We investigated the symbiont-bearing benthic foraminifer Palaeonummulites venosus to determine the chamber building rate (CBR), test diameter increase rate (DIR), reproduction time and longevity using the 'natural laboratory' approach. This is based on the decomposition of monthly obtained frequency distributions of chamber number and test diameter into normally distributed components. Test measurements were taken using MicroCT. The shift of the mean and standard deviation of component parameters during the 15-month investigation period was used to calculate Michaelis-Menten functions applied to estimate the averaged CBR and DIR under natural conditions. The individual dates of birth were estimated using the inverse averaged CBR and the inverse DIR fitted by the individual chamber number or the individual test diameter at the sampling date. Distributions of frequencies and densities (i.e., frequency divided by sediment weight) based on both CBR and DIR revealed continuous reproduction throughout the year with two peaks, a stronger one in June determined as the onset of the summer generation (generation 1) and a weaker one in November determined as the onset of the winter generation (generation 2). This reproduction scheme explains the presence of small and large specimens in the same sample. Longevity, calculated as the maximum difference in days between the individual's birth date and the sampling date, is approximately 1.5 yr, an estimation obtained by using both CBR and DIR.
The use of micro-computed tomography (μCT) provides a unique
opportunity to look inside the shells of larger benthic foraminifera to
investigate their structure by measuring linear and volumetric parameters. For
this study, gamonts/schizonts and agamonts of the species Heterostegina
depressa d'Orbigny were examined by μCT; each single
chamber's volume was digitally measured. This approach enables cell
growth to be recognised in terms of chamber volume sequence, which progressively
increases until reproduction occurs. This sequence represents the ontogeny of
the foraminiferal cell and has been used here to investigate controlling factors
potentially affecting the process of chamber formation. This is manifested as
instantaneous or periodic deviations of the realised chamber volumes derived
from modelled growth functions. The results obtained on naturally grown
specimens show oscillations in chamber volumes which can be modelled by sums of
sinusoidal functions. A set of functions with similar periods in all
investigated specimens points to lunar and tidal cycles.To determine whether such cyclic signals are genuine and not the effects
of a theoretical model, the same analysis was conducted on specimens held in a
closed laboratory facility, as they should not be affected by natural
environmental effects. Surprisingly, similar cyclicities were observed in such
samples. However, a solely genetic origin of these cycles couldn't be
verified either. Therefore, detailed analysis on the phase equality of these
growth oscillations have been done. This approach is pivotal for proving that
the oscillatory patterns discovered in LBF are indeed genuine signals, and on
how chamber growth might be influenced by tidal currents or lunar months.
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