Experimental and observational studies of radiolarian physiological ecology: 5. Temperature and salinity tolerance of Dictyocoryne truncatum

Abstract

The longevity and skeletal growth of Dictyocoryne truncatum, collected from surface waters near Barbados, were assessed in laboratory culture in relation to variations in temperature and salinity. The range of longevities for groups of organisms collected at different times, but maintained at 28°C and 35.0‰ salinity, exhibited a wide fluctuation (1–2 to 5–37 days). The amount of growth also showed a wide variation among the groups since it was generally proportional to the longevity. The causes of the variation in longevity and growth are not known, but we hypothesize that a combination of genetic variability and physical and biological factors intrinsic to the culture conditions produced the variability. D. truncatum showed a narrow temperature range for skeletal growth with an optimum in our culture conditions at about 28°C. Temperatures over 32°C or under 21°C suppressed skeletal growth. D. truncatum has a much wider temperature tolerance for survival than for growth. Longevity at temperatures as low as 15°C was comparable to that at 28°C. This indicates that D. truncatum can endure periods of relatively low temperatures. This may have survival advantage by conferring resistance to lower temperatures at great depths in the water column or during intrusion of colder water masses into warmer surface water regimes, but we have presently no evidence of their reproductive capacity at these lower temperatures. These results are consistent with a theory that D. truncatum is largely a surface-dwelling species surviving optimally in warmer water that supports optimum skeletal growth and maturation. The tolerance of cooler water, however, may also reflect biological adaptations related to variations in habitat during the reproductive cycle. Mature organisms may sink and release reproductive swarmers at greater depths in a water column, judging from the observation that mature individuals in culture withdrew their axopodia prior to swarmer release and settled to the bottom of culture vessels. The low temperature tolerance may also permit survival of juvenile organisms until they ascend into more warm surface water strata. Cultured D. truncatum has a wide growth and survival tolerance for variations in salinity. Mean growth and longevities were comparable at salinities of 27‰ and 35.0‰. A broad tolerance for variations in salinity can enhance survival at locations where wide variations in salinity occur regularly as happens near Barbados where the surface water is diluted by river outflows. The low salinity tolerance is also consistent with the interpretation that D. truncatum dwells in near-surface water in low latitude, open ocean locations.