FORAMINIFERAL REPRODUCTIVE STRATEGIES AND DIMORPHISM: A TEST OF ECOLOGICAL ADAPTATIONS AND TAXONOMIC CONSTRAINTS

Over the span of the Phanerozoic, the Foraminifera diversified, expanding into nearly all marine habitats and selected freshwater ones as well. Benthic marine environments typically support contemporaneous assemblages with several to many species that fill somewhat different niches, that is species occupy different microhabitats and/or utilize different nutritional resources. The question then is whether coexisting species also differ with regard to reproductive patterns or strategies. To what extent does environment control the mode of reproduction? The foraminiferal life cycle is among the most varied within the Protista and may differ from one species to another with regard to gamete morphology, mode of fertilization, test dimorphism, and the pattern with which the sexual and asexual generations alternate or are further modified. Such life cycle variations hypothetically may reflect adaptations to different environmental conditions, taxonomic constraints, or both. This study compares the dominant life cycle phases (a) across taxa within single, modern, contemporaneous assemblages and (b) across assemblages from very different marine environments. The methods employed (DAPI, removal of the test, fluorescence microscopy) necessitate an emphasis on Foraminifera with calcareous tests and selected monothalamous agglutinated species. Results show that not all species living in the same habitat utilize the same reproductive strategies. Some taxa may be dominated by gamonts while others are dominated by agamonts. Furthermore, those taxa that are most abundant within a single contemporaneous assemblage may employ quite different reproductive strategies. Environment alone therefore does not appear to determine the mode of reproduction. Taxonomic affiliations likewise do not seem to determine reproductive strategy in that species belonging to the same families or superfamilies may utilize different strategies. Results from the few previous investigations on seasonal patterns of reproduction in selected shallow-water species have been extrapolated to paleoceanographic interpretations based on deeper-water fossil assemblages. Results of this study indicate that no clear reproductive pattern is applicable to all taxa and that species should be treated individually.