Abstract
In Heterostegina depressa, the flagship species of laboratory investigations of larger benthic foraminifera (LBF) since the 70’s, the timing of reproduction, longevity and natural chamber building rates are still understudied. A recently developed method, the natural laboratory (sensu Hohenegger), has been applied on H. depressa populations from Sesoko Jima, NW Okinawa, Japan. An averaged chamber building rate and longevity of H. depressa were calculated based on 17 monthly samplings at fixed stations. All samples were collected at 20 and 50 m water depths using SCUBA. Live populations were dried and investigated by microCT. The monthly frequency distributions of chamber numbers and test diameters have been decomposed in normally distributed components. For each month, mean and standard deviations of the components were used to calculate the maximum chamber number and maximum test diameter. Based on these values, the natural chamber building rate (CBR) or diameter increase rate (DIR) could be estimated using the Michaelis-Menten function. CBR and DIR were inverted to estimate the ‘birthdate’ of all investigated individuals. Based on frequencies of these ‘birthdates’, main reproduction events could be detected and compared to the reproduction timing of other subtropical and tropical LBF taxa. Furthermore, peaks in reproduction could be linked to monsoon wet seasons (=“rainy seasons”) and winter rains.
Highlights
The nummulitid foraminifer Heterostegina depressa belongs to the non-taxonomic group of larger benthic foraminifera (LBF), which are characterized by their symbiosis with phototrophic microalgae
The 422 megalospheric specimens of Heterostegina depressa from ∼20 m water depth, which should be referred as schizonts according to Eder, Hohenegger & Briguglio (2017), show almost no significant correlation between NoC and test diameter (TD) (Table 2)
The 377 megalospheric specimens sampled at ∼50 m water depth, which mostly consist of gamonts, show highly significant correlation between NoC and TD with relatively high R2 scores except in one sample (May 9, 2014)
Summary
The nummulitid foraminifer Heterostegina depressa belongs to the non-taxonomic (paraphyletic) group of larger benthic foraminifera (LBF), which are characterized by their symbiosis with phototrophic microalgae. They are restricted to the photic zone of warm-temperate to tropic carbonate environments (Hallock, Röttger & Wetmore, 1991). Environmental constraints, like hydrodynamic energy, light penetration and nutrient influx, influence test morphology (Hohenegger, 2004; Briguglio & Hohenegger, 2009; Hohenegger, 2011). The complex test structures of LBFs have attracted scientific interest for a long time. A number of papers about their cell biology and ontogeny were published in the last decades
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