Bleaching compromises gamete ultrastructure and reproductive investment in the Southwestern Atlantic coral Mussismilia hispida

As coral bleaching intensifies across the world, understanding reproductive impacts is critical for predicting which populations can recover and which face local extinction. Here, we examined the ultrastructure and composition of gamete bundles, oocytes, and spermatozoa from unbleached and naturally bleached colonies of the endemic coral Mussismilia hispida using electron microscopy and three-dimensional volumetric analysis. Gamete bundles consisted of oocyte clusters and spermatozoa randomly distributed within amorphous mucus concentrated at the periphery and occasionally between gametes. Bundles from bleached colonies were 20% smaller than those from unbleached colonies, with a 66% reduction in spermatic area (p < 0.05). Critically, bleached colonies appeared to prioritize mucus production over sperm development, revealing a fundamental trade-off in resource allocation under thermal stress. Oocytes from bleached colonies showed 23% reduced lipid reserves compared to unbleached colonies (p < 0.05), indicating severely compromised energetic investment in gametogenesis. Our findings demonstrate that bleaching fundamentally alters reproductive resource allocation in M. hispida , compromising both gamete structure and fertilization potential. By providing the first ultrastructural evidence of bleaching-induced reproductive impairment in Southwestern Atlantic corals, this study reveals that gamete quality, not just coral survival, must be monitored to assess population recovery potential and guide effective conservation interventions in warming oceans. • Bleaching significantly reduces gamete bundle size in Mussismilia hispida . • Spermatic areas are markedly lower in bleached colonies than in non-bleached ones. • Bleaching shifts reproductive investment toward mucus rather than gamete structures. • Cytoplasmic and lipid content of gametes decrease under bleaching conditions. • Findings show how bleaching compromises coral reproduction in the South Atlantic.