Dataset: Urchin Respiration Rates

We performed the urchin physiology experiments in August 2018 at four different sites accessed via the R/V Queen Mabel. We recorded the temperature at each site by deploying one temperature logger (HOBO Water Temperature Pro v2 Data Logger- U22 001, Onset corporation, USA) during a previous research cruise in March 2018. Temperature was recorded at each site every 30 min at 7-12 m depth from March to August 2018. Punta Espinosa, located in the northeastern point of Fernandina Island in the western bioregion of the Archipelago, is within a major upwelling zone. La Botella and Punta Cormorant are located in the western and central-northern sides of Floreana, respectively, a southern island in the central-southeastern bioregion. Bartolomé is located in the south-eastern side of Santiago Island, in the central bioregion. Punta Espinosa and La Botella are located in high-upwelling zones; while Bartolomé and Punta Cormorant are located in low upwelling zones.

Site / Lat, Long
La Botella / 1.2914° S, 90.4965° W
Punta Cormorant / 1.2206° S, 90.4226° W
Punta Espinosa / 0.2703° S, 91.4358° W
Bartolomé / 0.2797° S, 90.5448° W

Using SCUBA at rocky reefs of depths of 7-12 m, eight individuals of Eucidaris galapagensis were hand-collected from each of the four sites during the August 2018 cruise aboard the R/V Queen Mabel. Selected sites displayed average urchin densities ranging from 2.5 to 5.0 ind·m-2. After collections, urchins were allowed to stabilize in a bucket on the ship with seawater and an aerator at ambient temperature for 30 min. Sea surface temperature was recorded for each collection site using a calibrated digital thermometer (Traceable High Accuracy ±0.2°C Digital Thermometer S/N 170718701).

The thermal sensitivity of each urchin (n=8 per site) was measured in a closed system of ten 620 ml acrylic respiration chambers with magnetic stir bars. In this respirometry setup, there were eight replicate chambers that contained sea urchins and two chambers with only seawater as controls. Oxygen consumption and temperature were monitored in each individual chamber with a fiber-optic oxygen probe (Presens dipping probes [DP-PSt7-10-L2.5-ST10-YOP], Germany) and a temperature probe (Pt1000), respectively. Measurements were taken using a Presens Oxygen Meter System (OXY-10 SMA (G2) Regensburg, Germany) with temperature correction made for each probe independently. Oxygen concentration in the urchins and control chambers was measured every 1 s during trials, that lasted 6 to 10 minutes for a given temperature. Temperature was controlled [±0.2°C] using a thermostat system (Apex Aquacontroller, Neptune Systems), bucket heaters (King Work Bucket Heater 05-742G 1000W), and a chiller (AquaEuroUSA Max Chill-1/13 HP). At each site, the initial (and lowest) temperature was the local ambient. After each trial, the temperature was increased by 1-3°C, depending on the temperature. We decreased the range between treatment temperatures around the expected respiration peak (based on pilot data) because increased resolution improves curve fitting. We used the following temperatures (in °C) for urchins tested from each of the four sites: Punta Espinosa (19, 23, 26, 28, 30, 31, 32, 33, 34, 36, 38, 42), La Botella (20, 23, 26, 28, 30, 31, 32, 33, 34, 36, 38, 42), Punta Cormorant (22, 26, 28, 30, 31, 32, 33, 34, 36, 38, 42) and Bartolomé (23, 26, 28, 30, 31, 32, 33, 34, 36, 38, 41). It took 10 to 20 minutes to warm the water bath between treatment levels (temperature ramping rates did not differ between sites). Once stabilized at the new temperature treatment level, the water inside the chambers was replaced with new seawater to ensure that it matched the temperature of the water bath, and to reset O2 and CO2 levels. The water volume in each chamber was measured indirectly by measuring urchin volume (as the volume of water displaced from a graduated cylinder) and subtracting that from the known chamber volumes. After all measurements had been made, urchins were frozen on the ship and brought to the Marine Ecology Laboratory of the Galápagos Science Center (GSC) on San Cristóbal Island. Respiration rates were normalized to urchin Ash-Free Dry Weight, which was determined by first drying each sample in a drying oven for 24 hrs at 60°C and then burning it in a muffle furnace (Optic Ivymen System Laboratory Furnace 8.2/1100) for 4 hrs at 500°C.