The cosmopolitan, symbiont-bearing, larger benthic foraminifer (LBF) Heterostegina sensu lato prefers oligotrophic environments in tropical and warm-temperate seas. Harboring diatoms enables this species to be found across a wide illumination gradient from intertidal pools, where H. depressa protects against strongest illumination by occupying cryptic habitat, down to the base of the euphotic zone. Sheltered cryptic habitat, such as in holes of boulders, allows this species to live in highly energetic zones down to the fairweather wave base. Dependence on light for photosynthesis of its endosymbionts is managed by increasing surface/volume ratios of the test correlated with decreasing light, resulting in test flattening. Hydrodynamics also influences reproductive strategies. In high energy environments, asexual reproduction by schizogony dominates, while sexual reproduction (gametogony) is the dominant mode under low energy conditions. Thus, there is a shift in proportions between schizonts with smaller proloculi and gamonts with larger proluculi along the hydrodynamic gradient. Because there is a negative correlation between proloculus size and the number of chambers undivided by septula (operculinid chambers), the latter character shows negative dependence along the hydrodynamic gradient. Both proloculus size and number of operculinid chambers have been used as metric characters not only in the evolution of Heterostegina lineages starting in the middle Eocene, but also in many other nummulitds (e.g., Nummulites, Spiroclypeus, Cycloclypeus), totally neglecting the environmental dependence. Additionally, proloculus size can differ between biogeographically different populations (e.g., Okinawa and Hawaii) taken under similar hydrodynamic conditions. Using growth-independent and growth-invariant characters to describe the internal test morphology can enhance interpretation of evolutionary tendencies as distinct from environmental and paleogeographic diversification.

Morphometry of the larger foraminifer Heterostegina explaining environmental dependence, evolution and paleogeographic diversification

Antonino Briguglio
2016

Abstract

The cosmopolitan, symbiont-bearing, larger benthic foraminifer (LBF) Heterostegina sensu lato prefers oligotrophic environments in tropical and warm-temperate seas. Harboring diatoms enables this species to be found across a wide illumination gradient from intertidal pools, where H. depressa protects against strongest illumination by occupying cryptic habitat, down to the base of the euphotic zone. Sheltered cryptic habitat, such as in holes of boulders, allows this species to live in highly energetic zones down to the fairweather wave base. Dependence on light for photosynthesis of its endosymbionts is managed by increasing surface/volume ratios of the test correlated with decreasing light, resulting in test flattening. Hydrodynamics also influences reproductive strategies. In high energy environments, asexual reproduction by schizogony dominates, while sexual reproduction (gametogony) is the dominant mode under low energy conditions. Thus, there is a shift in proportions between schizonts with smaller proloculi and gamonts with larger proluculi along the hydrodynamic gradient. Because there is a negative correlation between proloculus size and the number of chambers undivided by septula (operculinid chambers), the latter character shows negative dependence along the hydrodynamic gradient. Both proloculus size and number of operculinid chambers have been used as metric characters not only in the evolution of Heterostegina lineages starting in the middle Eocene, but also in many other nummulitds (e.g., Nummulites, Spiroclypeus, Cycloclypeus), totally neglecting the environmental dependence. Additionally, proloculus size can differ between biogeographically different populations (e.g., Okinawa and Hawaii) taken under similar hydrodynamic conditions. Using growth-independent and growth-invariant characters to describe the internal test morphology can enhance interpretation of evolutionary tendencies as distinct from environmental and paleogeographic diversification.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11567/893416
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