RESPONSE OF GREEN HYDRA TO FEEDING AND STARVATION AT FOUR IRRADIANCES

¹ School of Biological Sciences, University of Nebraska, Lincoln, Nebraska 68588-0118

The relationship between the productivity of symbiotic algae and growth of the hydra, Hydra viridissima (Florida strain), was investigated in hydra maintained at four irradiances (5, 10, 15, and 30 µE·m^-2·s^-1) and either fed or starved. Although the productivity of fed hydra increased from 0.3 to 1.15 µg C·mg protein^-1·h^-1 with increase in culture irradiance, there was no significant effect of culture irradiance on population growth or on the protein biomass of individual hydra. The survival of starved hydra was similarly not affected by culture irradiance. Algal-animal biomass parameters changed in response to feeding and culture irradiance. Numbers of symbiotic algae in fed hydra declined with increased culture irradiance. The protein content of hydra starved for 28 days declined to 10% of initial levels. The relative proportion of algal to animal biomass increased in starved hydra as both algal densities and algal cell volumes were almost twice those of fed hydra. Whereas culture irradiance and feeding alter the ratio of algal to animal biomass, growth of this green hydra is only affected by feeding.