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

Marine jellyfish Mastiglas sp. are abundant in marine lakes in Belau, Western Caroline Islands.They contain symbiobc dinoflagellates (= zooxanthellae) in cells of the mesoglea.The population density of zooxanthellae was measured as a function of host size and found to be sizeindependent, suggesting that it is regulated To determine how population density might be regulated, specific growth rates of algae (W, ) and host (F,) were measured independently as a funct~on of host size.The rates were dissimilar with ~l , less than F, in small medusae and p ., greater than F, in large medusae.Maintenance of medusae under laboratory conditions resulted in transient increases in W~ in small medusae u p 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.The general conclusion drawn is that zooxanthellae population density is maintained in small medusae by facultative, transient increase in K and in large medusae by expulsion and/or digestion of zooxanthellae.Factors which regulate zooxanthellae growth rate are not yet known.