For each temperature run (i.e., each experimental level) eight urchins of each species were collected from Cerro Tijieretas cove (0°53’16.78”S, 89°36’29.18”W) at an average depth of 4 m. Urchins were transported in plastic containers filled with sea water to the UNC/USFQ Galapagos Science Center. 

Each individual, whose volume has been calculated using a beaker (ie, via water displacement), was randomly assigned to a numbered mesocosm: green and pencil urchins were placed in 21x11x9 cm and 42x9 cm (circumference x depth), respectively, with 5g (wet weight) of fresh Ulva sp. each taken from the Cerro Tijieretas site. Ulva sp. was weighed after excess water was removed using a salad spinner that was cycled twice for 20 repetitions. The 16 mesocosms were then placed randomly within a large water bath of 130x60x35 dimensions at the temperature to be tested randomly. A chiller and two heaters, paired with an Apex aquarium thermostat, were used to control the water temperature inside the water bath. Each mesocosm closed with a wire mesh top so that the water temperature inside the mesocosm was the same as that in the aquarium and water could be exchanged freely. Before the test, the individuals were left to fast for 24 hours in basins with oxygenators. The water inside the basins was changed approximately every 12 hours with new sea water at an ambient temperature of ~23°C. This procedure was repeated for every temperature tested.