BIOMETRIC DIFFERENTIATION BETWEEN RECENT GLOBIGERINA BULLOIDES AND GLOBIGERINA FALCONENSIS IN THE SOUTHERN INDIAN OCEAN

Morphometric analysis has been carried out on the morphologically similar Recent planktic foraminiferal species Globigerina bulloides d’Orbigny and Globigerina falconensis Blow in southern Indian Ocean surface sediments from subtropical to northern subantarctic waters (30°S to 46°S). Previous non-biometric studies have suggested that these two forms grade into each other at such latitudes and that they may represent subspecies or phenotypic variants of a single species. Initial separation of the two forms is most readily based on the presence or absence of a distinct apertural lip in G. falconensis and G. bulloides respectively. A small percentage of specimens which have intermediate apertural characteristics and which do not allow clear taxonomic differentiation are allocated to either species by multivariate morphometric analysis. The studies have shown that the two forms exhibit distinct differences in patterns of variation in relative abundance, kummerform relative abundance, coiling direction and test size and shape. Essential morphologic differences are related to thickness (depth) of test and final chamber, and apertural dimensions with G. falconensis being more compressed and having smaller apertures than G. bulloides. Kummerform chambers in G. falconensis are more elongated than in G. bulloides, and this difference is amplified with cooler temperatures. Optimum environmental conditions for G. bulloides (relative abundances between 25 and 43%) coincide with the subantarctic water mass (yearly average surface-water temperatures between about 6 and 13°C) and for G. falconensis (relative abundances between 8 and 27%) with subtropical waters (temperatures above about 13°C). Kummerform relative abundances in G. falconensis samples (44 to 78%) are consistently higher than in G. bulloides samples (23 to 48%) but are not related to species relative abundances or surface­water temperature as in G. bulloides. Coiling in G. bulloides samples is normally dominantly sinistral and shows trends related to surface-water temperature. Globigerina falconensis is normally randomly coiled and coiling ratios lack relations with temperature. Globigerina bulloides is larger in size than G. falconensis at average annual surface-water temperatures less than about 18°C while G. falconensis is larger at warmer temperatures. Variations in test size of G. bulloides are related to changes in surface-water temperature whereas test size in G. falconensis varies randomly with temperature. Test size in normalform G. falconensis is positively correlated with relative abundances of G. falconensis. Likewise, size in both normalform and kummerform G. bulloides is positively correlated with its relative abundances. This is strong support for the hypothesis that optimum growth occurs in areas of optimum adaptation. Globigerina bulloides and G. falconensis do not show latitudinally correlated patterns with respect to: gross shape of test, relative size and shape of aperture and final chamber, rate of chamber expansion, or number of chambers in final whorl Size of aperture in kummerform G. falconensis increases with decreasing winter surface-water temperature. Relative apertural size in the same form increases with increasing latitude. Kummerform chambers in G. falconensis become gradually more elongated with decreasing temperature. Oxygen isotopic analysis suggests that normalform G. falconensis dwell at a shallower average depth in the water column than normalform G. bulloides. The biometric analyses thus show that G. bulloides and G. falconensis tend to react as different species in all essential foraminiferal parameters and therefore the taxonomic distinction between these forms should be retained.