Regulation of population density of symbiotic algae in a tropical marine jellyfish (Mastigias sp.)

Abstract

Marine jellyfish Mastigias sp., abundant in marine lakes in Belau, Western Caroline Islands, contain symbiotic dinoflagellates (= zooxanthellae) in cells of the mesoglea. The population density of zooxanthellae was measured as a function of host size and found to be size independent, suggesting that it is regulated. Maintenance of medusae under laboratory conditions resulted in transient increases in growth rate in small medusae up to 7 times greater than normal, suggesting that zooxanthellae in small medusae are capable of facultative increase in specific growth rate, thereby sustaining normal population density. Factors which regulate zooxanthellae growth rate are not yet known.