Morphological analysis of 70 globally distributed Holocene sediment assemblages of the calcareous marine phytoplankton genus Gephyrocapsa (Prymnesiophyta) displays significant correlations between the measured morphological parameters and environmental gradients. By extrapolating these essential findings, paleoecological and evolutionary interpretations can be obtained for the Pleistocene period. Measurements of various morphometric parameters of 3600 coccoliths showed that the morphologically most distinct features are bridge angle and placolith length. Bridge angle ranged from 120 degrees to 3 degrees and placolith length from 6.2 mu m to 0.9 mu m The combination of the mean bridge angle and the mean length of each sediment assemblage (Holocene) leads to six morphological groups which characterise different environments and geographic areas: Equatorial associations have a mean bridge angle larger than 56 degrees and a mean size between 3.1 mu m and 3.9 mu m. The temperature range of these associations is 25 degrees C to 29.5 degrees C mean sea surface temperature (MSST). Abbreviation GE: Gephyrocapsa Equatorial. Subtropical central water-mass associations have a mean bridge angle between 27 degrees and 56 degrees and a mean length larger than 3.1 mu m. These associations are only found in oligotrophic central gyre regions with a MSST between 22 degrees C and 25 degrees C. Abbreviation GO: Gephyrocapsa Oligotrophic. Transitional associations have a mean bridge angle between 27 degrees and 56 degrees and a mean length between 2.4 mu m and 3.1 mu m These associations are only found in areas with a mean SST between 19 degrees C and 20 degrees C. Abbreviation GT: Gephyrocapsa Transitional. Transitional to subarctic cold associations have a mean bridge angle less than 27 degrees and a mean length larger than 2.4 mu m. These associations are found in moderately productive regions with a mean SST lower than 21 degrees C. Abbreviation GC: Gephyrocapsa Cold. Moderately productive temperate associations have a mean bridge angle larger than 56 degrees and a mean length larger than 3.9 mu m These associations are dominant in upwelling regions, but also occur in non- upwelling areas. Their temperature range is 18 degrees C to 23 degrees C. The relative abundance of these associations within the Gephyrocapsa complex in individual samples decreases with increasing distance from the coast. Abbreviation: GL Gephyrocapsa Larger. Associations consisting of small placoliths with a mean bridge angle between 20 degrees and 50 degrees and a mean length less than 2.4 mu m show no well defined environmental or geographic preference. Such forms seem to be frequent near islands and coasts (neritic) in tropical to subtropical regions. Abbreviation: GM: Gephyrocapsa Minute. The observed morphological discontinuities along the environmental gradients are suggestive of distinct and possibly genetically isolated species populations. In individual assemblages, however, considerable morphological variability is observed and characteristic morphological traits show gradual change along environmental gradients. These features could be expected, if a single, globally homogeneous genotypic species Gephyrocapsa oceanica should show high phenotypic plasticity. Future morphological analysis of plankton samples from various areas and taken at various seasons are expected to confirm the conclusion of environmental dependence of Gephyrocapsa morphology drawn from this analysis.
