Acclimation capacity of Echinarachnius parma sand dollars enduring a highly seasonal sub-Arctic coastal system

Marine poikilotherm organisms have evolved several physiological strategies to acclimate to different thermal conditions occurring in coastal environments. This study aimed to describe the acclimation capacity of sand dollars Echinarachnius parma found in the highly seasonal sub-Arctic archipelago of Saint Pierre and Miquelon. Fatty acid composition was quantified in cell membranes and energy reserve of sand dollars collected at specific stations according to depth and seasons. We hypothesized that perceived temperatures, seasonal thermal conditions and the trophic environment may considerably influence E. parma fatty acid profiles. In the polar fraction, E. parma showed high relative contributions of eicosapentaenoic acid (20:5ω3; 28.5%) followed by 16:0 (10.83%), 14:0 (6.67%) and 18:4ω3 (3.49%). Whereas variations in 20:5ω3 relative contributions seemed to be mainly related to spawning events, docosahexaenoic acid (22:6ω3; 2.48%) and arachidonic acid (20:4ω6; 2.49%) varied according to the thermal gradient of the water column. Furthermore, the seasonal thermal internal wave acting throughout the archipelago appeared to lack sufficient intensity to trigger modifications in the polar concentration in individuals. In the neutral fraction, the strong dominance of diatom markers (20:5ω3; 24.2% and 16:1ω7; 21.9%) suggests that the diet of sand dollars mostly reflects the composition of the organic matter available over the sediment. Our study (1) highlights specific fatty acid functions in the metabolic regulation of echinoids, (2) defines the feeding ecology of sand dollars and (3) describes the effects of different thermal conditions on the physiology of locally adapted poikilotherm animals.