Size variations in planktonic foraminifera: implications for quantitative paleoclimatic analysis.

Populations of planktonic foraminiferal species and phenotypes, distinguished on the basis of color and coiling direction, reach maximum average test sizes in their regions of optimum development. Therefore, tropical species are largest in tropical waters, while polar species are largest in polar waters. Species living in subtropical and subpolar waters decrease in test size with both increasing and decreasing temperature.