Heterotrophic feeding, growth and nutrient budget in the scleractinian coral Galaxea fascicularis

Summary

Coral reefs are of tremendous ecological and economic importance, and are currently in global decline due to climate change and anthropogenic disturbances. Overfishing is a threat to reefs in Asia, where corals are collected for the aquarium trade. This trade appears unsustainable, as wild collection of reef organisms has led to elimination of local populations and significant changes in age structure. Therefore, a major incentive exists to grow corals sustainably, so that local degradation of coral reefs can be reduced. To optimise coral aquaculture, detailed knowledge of factors controlling growth is required. Zooplankton feeding is considered important to coral growth, as it supplies the coral with nutrients such as fatty acids and amino acids. However, a realistic quantification of the carbon, nitrogen and phosphorous acquisition from heterotrophic feeding is currently lacking, obscuring nutrient budgets for corals. In addition, contrasting short- and long-term effects of heterotrophy on coral growth have been found. To further complicate matters, (a)biotic factors, including water flow rate, coral size, the presence of episymbiontsand prey density affect zooplankton feeding, and thus nutrient input and growth, and knowledge of these factors is still limited. The relevance of addressing the knowledge gaps above is twofold. First, it contributes to our fundamental understanding of the role of heterotrophy in the coral nutrient budget and growth, and how (a)biotic factors affect this role. Second, these findings allow coral aquaculture protocols to be improved, which benefits the sustainable trade in these endangered species. The research questions for this thesis were: what is the potential role of heterotrophic feeding in the nutrient acquisition and budget for the scleractinian coral Galaxea fascicularis(Chapters 2 and 6)?;what mechanism explains the inhibitory short-term effect of zooplankton feeding on skeletal growth of G. fascicularis(Chapter 3)?; how does water flow rate affect zooplankton feeding by solitary polyps and colonies of G. fascicularis(Chapter 4)?; and finally, what is the effect of epizoic acoelomorph flatworms on zooplankton feeding by G. fascicularis, and is this effect dependent on prey availability (Chapter 5)?

In Chapter 2, the acquisition of organic compounds through heterotrophic feeding by the scleractinian coral G. fascicularisis presented. First, the feeding behaviour of single polyps was investigated using video analysis. A highly dynamic feeding process was observed. A single G. fascicularispolyp captured 558±67 Artemianauplii, and released 383±75 nauplii (N=3) over a 6 hour interval. On average, 98.6% of prey captured was not ingested. Instead, prey items were clustered in aggregates that were digested externally by mesenterial filaments. In addition, carbon, nitrogen and phosphorus analysis of zooplankton was conducted before and after digestion by G. fascicularis colonies (N=6). For total organic carbon (TOC), 43.1% (0.298±0.148 μg Artemia^-1) was lost after 6 hours of digestion. For total organic nitrogen (TON), total organic phosphorus (TOP) and orthophosphate (PO₄^3-), these values were 51.3% (0.059±0.028 μg Artemia^-1), 50.9% (0.009±0.004 μg Artemia^-1) and 84.6% (0.0019±0.0008 μg Artemia^-1), respectively. For extracoelenteric zooplankton feeding alone, total estimated nutrient inputs for G. fascicularis colonies were 76.5±0.0 μg organic carbon, 15.2±0.0 μg organic nitrogen, 2.3±0.2 μg organic phosphorus and 0.5±0.8 μg inorganic phosphorus per cm² coral tissue per day. These values exceed calculations based on intracoelenteric feeding by up to two orders of magnitude, and demonstrate that extracoelenteric zooplankton feeding is a key mechanism of nutrient acquisition for a scleractinian coral.

In Chapter 3, the short-term effects of zooplankton feeding on light and dark calcification rates of G. fascicularis colonies (N=4)at various oxygen saturation levels are discussed. Significant main and interactive effects of oxygen, heterotrophy and light on calcification rates were found. Light and dark calcification rates of unfed corals were severely affected by hypoxia and hyperoxia, with optimal rates at 110% saturation. Light calcification rates of fed corals exhibited a similar trend, with highest rates at 150% saturation. In contrast, dark calcification rates of fed corals were close to zero under all oxygen saturations. It is concluded that oxygen exerts a strong control over light and dark calcification rates of corals. Nevertheless, the inhibitory effect of heterotrophy on dark calcification appears to be oxygen-independent. A new hypothesis is that dark calcification is impaired during zooplankton feeding by a temporal decrease of the pH and aragonite saturation state of the calcifying medium, caused by increased respiration rates. This may invoke a transient reallocation of metabolic energy to soft tissue growth and organic matrix synthesis.

InChapter 4, the effects of water flow rate and polyp context (the presence of neighbouring polyps) on zooplankton feeding by G. fascicularisare described. Single polyps (N=4) and colonies (N=4) were incubated in a flow cell for 30 minutes with an ambient Artemianauplii concentration of 10,000 L^-1and water flow rates ranging from 1.25 to 40 cm s^-1.Water flow rate and polyp context showed significant main and interactive effects on feeding rates of G. fascicularispolyps. More specifically, feeding rates were optimal at flow rates of 1.25 and 5 to 10 cm s^-1for single polyps and those inhabiting colonies, respectively. These results demonstrate that flow affects coral feeding and thus heterotrophic nutrient input at both a polyp and colony level.

In Chapter 5, the effect of epizoic flatworms on zooplankton feeding by G. fascicularisis reported. The feeding behaviour of single polyps(N=9) was studied using video analysis, in the presence and absence of symbiotic flatworms. 18S DNA analysis revealed that flatworms inhabiting G. fascicularisbelonged to the genus Waminoa(Convolutidae), which were hosted at a density of 3.6±0.4 individuals polyp^-1.Polyps hosting flatworms exhibited prey capture rates of 2.2±2.5, 3.4±4.5 and 2.7±3.4nauplii polyp^-130 min^-1at prey concentrations of 250, 500 and 1,000 nauplii L^-1, respectively. Polyps that had their flatworms removed displayed prey capture rates of 2.7±1.6, 4.8±4.1 and 16.9±10.3nauplii polyp^-130 min^-1. Significant main and interactive effects of flatworm presence and ambient prey concentration were found, reflected by the fact that flatworms significantly impaired host feeding rates at the highest prey density of 1,000 naupliiL^-1. In addition, flatworms displayed kleptoparasitism, removing between 0.1±0.3 and 0.6±1.1 nauplii 30 min^-1from the oral disc of their host, or 5.3±3.3 to 50.0±2.1% of prey acquired by the coral. It is suggested to classify the coral-associated Waminoasp. as an epizoic parasite, as its presence may negatively affect growth and health of the host.

InChapter 6,the role of heterotrophic feeding in the nutrient budget and growth of G. fascicularisis discussed, including how (a)biotic factors affect this role. It is clear that the importance of heterotrophy in the coral nutrient budget has been underestimated, and that its relative contribution to the budget depends on water flow rate, coral size, flatworm presence and prey density. In addition, the short-term effects of heterotrophic feeding on coral growth are variable, and depend on ambient light and oxygen conditions. These insights are of relevance to coral ecology and optimisation of sustainable coral aquaculture. Future work will have to address important knowledge gaps, including the mechanism underlying impairment of dark calcification during feeding, the effect of epizoic flatworms on coral growth (both in in situand in aquaculture), and the interactive effects of oxygen and seawater pH on coral growth.