The life-cycle of Rhabditis marina Bastian 1865 was studied at 25°C and 20‰ salinity. The following demographic parameters were computed from life-table data: the intrinsic rate of natural increase (rm) = 0.914 day; net reproductivity (Ro) = 400; minimum generation time (Tmin) = 4.5 days; cohort generation time (Tc) = 7.2 days; mean generation time (T) = 6.6 days; and the age of an adult female when a median egg is deposited (T) = 6.1 days. Several approximate equations used to estimate rm were compared with these life-table calculations. Some give erroneous estimations and should be used with extreme caution. For iteroparous organisms (including most free-living nematodes), Tmin only gives an indication of the development time and is therefore unsuitable for describing 'mean generation time'.
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 pg wet weight):log Tmin =2.202-0.0461 t + 0.627 log W. When multiplied by the constant biomass turnover per generation (P/B),,, =3, development rate l/'T,i, 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.
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 betweenjuveniles and adults, and that the biomass of hatchlings, not of freshly laid eggs, is considered as generative production.
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