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.

Representatives of the foraminifer Nummulites are important in Earth history for timing Cenozoic shallow-water carbonates. Taphonomic complexity explains the construction of carbonate buildups, but reproduction and life span of the constructing individuals are unknown. During the 15-month investigation period, asexually reproduced schizonts and gamonts showed equal proportions in the first half of this period, whereas gamonts predominated in the second half. Oscillations in cell growth are mainly caused by light intensities during chamber construction when minor differences in water depth increase the photosynthetic rate of endosymbiotic diatoms during neap tides. The continuous reproduction rate of N . venosus throughout the year is increased in subtropical calms by higher summer temperatures and the marginal input of inorganic nutrients during rainy seasons. The expected life span of both gamonts and schizonts are 18 months.

In foraminifera, so-called "double tests" usually arise due to abnormal growth originating mainly from twinning, but may also be caused by irregularities in the early chambers and by regeneration after test injury that modifies the direction of growth. A fourth cause of double tests has only rarely been reported: the fusion of the tests of two adult individuals. We studied an early Eocene Nummulites double test consisting of two adult individuals that fused after an extended period of independent growth. The specimen was studied using computed tomography with micrometric resolution (micro-CT) that allowed bi-and three-dimensional visualization of the internal structure. Before fusion each individual test had 30-36 chambers, which, by comparison with growth rates in recent nummulitids, implies at least three months of independent growth. After fusion, the compound test grew in two spirals that fused after about one whorl and then continued in a single spiral.To fuse their tests, either adult individuals have to be forced to do so or the allorecognition (ability to distinguish between self and another individual) mechanisms must fail. A possible explanation for the merged Nummulites tests in this study is forced fusion in attached individuals after surviving ingestion and digestion by a metazoan. Alternatively, environmental stress could lead to a failure of allorecognition mechanisms and/or foraminiferal motility. Once fused, subsequent growth seems to be determined mainly by the relative orientation of individual tests. In any case, the frequency in which adult fusion occurs remains unknown.

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Cultures of larger benthic symbiont-bearing foraminifers have been used for studying growth and reproduction by long term investigations taking several months. For getting results that can be interpreted as 'natural growth', the culture medium must represent the same environmental conditions like the sea at sampling time. Growth of Nummulites venosus specimens was tested where about 40 chambers have been built in the natural environment at the time of sampling. After exposition to cultures with the same environmental conditions as those at the sampling stations, growth in the culture was compared with 'natural growth' of the initial part. This was done calculating deviations of empirical data obtained in the culture from the Gompertz function established for the cell part constructed in the sea. Three reaction types could be detected: 1) strong (negative) deviations just after exposure to the culture; 2) continuing natural growth in the culture, starting deviation in later chambers; 3) throughout continuing natural growth. The chamber building rate, calculated by the Michaelis-Menten function as the second parameter determining growth, differs significantly between growth in the nature and culture, where the velocity of chamber building rates decrease immediately after exposure into the cultures. It must be concluded, that the culture medium, although approximating natural conditions rather perfectly, always impact growth, especially in the chamber building rate. Therefore, studies checking influences of environmental factors using cultures must be carefully interpreted.