Description: The stomachs of most vertebrates operate at an acidic pH of 2 generated by the gastric H+/K+-ATPase located in parietal cells. The acidic pH in stomachs of vertebrates is believed to aid digestion and to protect against environmental pathogens. Little attention has been placed on whether acidic gastric pH regulation is a vertebrate character or a deuterostome ancestral trait. Here, we report alkaline conditions up to pH 10.5 in the larval digestive systems of ambulacraria (echinoderm + hemichordate), the closest relative of the chordate. Microelectrode measurements in combination with specific inhibitors for acid-base transporters and ion pumps demonstrated that the gastric alkalization machinery in sea urchin larvae is mainly based on direct H+ secretion from the stomach lumen and involves a conserved set of ion pumps and transporters. Hemichordate larvae additionally utilized HCO3- transport pathways to generate even more alkaline digestive conditions. Molecular analyses in combination with acidification experiments supported these findings and identified genes coding for ion pumps energizing gastric alkalization. Given that insect larval guts were also reported to be alkaline, our discovery raises the hypothesis that the bilaterian ancestor utilized alkaline digestive system while the vertebrate lineage has evolved a strategy to strongly acidify their stomachs.

Description: Ovigerous females were collected by scraping from inner Kiel Fjord in July -August 2017 and kept under control constant temperature room (~20°C) until egg hatching. Newly hatched larvae were reared under different salinity treatments in order to estimate their salinity tolerance,. Experiments were conducted until larvae reached the megalopae stage (last larval stage) or died, The results of such study could give us a primary outlook on the dispersal ability of H. takanoi larvae along the Baltic Sea, and insights about the life cycle of H. takanoi specifically in the south-western Baltic.

Description: Crabs were collected from inner Kiel Fjord by scraping in November 2018, in order to study their tolerance to salinity and consequently their ability to establish a population along the Baltic Sea, in addition, their feeding rates were estimated under laboratory condition. Experiments were conducted in constant climate temperature room (~12 °C) at GEOMAR during December 2018.

Description: Experimental ocean acidification leads to a shift in resource allocation and to an increased [HCO3-] within the perivisceral coelomic fluid (PCF) in the Baltic green sea urchin Strongylocentrotus droebachiensis. We investigated putative mechanisms of this pH compensation reaction by evaluating epithelial barrier function and the magnitude of skeleton (stereom) dissolution. In addition, we measured ossicle growth and skeletal stability. Ussing chamber measurements revealed that the intestine formed a barrier for HCO3- and was selective for cation diffusion. In contrast, the peritoneal epithelium was leaky and only formed a barrier for macromolecules. The ossicles of 6 week high CO2-acclimatised sea urchins revealed minor carbonate dissolution, reduced growth but unchanged stability. On the other hand, spines dissolved more severely and were more fragile following acclimatisation to high CO2. Our results indicate that epithelia lining the PCF space contribute to its acid–base regulation. The intestine prevents HCO3- diffusion and thus buffer leakage. In contrast, the leaky peritoneal epithelium allows buffer generation via carbonate dissolution from the surrounding skeletal ossicles. Long-term extracellular acid–base balance must be mediated by active processes, as sea urchins can maintain relatively high extracellular [HCO3-]. The intestinal epithelia are good candidate tissues for this active net import of HCO3- into the PCF. Spines appear to be more vulnerable to ocean acidification which might significantly impact resistance to predation pressure and thus influence fitness of this keystone species.

Description: Salinity in Kiel Fjord was recorded on mostly-weekly cruises of the research vessel FK Polarfuchs from 2005 to 2018 using a CTD48M probe (Sea and Sun technologies, Trappenkamp, Germany) at a station in front of the GEOMAR Pier (“Wittlingskuhle”, Position: 54°19'69 N, 10°09'06 E). These measurements are important for better understanding the salinity as one of the important abiotic factor determine the reproduction, establishment and has an effect on many species in the Baltic Sea in general and the fjord specifically.

Description: Anthropogenic CO2 emissions are acidifying the world's oceans. A growing body of evidence is showing that ocean acidification impacts growth and developmental rates of marine invertebrates. Here we test the impact of elevated seawater pCO2 (129 Pa, 1271 µatm) on early development, larval metabolic and feeding rates in a marine model organism, the sea urchin Strongylocentrotus purpuratus. Growth and development was assessed by measuring total body length, body rod length, postoral rod length and posterolateral rod length. Comparing these parameters between treatments suggests that larvae suffer from a developmental delay (by ca. 8%) rather than from the previously postulated reductions in size at comparable developmental stages. Further, we found maximum increases in respiration rates of + 100 % under elevated pCO2, while body length corrected feeding rates did not differ between larvae from both treatments. Calculating scope for growth illustrates that larvae raised under high pCO2 spent an average of 39 to 45% of the available energy for somatic growth, while control larvae could allocate between 78 and 80% of the available energy into growth processes. Our results highlight the importance of defining a standard frame of reference when comparing a given parameter between treatments, as observed differences can be easily due to comparison of different larval ages with their specific set of biological characters.

Description: In a warming ocean, temperature variability imposes intensified peak stress, but offers periods of stress release. While field observations on organismic responses to heatwaves are emerging, experimental evidence is rare and almost lacking for shorter-scale environmental variability. for two major invertebrate predators, we simulated sinusoidal temperature variability (±3 °C) around todays' warm summer temperatures and around a future warming scenario (+4 °C) over two months, based on high- resolution 15-year temperature data that allowed implementation of realistic seasonal temperature shifts peaking midpoint. Warming decreased sea stars' (Asterias rubens) energy uptake (Mytilus edulis consumption) and overall growth. Variability around the warming scenario imposed additional stress onto Asterias leading to an earlier collapse in feeding under sinusoidal fluctuations. High-peak temperatures prevented feeding, which was not compensated during phases of stress release (low-temperature peaks). In contrast, increased temperatures increased feeding on Mytilus but not growth rates of the recent invader Hemigrapsus takanoi, irrespective of the scale at which temperature variability was imposed. This study highlights species-specific impacts of warming and identifies temperature variability at the scale of days to weeks/months as important driver of thermal responses. When species' thermal limits are exceeded, temperature variability represents an additional source of stress as seen from future warming scenarios.

Description: Energy uptake of H. takanoi and A. rubens during the experimental period (72 and 64 days, respectively) measured under 4 temperature conditions: ambient ('constant'), warm ('constant'), ambient sinusoidal and warm sinusoidal. The experiments were conducted from July to September 2017 in the Kiel Indoor Benthocosms (KIBs), at GEOMAR Kiel, Germany. The energy uptake was evaluated at different scales summing the energy uptake for the particular scale of interest: (i) large-scale as the overall sum of the energy uptake, (ii) small-scale (Phase) of 8-day wavelength, for 2-day feeding periods were energy was summed at minimum (Min), maximum (Max) and mean temperatures after minimum (Descending) or maximum (Ascending) during each sinusoidal temperature cycle, and (iii) medium-scale (Period) of 16 days, where energy uptake was summed for 'Pre-heat' (July 19th to August 4th), 'Heat' (August 4th to August 20th) and 'Post-heat' (August 20th to September 5th). To estimate energy uptake, A. rubens and H. takanoi were fed M. edulis mussels every second day. Each individual of A. rubens received five mussels of 25 to 35 mm size (total length). For H. takanoi, 20 mussels of 9 to 12mm were offered. For A. rubens, the shell of every consumed mussel was measured. In the case of H. takanoi, the average size (10.5 mm) of the mussels offered was taken for further estimated on energy uptake. A predictive relationship between soft tissue dry weight of mussels (g) and mussel length (mm) was used to calculate dry mass and energy uptake according to Brey et al., as 18.85 Joules per mg dry mass of mussels.

Description: Size and weight of H. takanoi and A. rubens during the experimental period (72 and 64 days, respectively) measured under 4 temperature conditions: ambient ('constant'), warm ('constant'), ambient sinusoidal and warm sinusoidal. The experiments were conducted from July to September 2017 in the Kiel Indoor Benthocosms (KIBs), at GEOMAR Kiel, Germany. Wet and dry weight (g) were quantified at the start (July 12th) and at the end of the experiment (September 21st (day 72) for A. rubens and on September 14th (day 64) for H. takanoi). For this, individuals were weighed after gently blotting dry with a tissue. At the end of the experimental period, all individuals of both species were frozen (6 hours at -20°C), and dried (48hours, 80°C), and weighted to estimate dry weight.