Life History and Development of Oligocene Larger Benthic Foraminifera: A Test of the Environmental Control on Heterochrony

Abstract

Larger benthic foraminifera (LBF) comprise a heterogeneous group of protists that typically reach more than three cubic millimeters in test volume and have complex internal morphologies. For most extant species, large size and complex tests are related to algal symbiosis. Living forms are associated with coral reefs and related oligotrophic carbonate environments, where symbiotic relationships with algae are especially advantageous. Extinct larger foraminifera are almost invariably associated with similar (paleo)environments. It has been argued on theoretical grounds (and supported by some empirical data) that in stable (i.e., low energy) but stressful (e.g. , low light for photosymbionts) conditions, the semelparous LBF will delay reproduction, grow to larger sizes, and produce larger embryons (Juvenile tests) during asexual reproduction. To test these relations, test size and embryon size of the LBF Nummulites panamensis, Lepidocyclina mantelli, L. yurnagunensis, and L. undosa, were examined along a Lower Oligocene forereef-to-deep shelf transect across Georgia, Florida, and Alabama. The results of this study indicate that intraspecific LBF body size and embryon size are predicted to be greatest at the extremes of a species' range. At these limits, however, it is suggested that large size is attained for different reasons. At the deep end of the ecogradient, LBF delay reproduction and grow longer. Large juveniles (with much symbiont-rich protoplasm) are favored under these low light conditions. Such populations are under stress-selection. At the shallow end of the range, reproduction is also delayed (to increase either fecundity or juvenile size because of low juvenile survival in high energy conditions), but because of the optimality of the environment (i.e., low stress), growth rate is normal or even accelerated. These populations are under K-selection. Environmental variables exert a direct influence on the life history and development of larger foraminifera. By delaying reproduction (hypermorphosis) to increase fecundity or juvenile size at the extremes of a species' range, larger intraspecific body sizes result.