Contrasting metabolic strategies of two co-occurring deep-sea octocorals

Abstract

The feeding biology of deep-sea octocorals remains poorly understood, as attention is more often directed to reef building corals. The present study focused on two common deep-water octocoral species in the Azores Archipelago, Dentomuricea aff. meteor and Viminella flagellum, aiming at determining their ability to exploit different food sources. We adopted an experimental approach, with three different food sources, including live phytoplankton, live zooplankton and dissolved organic matter (DOM), that were artificially enriched with ¹³C and ¹⁵N (C and N tracers). The presence of tracers was subsequently followed in the coral tissue, C respiration and particulate organic C and N (POC and PON) release. In both species, feeding with zooplankton resulted in significantly higher incorporation of tracers in all measured variables, compared to the other food sources, highlighting the importance of zooplankton for major physiological processes. Our results revealed contrasting metabolic strategies between the two species, with D. aff. meteor acquiring higher amounts of prey and allocating higher percentage to respiration and release of POC and PON than V. flagellum. Such metabolic differences can shape species fitness and distributions and have further ecological implications on the ecosystem function of communities formed by different octocoral species.

Figure 1

(a) Mixed coral garden of the octocorals Viminella flagellum and Dentomuricea aff. meteor (Gavin Newman, Greenpeace); coral fragment of D. aff. meteor (b) and its polyps (c); coral fragment of V. flagellum (d) and its polyps (e). Scale bar 1 mm.

Figure 2

Tracer C (a) and N (b) incorporation (average ± SD) of the octocoral species Dentomuricea aff. meteor and Viminella flagellum upon provision of different food sources enriched with ¹³C and ¹⁵N. Axis breaks are used to highlight the large differences of tracer among treatments. PHYTO: phytoplankton; Chaetoceros calcitrans; DOM: dissolved organic matter; ZOO: zooplankton Branchionus plicatilis.

Figure 3

Oxygen consumption (average ± SD) of the octocoral species Dentomuricea aff. meteor and Viminella flagellum upon provision of different food sources. Coral fragments were fed with the respective food source for four days and oxygen was measured in closed-cell incubations that took place immediately after feeding on day four and lasted for approximately 12-14 h. FAST: no food provision; PHYTO: phytoplankton Chaetoceros calcitrans; ZOO: zooplankton Branchionus plicatilis; DOM: dissolved organic matter.

Figure 4

Tracer fluxes (average ± SD), including tracer C respiration (a), tracer C release (b) and tracer N release (c) of the octocoral species Dentomuricea aff. meteor and Viminella flagellum upon provision of different food sources. Numbers below bars represent the number of coral fragments for which positive estimates were obtained (max 7). Axis breaks are used to highlight large differences in scale among some treatments. PHYTO: phytoplankton Chaetoceros calcitrans; DOM: dissolved organic matter; ZOO: zooplankton Branchionus plicatilis.

Figure 5

Tracer utilized by fragments of two octocoral species Dentomuricea aff. meteor and Viminella flagellum, expressed as a percentage of the provided carbon of different food sources: PHYTO: phytoplankton Chaetoceros calcitrans; DOM: dissolved organic matter; ZOO: zooplankton Branchionus plicatilis.

Figure 6

Experimental design of the two feeding experiments with the octocoral species Dentomuricea aff. meteor and Viminella flagellum. Exp cycle: Experimental cycles; Aq: Aquaria; PHYTO: phytoplankton Chaetoceros calcitrans; ZOO: zooplankton Branchionus plicatilis; DOM: dissolved organic matter; FAST: no food provision. Rectangles represent experimental aquaria and black dots represent coral fragments.