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
