EVOLUTION AND ECOLOGY OF COCCOLITHOPHORES: IF LUMPING IS OUT, HOW DO WE SPLIT?

Understanding of plankton biodiversity today and in the past is based on morphologically defined species. Biodiversity can be characterized by species richness and species evenness, both of which were highly variable in space and through time. Potential controls for this variability may be of abiotic (e.g. temperature, nutrients, climate change) or biotic origin (e.g. predation, grazing, competition, infection etc.). Ecological studies at JGOFS time-series stations point to strong abiotic forcing. Up to 67% of the seasonal cell density variability of certain coccolithophore taxa can be explained by forcing factors such as temperature, light, and nutrients. Current taxonomic concepts seem to be adequate for such ecological characterization of certain species in single areas.

However, recent detailed morphometric analyses of a few taxa also suggest that the diversity of the coccolithophore community may be considerably underestimated because fine scale morphological variations in some species appear to be also reflected in their ecological preferences. Such variants have been successfully used in the development of transfer-functions, the documentation and interpretation of biogeographic abundance distributions, and the morphological evolution of some species through time.

The few genetic studies of single extant species and their morphotypes carried out so far indicate that morphotypes and subspecies are genetically distinct. To what extent the overal species diversity of coccolithophores may have been underestimated, however, remains to be determined.

A potential consequence of an inadequate taxonomic resolution might be the lack of obvious and consistent correlations between past changes in diversity and global climate and the anomalously long species longevities computed from stratigraphic analyses. Therefore, refinement of the species concept using combined morphometric and genetic analyses is needed. Efficient methods for the reevaluation of the ecological and evolutionary evidence, however, seem still in their nascent stage and will need further development before they can be applied widely and efficiently.