Intraspecific variation of Globigerinoides ruber and Globigerinoides trilobus-Globigerinoides sacculifer was investigated in 20 core top samples from the Atlantic Ocean and Gulf of Mexico. Such samples come from the top centimeter of cores and represent about 1,000 years accumulation. The average number of specimens measured for each sample was 50 and the size of the specimens was coarser than 250µ. Specimens are largest in the Gulf of Mexico and Caribbean Sea, smaller in the equatorial Atlantic, and smallest in the North Atlantic. Between 15 and 25°C, specimen size is correlated with mean sea surface temperature. Above 25°C large size variations occur within a narrow temperature range. Expansion rates, as measured by the relative increase in chamber diamet rs in both species, are correlative with available nutrients. The highest expansion rates occur in the Atlantic Undercurrent (0-5°N), and in the North Atlantic north of 30°N. Between 5 and 30°N in the Sargasso Sea, expansion rates are lower than in the equatorial or North Atlantic. Thus, for both species temperature and nutrient availability affect maximum size attained and rate of chamber growth.

Comparison of size and expansion rates for G. trilobus and G. sacculifer distinguished by the presence or absence of a saclike final chamber show the 2 phenotypes to be statistically similar. Within populations of G. ruber, the width/height ratio of the test, and of the primary aperture show a general trend of increasing values with increasing latitude. Variations in aperture size are linearly correlated with mean sea surface temperatures.

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