Lipids are central to crustacean development, yet lobster larvae's capacity to assimilate and modify dietary fatty acids (FAs) during ontogeny remains poorly understood. This study investigated the in vivo capacity for incorporation, esterification and structural modification of FAs in early-stage European lobster (Homarus gammarus). Larvae (stages I-III) and postlarvae (stage IV) were incubated in triplicate for 5 h under controlled seawater conditions with [1-14C]-labeled linoleic (LA, 18:2n-6); α-linolenic (ALA, 18:3n-3); arachidonic (ARA, 20:4n-6); eicosapentaenoic (EPA, 20:5n-3), or docosahexaenoic (DHA, 22:6n-3) acids. Unlabeled controls were maintained under identical conditions. For lobsters incubated with [1-14C]-FAs, incorporation into total lipids, esterification, and FA elongation/desaturation products were quantified. Lipid class and FA compositions were assessed in unlabeled controls. FA incorporation was strongly development-dependent. Stage I larvae preferentially incorporated n-3 polyunsaturated fatty acids (PUFAs), particularly EPA and ALA, whereas stage IV postlarvae favored incorporation of LA and ALA. At all stages, [1-14C]-FAs were mainly esterified into polar lipids classes, particularly the phospholipids phosphatidylcholine and phosphatidylethanolamine. Esterification into neutral lipids, mainly triacylglycerols, increased from stage I to III and declined in stage IV. FA elongation and desaturation were limited but detectable, peaking in stage III larvae. EPA was synthesized from ALA and ARA from LA. Detected LA-to-EPA conversion likely reflects peroxisomal β-oxidation and label redistribution. DHA synthesis was not detected. These results reveal stage-specific lipid metabolic capacities and limited endogenous production of long-chain PUFAs, underscoring the need for dietary LC-PUFAs supplementation to support optimal H. gammarus larval development.