ALBERT

Description: In situ experiments were run with the seastar Asterias rubens to investigate the influence of epibiosis on predation preferences. Mussels (Mytilus edulis) monospecifically fouled by different epibiont species (the barnacle Balanus improvisus, the red filamentous alga Ceramium strictum, the sponge Halichondria panicea and the hydrozoan Laomedea flexuosa) and macroscopically clean mussels were exposed and seastar predation was monitored by SCUBA. Asterias rubens preferred macroscopical unfouled mussels as prey. Fouling generally reduced predation pressure on the mussel hosts (associational resistance). Barnacles protected mussels less efficiently than hydrozoans or algae. We hypothesize that in top-down controlled communities this influence of epibiosis on predation pressure should affect mussel community patterns. A survey of natural mussel-epibiont distribution in the presence or absence of A. rubens showed that the prevalence of differently fouled mussels differed between predation-exposed and predation-protected habitats. Natural mussel-epibiont associations reflected the preferential predation of the major local predators. Additionally, higher epibiotic diversity and evenness could be observed at locations accessible to benthic predators as compared with habitats protected from predation. As blue mussels and seastars are important structuring and controlling elements in the shallow water community of Kiel Fjord, major consequences of epibiosis on the entire system are discussed.

Description: Cold-water corals are known to grow much slower than their tropical counterparts. However, this assumption is mainly based on laboratory measurements exposing specimens to conditions that differ from their natural environments. The cosmopolitan scleractinian Desmophyllum dianthus forms dense banks below 18 m in northern Patagonia, Chile. So as to measure in situ growth rates of this cold-water coral, specimens were collected from two sites, weighed and deployed on holders in their natural headlong orientation at the respective collecting site. Corals exhibited a calcium carbonate (CaCO3) mass increase of 5.44 ± 3.45 (mg (cm2 projected calyx area)–1 day–1) after 2 weeks, equivalent to a mass gain of 0.25 ± 0.18 s.d. % day–1. In comparison, D. dianthus specimens from the same collection sites maintained in an on-site flow-through aquarium system showed lower growth rates that were third of the in situ rates. In situ CaCO3 precipitation of D. dianthus extrapolated for 1 year (kg m2 year–1) displays the same order of magnitude as reported for massive growing tropical scleractinians, e.g. Porites sp.

Description: Donax hanleyanus Philippi, 1847 (Bivalvia: Donacidae) dominates fine to coarse sandy beach communities of the northern Argentinean Atlantic coast. The population biology of this intertidal wedge clam was studied by determining population structure, growth and production at the three locations Santa Teresita, Mar de las Pampas (both from December 2005 to December 2006) and Faro Querandí (from March 2005 to December 2006). Von Bertalanffy growth functions were established from length-frequency distributions using an asymptotic length (L∞) of 44 mm and the growth constants (K) of 0.46 and 0.47 y-1 respectively of Mar de las Pampas and Faro querandí. Compared with growth studies four decades ago, D. hanleyanus today is growing more slowly, but is reaching a higher maximum length. Longevity is estimated to be approximately five years. The present study confirms that the overall growth performance index is habitat-specific, grouping Donacidae into tropical/subtropical, temperate and upwelling species. The intertidal biomass of D. hanleyanus ranged between 0.04 and 1.32 g ash-free dry mass (AFDM) m-2yr-1. Individual production revealed the highest value at 30 mm length (0.16 g AFDM m-2yr-1) and annual production ranged between 0.08 and 0.99 g AFDM m-2yr-1, resulting in renewal rate values (P/) between 0.82 and 2.16. The P/ ratios of D. hanleyanus populations increased with decreasing latitude from temperate to tropical regions. Only at Santa Teresita D. hanleyanus was found living with the sympatric yellow clam Mesodesma mactroides. A significant negative correlation between abundances of both surf clams suggests that abundance peaks of D. hanleyanus are related with population crashes of M. mactroides. Spatial differences in abundance are significantly related to sand texture as confirmed by nonmetrical multidimensional scaling, but not to sea surface temperature. However, the decrease of D. hanleyanus seems to be principally related to human activities.

Description: The Humboldt Current System is a highly productive ecosystem that is subject to the dynamics of the El Niño Southern Oscillation (ENSO). El Niño (EN, the warm phase of ENSO) causes vital changes in surface water temperature, oxygen levels, and salinity conditions, which arc reflected in various responses of coastal pelagic and benthic organisms. For very shallow habitats such as sandy beaches, temperature and salinity are considered the principal parameters changing during strong EN. However, the mechanisms by which these changes effect change on the structure of coastal populations remains largely unknown. The surf clam Donax ohesulus is dominant on large sandy beaches of the Humboldt Current System. Its biogeographical distribution is largely influenced by EN-induced environmental changes. Despite the species' key role in the beach ecosystem, the effects of modified abiotic conditions on the meroplanktonic larval stages and threshold temperatures involved have not yet been investigated. After EN episodes, meroplanktonic larval stages play a crucial role in the medium- and long-term stability of shallow-water species. Thus, this study makes a first attempt to describe the ontogeny of D. ohesulus and examines the effects on development of EN temperature conditions (ENTC) in comparison with normal temperature conditions (NTC). Results indicate that early life history follows a pattern previously described for other donacid bivalves. Development, growth, and mortality of larvae were assessed during a 3-wk in vitro experiment, indicating that larvae reared under ENTC grew and developed faster in comparison with those reared under NTC; mortality was slightly higher under ENTC. During a 2nd experiment, larvae were exposed for 48 h to a distinct range of different salinities (35, 25, 15, and 5 ± 1) at 2 different temperatures (NTC and ENTC). At both temperatures, larvae suffered no mortality at medium and low salinity (35, 25, and 15 ± 1) but showed 100% mortality at very low salinity (5 ± 1) after 16 h at NTC and 32 h at ENTC. Activity of larvae was highest at medium salinity (25 ± 1) and lowest at normal salinity (35 ± 1). The results of this study indicate that early larval stages of D. ohesulus can cope with temperature and salinity changes induced during EN. Only extremely low salinity (5 ± 1) such as that observed close to river mouths may cause high mortality rates in D. ohesulus offspring.

Description: In the naturally acidified Comau Fjord (Chile), high densities of the cosmopolitan cold-water coral (CWC) Desmophyllum dianthus are found at or below aragonite saturation (Ωar ≤ 1). However, it is not known so far if seasonal changes in Ωar lead to seasonal differences in calcification rates and the corals’ ability to up-regulate the pH in the calcifying fluid (pHcf). In the present study, corals were sampled along both horizontal and vertical pH gradients (pHT = 7.6-7.9, Ωar = 0.76-1.45) in Comau Fjord. We compared D. dianthus’ calcification rates (buoyant weight technique) with the physico-chemical conditions in the water column (T, Ωar) in austral summer 2016/2017 and winter 2017. In order to determine the biological pHcf up-regulation of D. dianthus, the skeletal boron isotopic composition (δ11B) was measured in the upper part of the calyx between the septa, using a UV femtosecond laser ablation system connected to a multicollector inductively coupled plasma mass spectrometer (LA-MC-ICP-MS). Higher growth rates of D. dianthus were found in summer than in winter. Surprisingly, growth of D. dianthus was highest in undersaturated waters in both seasons (Ωar = 0.76 and 0.84) and cross-transplanted specimens were able to acclimatise to Ωar 〈 1. Therefore, the present study shows that Ωar alone is a poor predictor of D. dianthus growth. Skeletal analyses show a complex relationship between δ11B and the structure of the coral skeletons. δ11B measurements were highly variable, which may be attributed to the high calcification rates in the upper part of the coral calyx. Therefore, high resolution analyses of the skeletal composition and micro-structure will be conducted along the entire longitudinal section of D. dianthus skeletons using Raman microscopy and scanning electron microscopy (SEM). In addition, δ11B will be measured in different skeletal parts and compared to skeletal structure analyses for a reliable reconstruction of seawater pH at high temporal resolution using skeletons of D. dianthus grown under laboratory and field conditions (Comau Fjord, Chile).

Description: The fibrous calcite layer of modern brachiopod shells is a hybrid composite material and forms a substantial part of the hard tissue. We investigated how cells of the outer mantle epithelium (OME) secrete calcite material and generate the characteristic fibre morphology and composite microstructure of the shell. We employed AFM, FE-SEM, and TEM imaging of embedded/etched, chemically fixed/decalcified and high-pressure frozen/freeze substituted samples. Calcite fibres are secreted by outer mantle epithelium (OME) cells. Biometric analysis of TEM micrographs indicates that about 50% of these cells are attached via hemidesmosomes to an extracellular organic membrane present at the proximal, convex surface of the fibres. At these sites, mineral secretion is not active. Instead, ion transport from OME cells to developing fibres occurs at regions of closest contact between cells and fibres, however only at sites where the extracellular membrane at the proximal fibre surface is not developed yet. Fibre formation requires the cooperation of several adjacent OME cells. It is a spatially and temporally changing process comprising of detachment of OME cells from the extracellular organic membrane, mineral secretion at detachment sites, termination of secretion with formation of the extracellular organic membrane, and attachment of cells via hemidesmosomes to this membrane.

Description: CO2-induced ocean acidification and associated decrease of seawater carbonate saturation state contributed to multiple environmental crises in Earth’s history, and currently poses a major threat for marine calcifying organisms. Owing to their high abundance and good preservation in the Phanerozoic geological record, brachiopods present an advantageous taxon of marine calcifiers for palaeo-proxy applications as well as studies on biological mechanism to cope with environmental change. To investigate the geochemical and physiological responses of brachiopods to prolonged low-pH conditions we cultured Magellania venosa, Terebratella dorsata and Pajaudina atlantica under controlled experimental settings over a period of more than two years. Our experiments demonstrate that brachiopods form their calcite shells under strong biological control, which enables them to survive and grow under low-pH conditions and even in seawater strongly undersaturated with respect to calcite (pH = 7.35, Ωcal = 0.6). Using boron isotope (δ11B) systematics including MC-ICP-MS as well as SIMS analyses, validated against in vivo microelectrode measurements, we show that this resilience is achieved by strict regulation of the calcifying fluid pH between the epithelial mantle and the shell. We provide a culture-based δ11B−pH calibration, which as a result of the internal pH regulatory mechanisms deviates from the inorganic borate ion to pH relationship, but confirms a clear yet subtle pH dependency for brachiopods. At a micro-scale level, the incorporation of 11B appears to be principally driven by a physiological gradient across the shell, where the δ11B values of the innermost calcite record the internal calcifying fluid pH while the composition of the outermost layers is also influenced by seawater pH. These findings are of consequence to studies on biomineralisation processes, physiological adaptations as well as past climate reconstructions.

Description: Throughout the last few decades and in the near future CO2–induced ocean acidification is potentially a big threat to marine calcite-shelled animals (e.g., brachiopods, bivalves, corals and gastropods). Despite the great number of studies focusing on the effects of acidification on shell growth, metabolism, shell dissolution and shell repair, the consequences on biomineral formation remain poorly understood, and only few studies addressed contemporarily the impact of acidification on shell microstructure and geochemistry. In this study, a detailed microstructure and stable isotope geochemistry investigation was performed on nine adult brachiopod specimens of Magellania venosa (Dixon, 1789), grown in the natural environment as well as in controlled culturing experiments at different pH conditions (ranging 7.35 to 8.15±0.05) over different time intervals (214 to 335 days). Details of shell microstructural features, such as thickness of the primary layer, density and size of endopunctae and morphology of the basic structural unit of the secondary layer were analysed using scanning electron microscopy (SEM). Stable isotope compositions (δ13C and δ18O) were tested from the secondary shell layer along shell ontogenetic increments in both dorsal and ventral valves. Based on our comprehensive dataset, we observed that, under low pH conditions, M. venosa produced a more organic-rich shell with higher density of and larger endopunctae, and smaller secondary layer fibres, when subjected to about one year of culturing. Also, increasingly negative δ13C and δ18O values are recorded by the shell produced during culturing and are related to the CO2–source in the culture setup. Both the microstructural changes and the stable isotope results are similar to observations on brachiopods from the fossil record and strongly support the value of brachiopods as robust archives of proxies for studying ocean acidification events in the geologic past.