Biomass, metabolic rates and feeding of some common reef zoantharians and octocorals

Abstract

The biomass of individual polyps in most common octocorals ranged from 40 to 90 µg C, and the biomass of polyps per 1 g wet weight of colony ranged from 15 to 30 mg C. In the case of zoantharians, the corresponding values were 0.6 mg C and 3.7 mg C g^-1 for Palythoa caesia and 9 mg C and 73 mg C g^-1 for Zoanthus sociatus. In both octocorals and zoantharians, the percentage ratios of the biomass of zooxanthellae to the biomass of polyps were similar to those in scleractinians: 1.5-4.0%. The rate of photosynthesis ranged from 0.1 to 3.2 mg C g^-1 dry weight of colony per day. The ratio of photosynthesis to respiration in most symbiotic octocorals was more than 1 (1.4-2.7); it was less than 1 only in Alcyonium molle and Lobophytum roxasi. Daily respiration in symbiotic octocorals and zoantharians represented 0.6-3.0% of the total biomass of their polyps; in asymbiotic species, it was around 2-5%. Heterotrophic predatory feeding, sedimentary feeding, and feeding on dissolved organic matter were quantified by radiocarbon labelling. The ability to prey on and use different kinds of food varied with taxonomic group. Also, zoantharians and octocorals were much poorer heterotrophic feeders than are scleractinians. By heterotrophic feeding at optimum food concentrations, most zoantharians and octocorals could satisfy 10-100% of their energy requirements (cf. 200-300% for scleractinian corals). The extent of success of zoantharians and octocorals in competition with scleractinians is discussed.