Chemical versus structural defense against fish predation in two dominant soft coral species (Xeniidae) in the Red Sea

Soft corals of the family Xeniidae are particularly abundant in Red Sea coral reefs. Their success may be partly due to a strong defense mechanism against fish predation. To test this, we conducted field and aquarium experiments in which we assessed the antifeeding effect of secondary metabolites of 2 common xeniid species, Ovabunda crenata and Heteroxenia ghardaqensis. In the field experiment, the metabolites of both investigated species reduced feeding on experimental food pellets in the natural population of Red Sea reef fishes by 86 and 92% for O. crenata and H. ghardaqensis, respectively. In the aquarium experiment, natural concentration of crude extract reduced feeding on experimental food pellets in the common reef fish Thalassoma lunare (moon wrasse) by 83 and 85%, respectively. Moon wrasse feeding was even reduced at extract concentrations as low as 12.5% of the natural concentration in living soft coral tissues. To assess the potential of a structural anti-feeding defence, sclerites of O. crenata were extracted and mixed into food pellets at natural, doubled and reduced concentration without and in combination with crude extract at 25% of natural concentration, and tested in an aquarium experiment. The sclerites did not show any effect on the feeding behavior of the moon wrasse indicating that sclerites provide structural support rather than antifeeding defense. H. ghardaqensis lacks sclerites. We conclude that the conspicuous abundance of xeniid soft coral species in the Red Sea is likely a consequence of a strong chemical defence, rather than physical defences, against potential predators.

Soft-bottom macrobenthic faunal associations in the southern Chilean glacial fjord complex

Macrobenthic associations were investigated at 29 sampling stations with a semi-quantitative Agassiz trawl, ranging from the South Patagonian Icefield to the Straits of Magellan in the South Chilean fjord system. A total of 1,895 individuals belonging to 131 species were collected. 19 species belong to colonial organisms, mainly Bryozoa (17 species) and Octocorallia (2 species). The phylum Echinodermata was the most diverse in species number (47 species), with asteroids (25 species) and ophiuroids (13 species) being the best represented within this taxon. Polychaeta was the second dominant group in terms of species richness (46 species). Multidimensional scaling ordination (MDS) separated two station groups, one related to fjords and channels off the South Patagonian Icefield and the second one to stations surrounding the Straits of Magellan. 45 species account for 90% of the dissimilarity between these two groups. These differences can mainly be explained by the influence of local environmental conditions determined by processes closely related to the pres- ence/absence of glaciers. Abiotic parameters such as water depth, type of sediment and chemical features of the superficial sediment were not correlated with the numbers of individuals caught by the Agassiz trawl in each group of sampling stations.

Spatial community shift from hard to soft corals in acidified water

Anthropogenic increases in the partial pressure of CO2 (pCO2) cause ocean acidification, declining calcium carbonate saturation states, reduced coral reef calcification and changes in the compositions of marine communities. Most projected community changes due to ocean acidification describe transitions from hard coral to non-calcifying macroalgal communities; other organisms have received less attention, despite the biotic diversity of coral reef communities. We show that the spatial distributions of both hard and soft coral communities in volcanically acidified, semi-enclosed waters off Iwotorishima Island, Japan, are related to pCO2 levels. Hard corals are restricted to non-acidified low- pCO2 (225 µatm) zones, dense populations of the soft coral Sarcophyton elegans dominate medium- pCO2 (831 µatm) zones, and both hard and soft corals are absent from the highest- pCO2 (1,465 µatm) zone. In CO2-enriched culture experiments, high- pCO2 conditions benefited Sarcophyton elegans by enhancing photosynthesis rates and did not affect light calcification, but dark decalcification (negative net calcification) increased with increasing pCO2. These results suggest that reef communities may shift from reef-building hard corals to non-reef-building soft corals under pCO2 levels (550-970 µatm) predicted by the end of this century, and that higher pCO2 levels would challenge the survival of some reef organisms.

Influence of sediment acidification and water flow on sediment acceptance and dispersal of juvenile soft-shell clams (Mya arenaria L.)

Although ocean acidification is expected to reduce carbonate saturation and yield negative impacts on open-ocean calcifying organisms in the near future, acidification in coastal ecosystems may already be affecting these organisms. Few studies have addressed the effects of sedimentary saturation state on benthic invertebrates. Here, we investigate whether sedimentary aragonite saturation (Omega aragonite) and proton concentration ([H+]) affect burrowing and dispersal rates of juvenile soft-shell clams (Mya arenaria) in a laboratory flume experiment. Two size classes of juvenile clams (0.5-1.5 mm and 1.51-2.5 mm) were subjected to a range of sediment Omega aragonite and [H+] conditions within the range of typical estuarine sediments (Omega aragonite 0.21-1.87; pH 6.8-7.8; [H+] 1.58 × 10**-8-1.51 × 10**- 7) by the addition of varying amounts of CO2, while overlying water pH was kept constant ~ 7.8 (Omega aragonite ~ 1.97). There was a significant positive relationship between the percent of juvenile clams burrowed in still water and Omega aragonite and a significant negative relationship between burrowing and [H+]. Clams were subsequently exposed to one of two different flow conditions (flume; 11 cm/s and 23 cm/s) and there was a significant negative relationship between Omega aragonite and dispersal, regardless of clam size class and flow speed. No apparent relationship was evident between dispersal and [H+]. The results of this study suggest that sediment acidification may play an important role in soft-shell clam recruitment and dispersal. When assessing the impacts of open-ocean and coastal acidification on infaunal organisms, future studies should address the effects of sediment acidification to adequately understand how calcifying organisms may be affected by shifting pH conditions.

Relative sensitivity of soft-bottom intertidal macrofauna to increased CO2 and experimental stress

We used a controlled CO2 perturbation experiment to test hypotheses about changes in diversity, composition and structure of soft-bottom intertidal macrobenthic assemblages, under realistic and locally relevant scenarios of seawater acidification. Patches of undisturbed sediment were collected from 2 types of intertidal sedimentary habitat in the Ria Formosa coastal lagoon (South Portugal) and exposed to 2 levels of seawater acidification (pH reduced by 0.3 and 0.6 units) and 1 unmanipulated (control) level. After 75 d the assemblages differed significantly between the 2 types of sediment and between field controls and the ex situ treatments, but not among the 3 pH levels tested. The naturally high values of total alkalinity buffered seawater from the changes imposed on carbonate chemistry and may have contributed to offsetting acidification at the local scale. Observed differences on biota were strongly related to the organic matter content and grain-size of the sediments, particularly to the fractions of medium and coarse sand. Soft-bottom intertidal macrofauna was significantly affected by the stress of being held in an artificial environment, but not by CO2-induced seawater acidification. Given the previously observed variations in the sensitivities of marine organisms to seawater acidification, direct extrapolations of the present findings to different regions or other types of assemblages do not seem advisable. However, the contribution of ex situ studies to the assessment of ecosystem-level responses to environmental disturbances could generally be improved by incorporating adequate field controls in the experimental design.