Influence of the thermocline on the vertical migration of medusae during a 48 h sampling period

The vertical distribution and migratory movements of epiplanktonlc medusae were studied at an oceanic station off northern Namibia (18°00’S /10°30’E). Samples were collected from five separate depth strata (200–100 m, 100–60 m, 60–40 m, 40–20 m, and 20–0 m) using a multiple opening and closing RMT 1 x 6 net over a 48–h period. The area was characterized by mixing of the Angola Current surface waters with the northernmost subsurface waters of the Benguela Current. During the sampling period there was a continuous flow of water from Angola, giving rise to a strong thermocline between 20 and 40 m depth. A total of 17 species of medusae were captured, of which Aglaura hemistoma, Liriope tetraphylla, and Solmundella bitentaculata were the most abundant. Both the number of species and the number of individuals were higher on the second than on the first day of sampling, chiefly in the 20–0 stratum. Differences in abundance appeared to be related to the patch size of each species. The depth diistributlon of the medusae population was characterized by the existence of two assemblages, one associated with each current, and by the presence of the thermocline, which acted as a boundary between the two water masses and did not facilitate migratory movements of the most abundant species. The non-migratory distribution pattern of most of the species was attributable to the high concentration of potential prey items and to the absence of predators in the layer above the thermocline throughout the sampling period

Reproduction in the externally brooding sea anemone Epiactis georgiana in the Antarctic Peninsula and the Weddell Sea

14 pages, 8 figures, 2 tablesExternal parental care is uncommon among actiniarians but common in Epiactis species. Here, several aspects of reproduction are analyzed for of one of them, Epiactis georgiana. Samples were collected in December, January, February, March, and April in the Antarctic Peninsula and the eastern Weddell Sea, during 1998, 2000, 2002, and 2003. Most sexually mature individuals of E. georgiana are male or female, but some are hermaphrodites. This is the first report of hermaphroditism in E. georgiana, which is the third species of the genus with this sexual pattern. The results suggest that oogenesis starts in December and that at least two generations of oocytes overlap; a third generation is often brooded externally. Putative fertilization is likely internal, and larvae and/or embryos are externally brooded on the distal part of the adult column until an advanced developmental stage. Apparently E. georgiana reproduces seasonally, probably releasing the embryos/larvae in the last months of the austral spring (December). Inter-individual variability was observed in gametogenesis. In addition, specimens from the Antarctic Peninsula were larger than those from the Weddell Sea. This study represents the first step in understanding the reproductive mode of E. georgianaSpecial thanks are addressed to Prof. Dr. Wolf Arntz (Alfred-Wegener-Institute, Bremerhaven, Germany) who made possible our participation in several Antarctic projects and cruises. We extend our acknowledgements to the officers and crew of the R/V Polarstern and many colleagues on board during the EASIZ, ANDEEP, and BENDEX cruises for their valuable assistance. Thanks to M. Conradi (Universidad de Sevilla) who collected a considerable amount of the material analyzed in this manuscript. Comments from M. Daly, D. Fautin, and an anonymous reviewer substantially improved this manuscript. Support was provided by a MCT-CSICgrant (I3P-BPD2001-1) to E. Rodríguez and Spanish CICYT projects: ANT97-1533-E, ANT98-1739-E, ANT99-1608-E, REN2001-4269-E/ANT, REN2003-04236, and CGL2004-20141-E. This is a contribution to the SCAR program, Ecology of the Antarctic Sea Ice Zone (EASIZ) and ANDEEP contribution 159Peer reviewe

Living in close quarters: Epibionts on Dendrophyllia ramea deep-water corals (Cyprus and Menorca Channel)

In sharp contrast to shallow and/or tropical coral habitats, the role of deep-water corals (DWC) as habitat providers is not well known and even less understood. For this purpose, epibionts on the deep-water coral Dendrophyllia ramea were studied from samples collected in Cyprus and compared to those from Menorca Channel. A total of 63 species were found; bryozoans (ca. 60%) and serpulid polychaetes (ca. 10%) dominated the assemblage of species. Cyprus (48 species in total) and Menorca (22) corals shared few epizoic species (7). Several of these species were previously thought absent from the Levantine basin. These results are important contributions to the knowledge on the deep-water epibiotic biodiversity of the Levantine Basin and the Mediterranean Sea in genera

Physiological performance of the cold-water coral Dendrophyllia cornigera reveals its preference for temperate environments

Cold-water corals (CWCs) are key ecosystem engineers in deep-sea benthic communities around the world. Their distribution patterns are related to several abiotic and biotic factors, of which seawater temperature is arguably one of the most important due to its role in coral physiological processes. The CWC Dendrophyllia cornigera has the particular ability to thrive in several locations in which temperatures range from 11 to 17 °C, but to be apparently absent from most CWC reefs at temperatures constantly below 11 °C. This study thus aimed to assess the thermal tolerance of this CWC species, collected in the Mediterranean Sea at 12 °C, and grown at the three relevant temperatures of 8, 12, and 16 °C. This species displayed thermal tolerance to the large range of seawater temperatures investigated, but growth, calcification, respiration, and total organic carbon (TOC) fluxes severely decreased at 8 °C compared to the in situ temperature of 12 °C. Conversely, no significant differences in calcification, respiration, and TOC fluxes were observed between corals maintained at 12 and 16 °C, suggesting that the fitness of this CWC is higher in temperate rather than cold environments. The capacity to maintain physiological functions between 12 and 16 °C allows D. cornigera to be the most abundant CWC species in deep-sea ecosystems where temperatures are too warm for other CWC species (e.g., Canary Islands). This study also shows that not all CWC species occurring in the Mediterranean Sea (at deep-water temperatures of 12-14 °C) are currently living at their upper thermal tolerance limit. © 2014 Springer-Verlag Berlin Heidelber

Predictive Ensemble Maps for cold-water coral distributions in the Cap de Creus Canyon (NW Mediterranean)

Predictive habitat mapping has shown great promise to improve the understanding of the spatial distribution of benthic habitats. However, although they surely represent an important step forward in process-based ecosystem management, their predictive efficiency is not always tested by independent groundtruthing data. This is particularly true for the deep-sea environment, where sample data are always limited compared to the large extent of the areas to be mapped. The aim of this study is to apply and test different spatial models to statistically predict the distribution of three Cold-Water Coral (CWC) species (Madrepora oculata, Lophelia pertusa and Dendrophyllia cornigera) in the Cap de Creus Canyon (NW Mediterranean), based on high-resolution swath-bathymetry data and video observations from the submersible JAGO (IFM-GEOMAR). Submarine canyons act as specific hosting areas for CWCs, owing to their favourable environmental conditions, which provide habitat and shelter for a wide range of species, including commercially viable fish. Maximum Entropy (MaxEnt), General Additive Model (GAM) and decision tree model (Random Forest) were independently applied to represent non-linear species-environment relationships using terrain variables derived from multibeam bathymetry (slope, geomorphologic category, rugosity, aspect, backscatter). Relevant differences between the three models were observed. Nonetheless, the predicted areas where CWCs should be found with higher probabilities coincided for the three methods when a lower spatial scale was considered. According to the models, CWCs are most likely to be found on the medium to steeply sloping, rough walls of the southern flank of the canyon, aligning with the known CWC ecology acquired from previous studies in the area. As a final step, a probabilistic predictive ensemble has been produced merging the outcomes of the three models considered, providing a more robust prediction for the three species. The main insight is that important discrepancies can arise in using different species distribution models, especially when high spatial resolutions are considered. This could in part be the result of the different statistical assumptions behind each of the models. We suggest that a more reliable prediction could be obtained by merging models into spatial ensembles, able to reduce differences and associated uncertainties, showing hence a strong potential as an objective approach in the planning and management of natural resources

Operator mixing and three-point functions in N=4 SYM

We study the three-point functions between two BPS and one non-BPS local gauge invariant operators in N=4 Super Yang-Mills theory. In particular we show, in explicit 1-loop examples, that the operator mixing discussed in arXiv:0810.0499 plays an important role in the computations of the correlators and is necessary to cancel contributions that would violate the constraints following from the superconformal and the bonus U(1)_Y symmetries. We analyse the same type of correlators also at strong coupling by using the BMN limit of the AdS_5xS^5 string theory. Again the mixing between states with different types of impurities is crucial to ensure the cancellation of various amplitudes that would violate the constraints mentioned above. However, on the string side, we also find some examples of interactions between one non-BPS and two BPS states that do not satisfy expectations based on the superconformal and the bonus U(1)_Y symmetries.Comment: 28 pages, 5 figure

Operator Mixing and the AdS/CFT correspondence

We provide a direct prescription for computing the mixing among gauge invariant operators in N=4 SYM. Our approach is based on the action of the superalgebra on the states of the theory and thus it can be also applied to resolve the mixing in the dual string description. As an example, we focus on the supermultiplet containing the BMN operators with two impurities. On the field theory side, we derive the leading planar quantum corrections to the naive expression of the highest weight state. Then we use the same prescription in the BMN limit of the AdS5xS5 string theory and derive the form of the 2-impurity highest weight state. The string expression matches nicely the SYM result and provides a prediction for the mixing due to higher order quantum corrections in field theory.Comment: 21 pages, 3 figures. Typos correcte

The benthic communities of the Cap de Creus canyon

Publicad