Depth-related morphoclines of megalospheric tests of heterostegina depressa d'orbigny: Biostratigraphic and paleobiological implications

Morphometric characters of equatorial sections of larger benthic foraminifera have been widely applied to define species for both biostratigraphic and evolutionary studies. In order to test the hypothesis that some of the observed morphological differences may reflect environmental conditions rather than evolutionary changes, we applied morphometric analysis to equatorial sections of megalospheric Heterostegina depressa tests from the reef slope of Sesoko-Jima, NW-Okinawa. Only living specimens were analyzed, thereby eliminating any postmortem alteration of the distribution of H. depressa along the water depth gradient. The analyses clearly differentiated two morphogroups corresponding to two megalospheric generations: gamonts with significantly larger proloculi and schizonts with smaller proloculi. Due to their asexual reproduction strategy, schizonts dominate in high-energy shallower environments. After a transition zone between 35 to 55 m, where both generations are present, schizonts are replaced by gamonts deeper on the slope. Both generations retain the characters of their initial tests regardless of depth. Where both megalospheric generations co-occur, the change in proportion of generations with depth results in an environmental morphological trend that matches apparent fossil evolutionary trends. These results are important for understanding relationships among fossil Heterostegina species, where continuous changes in morphological characters of the initial test part are interpreted as evolutionary trends.