Abstract
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.
Highlights
Larger benthic foraminifera (LBF) are an informal group of benthic, symbiont-bearing, marine shallow-water foraminifera that commonly possess a volume larger than 3 mm3 (Ross, 1974)
The species selected for these analyses is H. depressa d'Orbigny: it constructs chambers divided into chamberlets, which are arranged in a coil that can be approximated by a modified logarithmic spiral (Hohenegger, 2011a; Fig. 3)
Computed micro-tomography and 3D reconstruction successfully quantifies the ontogeny of foraminiferal cells volumetrically, enabling the volumes of the whole chamber sequence to be accessed
Summary
Larger benthic foraminifera (LBF) are an informal group of benthic, symbiont-bearing, marine shallow-water foraminifera that commonly possess a volume larger than 3 mm (Ross, 1974). They host phototrophic algal symbionts within their shells, functioning as greenhouses (Lee and Hallock, 1987; Lee, 2006; Hohenegger, 2011b). Several studies on the functional morphology and external ornamentation of the shells yielded important information on their ecological niches and distribution (Renema and Troelstra, 2001) in terms of water depth (Hottinger, 2006b), trophic resources (Hallock, 1988) and light intensity (Hohenegger, 2009). In tropical and subtropical regions, the water depth characterising this temperature limit is much deeper than the depth limit based on light
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