Heterotrophic nutrition and control of bacterial density in the coelenteron of the giant sea anemone Stoichactis giganteum

Abstract

Contents of gastral cavities of giant seaanemones Stoichactjs giganteum Forskal, sucked out in situby means of a syrlnge, revealed that echinoids (Tripneustessp.), brittle stars (Ophiothrix sp.) and large benthic decapodsconstitute the most important heterotrophic food source of S.giganteurn. Additionally, a bacterial population within thegastral cavity was observed. Mean bacterial density was2.49 X 106 cells ml-l. Rod-shaped bacteria contributed about90% to the number of bacteria in the gastral cavity; theirdensity was 14 times higher than that of rods in ambientwaters. Laboratory experiments with starved and fedanemones revealed a rhythmic variation pattern in the bacterialdensity. During starvation bacterial density dropped to0.98 X 105 cells ml-l. A net increase in density was observedafter feeding within 4 to 5 h, followed by a decrease in thenext 2 to 3 h for both rods and cocci, Incubation experimentswith coelenteric fluid revealed its regulat~vefu nction in controllingbacterial density in the coelenteron. During incubationof the coelenteric fluid of starved anemones, a build up ofbacterial density was observed to 35.3 X 105 cells ml-' w~thin4 h, while the coelenteric fluid of fed anemones demonstrateda bacterial digestion ability which increased with increasingbacterial density of the coelenteron. Evidence is provided forthe fact that these bacteria contribute only a minor portion ofthe total energy requirements of the anemone.