Globorotalia tumida (Brady, 1877)
This species (Fig. 26) occurs in tropical and subtropical environments but has distinct preferences for warm sea surface temperatures, near-average sea surface salinities, and consequently for low water densities in summer. The relations with vertical gradients in temperature and density are relatively broad and do not suggest a large effect of these parameters on the distribution of G. tumida. The species is rare in the subtropical Indian Ocean. The reason for this biogeographic limitation may be seen in the lower sea surface temperature between 20-40 ° southern latitude in the Indian Ocean. The species occurs where summer salinities exceed about 34.5 psu and it has a distinct maximum between 35-36 psu which would be met in the southern subtropical salinity maximum in the Indian Ocean.
The subspecies G. tumida flexuosa is a rare component of Atlantic and Indian Ocean assemblages (<1.5 % relative abundance, Fig. 27). There is no correlation between relative abundances of G. tumida and G. tumida flexuosa, however, the
relative abundance of the subspecies is low. Large statistical uncertainty may mask any correlation. Globorotalia tumida flexuosa occurs only where G. tumida has relative abundances of less than 5%. The preferences for physical conditions
seem to be comparable but the clear biogeographic restriction of G. tumida is not expressed in the occurrences of G. tumida flexuosa. In the Atlantic and Indian Oceans G. tumida flexuosa may be a rare phenotype of G. tumida under suboptimum
Figure 26: Globorotalia tumida (Brady, 1877). Plots of relative abundance vs. selected physical parameters and latitude. For preferences in numeric form see appendix.
Figure 27: Correlation of relative abundances of Globorotalia tumida and G. tumida flexuosa; relations of G. tumida flexuosa with sea surface temperature in winter and summer, and its biogeographic distribution.