G. Vranken

The productivity of marine nematodes

Abstract The productivity of marine nematodes was studied from laboratory experiments investigating the relationship between minimum generation time and temperature, the daily birth rate as calculated from life-tables taking fecundity and survivorship into account and the temperature regime in the field. The life cycle of Monhystera disjuncta is described. Females produce about 200 eggs in agnotobiotic conditions over about 70 days: this represents 17 times their own body weight. The mean generation time is 20 days at 12 °C and this species can produce 23 generations in the field each year. The maximum annual P/B is equal to 69. The annual P/B calculated from the birth rate is 60. From similar studies on five other nematode species it is concluded that the life-cycle turnover is equal to three but the number of generations annually produced in the field varies from one to twenty. Annual P/B for the species studied and from literature data on other species lies between 4 and 69. The use of a single P/B val...

Studies of the life-history and energetics of marine and brackish-water nematodes

SummaryAspects of the demography of Monhystera disjuncta were investigated at different temperatures (in agnotobiotic cultures) and in different feeding conditions (monoxenic cultures with different bacterial strains, and different densities in the feeding suspension with one strain). Embryonic development time, minimum generation time, egg deposition rate and adult longevity depend on temperature, quality and quantity of food offered. Body mass at maturity is an allometric function of food density. It is shown that a previously inferred selectivity in food uptake is an artifact of culture conditions. pH buffering and addition of sterols permit culture of the species on a wide variety of bacterial strains. M. disjuncta is less well adapted to take advantage of high food density than are nematodes from polysaprobic environments. The animals channel surplus energy intake into a larger body mass, without being able to increase their rate of population growth accordingly.

A re-evaluation of marine nematode productivity

Nematodes are the most abundant multicellular animals in marine sediments but their role in the benthos has not been properly quantified yet. In nearly all energy-flow budgets of marine systems their annual production P is given as about nine times their mean biomass B and their part in the total energy-flow is consequently estimated as anywhere between 3 and 30% of the total (carbon) input in the benthic system. Our laboratory experiments demonstrate that nematode productivity is much higher than P/B ∼ 9 per year and may reach values of over 60 for bacterial grazers. To obtain more reliable estimates for field populations we propose a regression equation relating egg-to-egg development time Tmin to temperature (t) and adult female weight (W in µg wet weight):log Tmin = 2.202−0.0461 t + 0.627 log W. When multiplied by the constant biomass turnover per generation (P/B)gen = 3, development rate 1/Tmin is a good predictor of daily P/B. This method was applied to two series of field data. A rather stable community from a sublittoral mud in the North Sea had an annual P/B = 20. A less stable Aufwuchs community from Sargassum in Japan had an annual P/B = 58.

Problems in meiofauna energy-flow studies

The direct estimation of energy flow through marine meiobenthic populations poses several difficulties, mainly relating to sampling problems. The usefulness of some indirect estimation methods is discussed.Direct production estimates and respiration measurements for three brackish water crustacean populations are given, indicating a relative constant proportion between population production and respiration. The production: assimilation ratio for these populations fluctuates between 0.3 and 0.4. This is contrasted to literature data revealing much higher production: assimilation ratios as determined in the laboratory for nematode populations. Using data on laboratory cultures of the nematode Monhystera disjuncta some factors that can possibly generate this discrepancy are discussed. An analysis of P:B in different life stages of this population justifies the use of a life-cycle turnover of about 3 for meiobenthic populations, provided some conditions are met. Among these is that no drastic change in productivity occurs between juveniles and adults, and that the biomass of hatchlings, not of freshly laid eggs, is considered as generative production.