Niche overlap between a cold-water coral and an associated sponge for isotopically-enriched particulate food sources.

The cold-water coral Lophelia pertusa is an ecosystem engineer that builds reef structures on the seafloor. The interaction of the reef topography with hydrodynamics is known to enhance the supply of suspended food sources to the reef communities. However, the reef framework is also a substrate for other organisms that may compete for the very same suspended food sources. Here, we used the passive suspension feeder Lophelia pertusa and the active suspension feeding sponge Hymedesmia coriacea as model organisms to study niche overlap using isotopically-enriched algae and bacteria as suspended food sources. The coral and the sponge were fed with a combination of 13C-enriched bacteria/15N-enriched algae or 15N-enriched bacteria/13C-enriched algae, which was subsequently traced into bulk tissue, coral skeleton and dissolved inorganic carbon (i.e. respiration). Both the coral and the sponge assimilated and respired the suspended bacteria and algae, indicating niche overlap between these species. The assimilation rates of C and N into bulk tissue of specimens incubated separately were not significantly different from assimilation rates during incubations with co-occurring corals and sponges. Hence, no evidence for exploitative resource competition was found, but this is likely due to the saturating experimental food concentration that was used. We do not rule out that exploitative competition occurs in nature during periods of low food concentrations. Food assimilation and respiration rates of the sponge were almost an order of magnitude higher than those of the cold-water coral. We hypothesize that the active suspension feeding mode of the sponge explains the observed differences in resource uptake as opposed to the passive suspension feeding mode of the cold-water coral. These feeding mode differences may set constraints on suitable habitats for cold-water corals and sponges in their natural habitats.

Embryogenesis and larval biology of the cold-water coral Lophelia pertusa.

Cold-water coral reefs form spectacular and highly diverse ecosystems in the deep sea but little is known about reproduction, and virtually nothing about the larval biology in these corals. This study is based on data from two locations of the North East Atlantic and documents the first observations of embryogenesis and larval development in Lophelia pertusa, the most common framework-building cold-water scleractinian. Embryos developed in a more or less organized radial cleavage pattern from ∼ 160 µm large neutral or negatively buoyant eggs, to 120-270 µm long ciliated planulae. Embryogenesis was slow with cleavage occurring at intervals of 6-8 hours up to the 64-cell stage. Genetically characterized larvae were sexually derived, with maternal and paternal alleles present. Larvae were active swimmers (0.5 mm s(-1)) initially residing in the upper part of the water column, with bottom probing behavior starting 3-5 weeks after fertilization. Nematocysts had developed by day 30, coinciding with peak bottom-probing behavior, and possibly an indication that larvae are fully competent to settle at this time. Planulae survived for eight weeks under laboratory conditions, and preliminary results indicate that these planulae are planktotrophic. The late onset of competency and larval longevity suggests a high dispersal potential. Understanding larval biology and behavior is of paramount importance for biophysical modeling of larval dispersal, which forms the basis for predictions of connectivity among populations.

Rapid glass sponge expansion after climate-induced Antarctic ice shelf collapse.

Over 30% of the Antarctic continental shelf is permanently covered by floating ice shelves, providing aphotic conditions for a depauperate fauna sustained by laterally advected food. In much of the remaining Antarctic shallows (<300 m depth), seasonal sea-ice melting allows a patchy primary production supporting rich megabenthic communities dominated by glass sponges (Porifera, Hexactinellida). The catastrophic collapse of ice shelves due to rapid regional warming along the Antarctic Peninsula in recent decades has exposed over 23,000 km(2) of seafloor to local primary production. The response of the benthos to this unprecedented flux of food is, however, still unknown. In 2007, 12 years after disintegration of the Larsen A ice shelf, a first biological survey interpreted the presence of hexactinellids as remnants of a former under-ice fauna with deep-sea characteristics. Four years later, we revisited the original transect, finding 2- and 3-fold increases in glass sponge biomass and abundance, respectively, after only two favorable growth periods. Our findings, along with other long-term studies, suggest that Antarctic hexactinellids, locked in arrested growth for decades, may undergo boom-and-bust cycles, allowing them to quickly colonize new habitats. The cues triggering growth and reproduction in Antarctic glass sponges remain enigmatic.

The symbiosis between Lophelia pertusa and Eunice norvegica stimulates coral calcification and worm assimilation.

We investigated the interactions between the cold-water coral Lophelia pertusa and its associated polychaete Eunice norvegica by quantifying carbon (C) and nitrogen (N) budgets of tissue assimilation, food partitioning, calcification and respiration using (13)C and (15)N enriched algae and zooplankton as food sources. During incubations both species were kept either together or in separate chambers to study the net outcome of their interaction on the above mentioned processes. The stable isotope approach also allowed us to follow metabolically derived tracer C further into the coral skeleton and therefore estimate the effect of the interaction on coral calcification. Results showed that food assimilation by the coral was not significantly elevated in presence of E. norvegica but food assimilation by the polychaete was up to 2 to 4 times higher in the presence of the coral. The corals kept assimilation constant by increasing the consumption of smaller algae particles less favored by the polychaete while the assimilation of Artemia was unaffected by the interaction. Total respiration of tracer C did not differ among incubations, although E. norvegica enhanced coral calcification up to 4 times. These results together with the reported high abundance of E. norvegica in cold-water coral reefs, indicate that the interactions between L. pertusa and E. norvegica can be of high importance for ecosystem functioning.

Two brominated cyclic dipeptides released by the coldwater marine sponge Geodia barretti act in synergy as chemical defense.

The current work shows that two structurally similar cyclodipeptides, barettin (1) and 8,9-dihydrobarettin (2), produced by the coldwater marine sponge Geodia barretti Bowerbank act in synergy to deter larvae of surface settlers and may also be involved in defense against grazers. Previously, 1 and 2 were demonstrated to bind specifically to serotonergic 5-HT receptors. It may be suggested that chemical defense in G. barretti involves a synergistic action where one of the molecular targets is a 5-HT receptor. A mixture of 1 and 2 lowered the EC(50) of larval settlement as compared to the calculated theoretical additive effect of the two compounds. Moreover, an in situ sampling at 120 m depth using a remotely operated vehicle revealed that the sponge releases these two compounds to the ambient water. Thus, it is suggested that the synergistic action of 1 and 2 may benefit the sponge by reducing the expenditure of continuous production and release of its chemical defense substances. Furthermore, a synergistic action between structurally closely related compounds produced by the same bioenzymatic machinery ought to be the most energy effective for the organism and, thus, is more common than synergy between structurally indistinct compounds.