Globorotalia truncatulinoides (d'Orbigny, 1839)
Globorotalia truncatulinoides (Fig. 24) is a typically subtropical species which occurs over a broad range of sea surface temperatures and salinities. It prefers areas with little seasonality in salinity in contrast to broad tolerances for seasonal change in sea surface temperatures. Most noteworthy is a distinctly lower tolerance for the vertical temperature gradient and stratification in winter, compared to summer conditions. The preference for low vertical gradients in winter argues for a relationship with the known annual ontogenetic cycle. G. truncatulinoides is a deep-dwelling species which ascends to shallower depths during its reproduction period in winter. The migration through the water column may be less energy consumptive when low vertical gradients exist in the water column (see G. hirsuta and discussion section).
There are no substantially different preferences of dextral and sinistral coiling forms of G. truncatulinoides for the physical conditions analysed (Fig. 25). In warm subtropical and tropical environments dextral coiling specimens occur as
rare components of assemblages under a broader range of physical conditions. In the Indian Ocean dextral coiling forms reveal a distinct maximum in relative abundance offset towards lower latitudes compared to the abundance maximum of
sinistral coiling G. truncatulinoides. The change in relative abundances of dextral and sinistral forms appears to be gradual along environmental gradients (clinal variation).
Figure 24: Globorotalia truncatulinoides (d'Orbigny, 1839). Plots of relative abundance vs. selected physical parameters and latitude. For preferences in numeric form see appendix.
Figure 25: Preferences for vertical temperature gradients and biogeographic distribution of Globorotalia truncatulinoides differentiated for sinistral and dextral coiling variants. For discussion see text.