Surf smelt accelerate usage of endogenous energy reserves under climate change

[42] found a Q10 of 4 for yolk absorption between environmental temperature range of 15 to 21˚C. The fact that our observed Q10 values are slightly below the averages summarized in Heming and Buddington [37] and well-below those reported in Lasker [43] suggests that surf smelt, while not immune to temperature effects, are less sensitive to temperature stress com- pared to other fish species. More robust tolerance to elevated temperature compared to other fish species may be an adaptation to withstand the wide-ranging temperatures surf smelt embryos experience during their high intertidal incubation, which can last between 10 and 21 days [4]. Elevated temperature was associated with a significant decrease in oil globule size in addi- tion to decreased larval yolk area in this study, indicating ongoing taxation on endogenous energy in the larval stage. While fish embryos use free amino acids found in their yolk as their primary energy source prior to hatching, they switch to using fatty acids from their oil globule post hatching [44]. Ehrlich and Muszynski [45] proposed that in addition to being a source of energy for metabolism, the lipids that make up the oil globules in surf smelt likely aid in larval buoyancy. This was supported by findings in Blaxter and Ehrlich [46], who observed that Atlantic herring and European plaice were oriented head down in the water column post yolk absorption. These larvae were unable to feed even when offered food due to their heads sinking when attempting to follow prey. The rapid use of the oil globule in surf smelt at higher temper-

atures could result in compromised fitness when larvae enter the plankton, and buoyancy effects likely reduce exogenous energy acquisition through decreased swimming performance and prey capture efficiency. Swimming is arguably the largest sink for energy in fish larva [46], and an essential behavior for acquiring prey and for locating abiotic conditions that maximize fitness and survival. Thus, any environmental factor that accelerates the use of this endogenous energy source prior to the onset of feeding may affect larval survival and, ultimately, adult recruitment. Fig 3. Larvae oil globule usage. Oil globule area of surf smelt larvae (n = 144/day) that were submerged in treatment water for 3 days. Each point is an average measurement representative of the temperature treatment, pCO2 treatment (points offset for visualization), and day with standard error shown as whiskers. Day, and Temperature were significant factors predicting larvae oil globule usage. https://doi.org/10.1371/journal.pone.0270491.g003 PLOS ONE Surf smelt response to climate co-stressors PLOS ONE | https://doi.org/10.1371/journal.pone.0270491 June 27, 2022 9 / 14