We measured larval mortality for S. droebachiensis at 3 different temperatures (3, 10 and 21 °C) reared in 4-l culture jars (n = 3). Temperatures in the jars were maintained either by placing them in a water bath (3 and 21 °C) or in a temperature-controlled room (10 °C). These temperatures encompass ambient and the extreme temperatures used in the experimental thermoclines. Six-day old larvae (each replicate was from a single parental pair, and reared at 10 °C) were used for this experiment. To quantify mortality, 30 larvae from every treatment for each replicate were transferred to a Petri dish at 0, 24 and 48 h, and categorized as live if swimming was observed using a Nikon SMZ 1500 dissecting microscope. Data set includes jar (replicate 4-l cuture jars), temperature (°C), time (h), and the number of live and dead larvae from a sample of 30 larvae