Experimental and observational studies of radiolarian physiological ecology: 4. Factors determining the distribution and survival of Didymocyrtis tetrathalamus tetrathalamus with implications for paleoecological interpretations

Abstract

The mean growth, longevity and vitality (based on axopodial morphology and cytoplasmic flow) of Didymocyrtis tetrathalamus tetrathalamus were assessed in laboratory culture in relation to variations in temperature, salinity and light. Maximum longevity and vitality occurred in a temperature range of 21°C to 27°C. However, some individuals survived for extended periods (up to 18 days) with normal axopodia and cytoplasmic flow at temperatures as low as 10°C. Though abundant in tropical and subtropical locations and often thought to be exclusively warmer-water species, these data suggest D. tetrathalamus may have a lower temperature tolerance than indicated by it geographical location. This is in contrast to Spongaster tetras (a species found in the same locale) that is decidedly disabled by temperatures below approximately 20°C. Moreover, abundance of D. tetrathalamus in the natural environment near Barbados increased substantially during periods of cooler surface water temperatures associated with changes in temperature stratification of the upper water column. These data suggests that although D. tetrathalamus is typically found in low to mid latitude tropical and subtropical locations, they may not be restricted to warmer water masses assuming other environmental and trophic factors are favorable. Food vacuole contents, based on transmission electron microscopic evidence, include large proportions of diatoms, other algae and small masses of cytoplasm that lack plastids and appear to be microheterotrophs. Microflagellates (Kinetoplastida) are observed within the peripheral cytoplasm and may be consumed as prey. Eubacteria are only occasionally observed in digestive vacuoles. Partially dissolved fragments of diatom frustules are incorporated into the skeleton of D. tetrathalamus and may be used as a source of skeletal material in low silicate environments. The composite data from this research suggests that D. tetrathalamus is highly adaptive and may be capable of surviving in a broader range of trophic and physical environmental conditions than previously assumed.