in advance. This permits us to require, for example, that if each of two bodies of voters prefers candidate A to candidate B under a given procedure, then the combination of these bodies should prefer A to B under the same procedure. In Section 1 we give the formal definitions of an aggregation procedure and discuss certain desirable features, namely "neutrality" (treats candidates symmetrically), "separability" (the condition mentioned above), "monotonicity," and an "Archimedean" property which says, roughly, that a sufficiently large body with a given distribution of preferences can impose its will on any body of fixed size. In Section 2 we introduce certain procedures: "point systems" and "generalized point systems" (roughly, point systems allowing "infinitesimal points"), which are neutral and separable. They are monotonic if and only if the points are arranged in the "natural" order, and the point systems are, in addition, Archimedean. In Section 3 we prove a converse, namely that any neutral and separable procedure can be realized by a generalized point system and, if it is Archimedean, by a point system. This part requires some familiarity with the notions of least upper bound of a set of real numbers and bases of vector spaces. In Section 4 (which is largely independent of Section 3), we consider "point runoff" systems which use point systems in a succession of eliminations. Such systems are neither separable nor monotonic but do satisfy some very weak separability and monotonicity conditions. While these probably do not characterize point runoff systems, we know of no other systems satisfying them.

The refugee in flight: Kinetic models and forms of displacement.

Abstract: After the publication of Fairchild’s early Immigrant Backgrounds in which he emphasised the importance of such immigrant characteristics as race culture and nationality it became an axiom of immigration studies that the immigrant’s background affects his future as settler. Work following in the next decades confirmed Fairchild’s observation and enlarged on it. By 1958 Petersen listed the "emigrantsmotives and the social causes of emigration" as an important background factor affecting aspirations and migrant outcomes. The growing recognition that the refugeesmotivation to seek a new place of settlement differs from that of the voluntary migrant was also accompanied by mounting evidence that the refugeesexperiences may vary one from the other the variance resulting in different refugee outcomes. However observations on individual refugee groups were neither compared nor operationally utilised in any comprehensive way. Such a comprehensive study cannot be undertaken without the existence of a workable theory of refugee movements based on a fruitful refugee typology and resting on suitable conceptual categories. This study intends to help lay these foundations. (excerpt)

Ecology of Vibrio parahaemolyticus in Chesapeake Bay

Abstract: A study of the ecology of Vibrio parahaemolyticus and related vibrios in the Rhode River area of Chesapeake Bay was carried out over the period December 1970 through August 1971. The incidence of V. parahaemolyticus and related vibrios was found to be correlated with water temperature. The vibrios could not be detected in the water column during the winter months, although they were present in sediment. From late spring to early summer, when water temperatures were 14 ± 1 C, vibrios over-wintering in sediment were released from the bottom communities and attached to zooplankton, proliferating as the temperature rose. The number of vibrios in and on plankton was reflected in the water column bacterial population densities at water temperatures of ca. 19 C. Thus, temperature of the water column in the range of 14 to 19 C was found to be critical in the annual cycle of the vibrios. Interaction between sediment, water, and zooplankton was found to be essential in the natural estuarine ecosystem. Bacterial counts of zooplankton were found to be temperature dependent. The bacterial population associated with zooplankton was found to be predominantly on external surfaces and was specific, differing from that of the sediment. Vibrio spp. and related organisms comprised the total bacterial population associated with zooplankton in summer months. The ecological role of Vibrio spp., including V. parahaemolyticus, was found to be significant, with respect to their property of chitin digestion and in relation to the population dynamics of zooplankton in Chesapeake Bay.