Analyse von Wachstumskurven von Heterostegina depressa (Foraminifera: Nummulitidae)

Chamber formation in individuals of the large foraminifer Heterostegina depressa was studied with the intention of explaining the different shapes of growth curves established for groups of individuals. The material was derived from clone cultures and from the natural habitat. Chamber formation was observed by daily control of many individuals. The growth increments of a group of individuals depend mainly upon the frequency of chamber formation of the individuals, they also depend upon the percentage of growing individuals present and the sizes of chambers. Within a prolifically growing group, each individual builds a new chamber every second or third day, and 81 to 86% exhibit chamber-building activities. Further features of quickly growing individuals are: formation of chambers in a regular sequence, and short periods of rest between long periods of growth. During low illumination intensity and in darkness, growth of H. depressa may cease for several months (up to 81/2 months). Cessation of growth is connected with a marked reduction in all other physiological processes. After month-long inhibition of growth, different individuals responded with different growth behaviour under the same environmental conditions. For example, smaller individuals needed longer periods of physiological regeneration before building new chambers. Prior darkexposure resulted in intermittent growth in many individuals. Periods of rest were followed by periods of unusually intensive growth. Lower temperature resulted in a reduced frequency of chamber formation. The size of chambers, however, was not influenced; accordingly the shape of the test did not vary. Immediately after multiple fission, rapid growth begins. Growth curves of H. depressa, therefore, do not show initial periods of slow growth which is characteristic for general exponential growth. H. depressa exhibited sigmoidal growth curves only after periods of growth inhibition; it displayed typical regenerative growth. During these periods, up to 4 chambers in 4 days, or 9 chambers in 12 days, were built by 11-month-old individuals.