Observations on the distribution and ecology of the mammals of the cerrado grasslands of central Brazil

Abstract

Polycystine radiolarians are considered to be herbivores and/or suspended organic feeders and a significant component of the pelagic ecosystem. Their life spans are probably less than a month and their densities may reach as high as 82 000 individuals per cubic meter, with the highest densities usually below the upper 25 m and above a few hundred meters depth. They exhibit greatest diversity in the equatorial waters with the diversity decreasing and the tendency towards dominancy increasing poleward. Daily fluctuations in polycystine distributions are currently poorly understood; however, seasonal fluctuations are at least in some cases easily correlated with seasonal fluctuations in the physical oceanography of the area, such as the seasonal invasions of water masses and currents. The polycystine radiolarians do exhibit vertical and horizontal zonations which in some species appear to be related to such physical factors as water mass zonations, convergence zones, circulation patterns, and pycnoclines, and such biological factors as their association with symbiotic zooxanthellae and probably predator-prey and more subtle associations. Enough is known about many of the living polycystine radiolarians so that they may be placed in their correct biogeographic zones. Knowledge of their present day distribution and ecology has resulted in their use as palaeoecologic indicators in determining palaeotemperatures and recent investigations using radiolarians have resulted in attempts to evaluate palaeodiversities, evolutions, mass mortalities, extinctions and selective extinctions in the geologic record as related to palaeo-oceanographic conditions.