Temperature spikes and marine heatwaves trigger rapid larval development in Pisaster ochraceus (Echinodermata: Asteroidea)

Abstract

In the North Pacific, marine heatwaves (MHWs) have been increasing in intensity, frequency, and duration, and have resulted in large ecological shifts in the rocky intertidal. The sea star Pisaster ochraceus, a keystone species in the rocky intertidal, has seen declines of up to 90% associated with MHWs. This study focused on the effect of MHWs and temperature “spikes” on the larval stage of P. ochraceus. Larvae were reared in control ambient (CA) temperatures of 11.0-16.0℃, constant control (CC) temperatures 11.0-11.7 (±1℃), and MHW temperatures that rose to a maximum of 20°C (±1℃) over a 12-day period and remained high for 40 days before returning to ambient conditions. Morphological measurements and development of larvae from each treatment were obtained once a week. Linear regression and PCA analysis were conducted to summarize and visualize the morphological data. Linear regression revealed strong correlations between total length and total width and stronger correlations between total length and preoral process length for CA and MHW treatments. PCA showed that at 15 days post fertilization (dpf) and 22 dpf, the morphological space occupied by MHW larvae differed significantly from both control treatments, while the two control groups (CA and CC) were similar. By 29 dpf, the morphological space occupied by larvae in all three treatments diverged significantly. The separation of CC larvae in morphological space from CA larvae was evident. Spicule formation and the appearance of brachiolar arms and adhesive disk appeared much earlier for MHW larvae than for larvae in the control treatments (CA and CC). This study shows that both MHWs and temperature spikes can trigger rapid larval growth and development, potentially impacting settlement through their effects on the development of critical juvenile structures.