Many species of Radiolaria inhabit masses of ocean water, and occupy faunal niches or biographical zones comparable with other zooplankton. Radiolarian species are non-motile; they drift along water currents while those currents compartmentalize the ocean into finer ecological domains. Ocean currents carry radiolaria from one water mass to another, so that species tend to have variable reproductive success.
One of the major controlling factors of Radiolaria distribution is water temperature and salinity. Radiolaria appear to be most abundant in warm waters of the equatorial zone. As a result of this trend, total Radiolaria diversity declines as latitude increases. Besides water temperature and salinity, there is evidence that maximum concentrations of radiolarian below the surface are associated with maximum concentrations of chlorophyll. This leads biologists to believe that chlorophyll is somehow necessary for radiolarian survival. The niches of Radiolaria are rather complex and species may vary within any given niche based on a given season, depth, and/or the availability of food.
Radiolaria exhibit several different types of behavior that should be noted. They often share relationships with dinoflagellate symbionts. These dinoflagellate symbionts are enclosed in a thin envelope of cytoplasm produced by the host's rhizopodial system. Radiolaria provide ammonium and carbon dioxide for the dinoflagellate symbionts, and in return the dinoflagellates provide their radiolarian host with a jelly-like layer that serves as both for protection and capturing prey. Another symbiotic relationship for in radiolarians is with algal symbionts. When food is scarce, an algal symbiont can provide its host radiolarian with much needed nourishment. There is very little known about the Radiolaria-algal symbiont symbiotic relationship, but it is known that algal symbionts are found in the extracapsulum of many radiolarians, and can provide a host with enough nourishment to allow it to maintain nutritional autonomy. This simply means that the radiolarian can go several weeks with out food. Algal symbionts can also alleviate competition among individuals of a Radiolaria colony. Algal symbionts are generally found only within Radiolaria who dwell in areas that receive sufficient light.
Potential sources of food can also influence Radiolaria behavior. Although they may possess symbiotic relationships and can act as particle feeders (meaning they just hang out and wait for food), radiolarians have a darker side to them -- they can also act as predators. When feeding as predators, Radiolaria may capture diatoms, tintinnids, and other calcareous organisms by ingesting them into their central cavity. They trap their prey on the peripheral network of rhizopodia. The symbiotic relationship that radiolarians share with dinoflagellates enable them to use a jelly-like layer to trap their prey.
Observations show that polycystine Radiolaria have their greatest density and diversity in the eutrophic, nutrient rich, waters of the California Current, with decreasing densities in the Gulf Stream and Gulf of Mexico. Radiolaria populations are very low in the eutrophic shallow waters, which tend to lack symbiotic algae. According to this observation, the eutrophic environment can provide nutrients, enhance symbiont productivity, and lead to increasing productivity of the Radiolaria.
