Laminarin, one of the main storage polysaccharides of brown algae and diatoms, was recently found to be an important molecule in the world’s oceans by accounting for up to 26 ± 17 % of the particulate organic carbon. While digestive enzymes breaking down laminarin (laminarinases) have been intensely studied in a number of adult marine invertebrates, data on larval stages remain scarce. Abundantly inhabiting kelp forests as ecosystem engineers, especially sea urchins are found in laminarin-rich areas, suggesting that laminarin digestion plays a significant role throughout their life cycle. Considering the high laminarin abundance in microalgae, i.e. the food source of echinoderm larvae, digestive degradation might be beneficial, if not crucial, not only in adult sea urchins but also for larval performance within plankton. To disentangle the ecological and physiological role of laminarin for sea urchin larvae we conducted perturbation experiments with different laminarin concentrations and food sources (laminarin-rich and -poor microalgae) to evaluate the impact onto larval nutrition and performance. We analyzed (I) growth, survival and settlement success, as well as (II) laminarinase activity and expression patterns during larval development of the painted sea urchin Lytechinus pictus in response to the feeding regimes. The results contribute to a better understanding of laminarin dependent digestive dynamics in the early life stages of echinoderms, potentially affecting the recruitment of these important ecosystem engineers.