In the life cycle of Pyropia yezoensis, sporangial filaments connect conchocelis and thallus, but the mechanisms of maturation and conchospore release of sporangial filaments are poorly understood. We found that the morphological change from vegetative growth form (hollow cells) to reproductive form (bipartite cells), and the release of conchospores from bipartite cells were all closely correlated with culture density and light intensity. Bipartite cells formed at low density (50–1,000 fragments/mL) and when stimulated by high light levels (40–100 µmol photons m−2 s−1), but conchospore release was inhibited at such light intensities. At high densities (5,000–10,000 fragments/mL), sporangial filaments retained the hollow cell morphology and rarely formed bipartite cells. Ultrastructural observation showed that the degradation of autophagosome-like structures in vacuoles caused the typical hollow form. Transcriptome analysis indicated that adaptive responses to environmental changes, mainly autophagy, endocytosis and phosphatidylinositol metabolism, caused the morphological transformation of free-living sporangial filaments. Meanwhile, the extensive promotion of energy accumulation under high light levels promoted vegetative growth of sporangial filaments, and thus inhibited conchospore release from bipartite cells. These results provide a theoretical basis for maturation of sporangial filaments and release of conchospores in P. yezoensis and other related species.