Many marine invertebrates brood their young and use various methods to supply energy to their offspring, such as yolk in the egg, parental secretion of nutrients, or trophic eggs. In sea spiders (Arthropoda: Pycnogonida), the male broods the eggs. Brooding is prolonged in many Antarctic species, and larvae molt through several stages while clinging on to the egg mass. The energetic cost of development and the source of this energy are both unknown. We characterized patterns of organic mass loss in the developing embryos and larvae of two species of Antarctic sea spider, Ammothea glacialis and Nymphon australe. Both species brood larvae to an advanced stage, with larvae undergoing three molts while clinging to the egg mass. We collected egg-bearing males from four dive sites in McMurdo Sound, Antarctica, between October 2019 and January 2020. We measured the ash-free dry weight of multiple subsets of five offspring from each stage, with each subset sampled from a different egg mass (n = 10, 8, 8, 8 and n = 11, 9, 14, 13 for the eggs, Stage 1, Stage 2 and Stage 3 larvae of A. glacialis and N. australe respectively). Organic mass declined significantly between each consecutive stage for A. glacialis (total loss of 15.4 ± 3.2µg, 28.6% of the egg’s AFDW), and between the egg, Stage 1 and Stage 2 larva of N. australe (total loss of 25.3 ± 3.7µg, 34.6% of the egg’s AFDW). However, the Stage 3 larvae of N. australe displayed a significant increase in biomass as compared to the Stage 2s. While the source of this energy is yet unknown, it may be due to feeding, either on siblings or organic detritus. Our findings suggest that development is fueled largely by yolk reserves in brooding sea spiders, but that later stages of some species supplement nutrition by exogenous feeding. Funded by NSF-OPP-1745130 to ALM.