First Frozen Repository for Coral Sperm in the Southwestern Atlantic: A Tool to Support Reef Conservation

ABSTRACT Coral reefs are among the most biodiverse ecosystems on Earth, yet they are rapidly declining due to climate change and other anthropogenic stressors. Population reductions threaten sexual reproduction in sessile coral species, as increased distances between colonies hinder gamete encounters. Once populations fall below critical density thresholds, natural recovery may become impossible even after stressors are removed. Biobanking of coral germplasm has therefore emerged as a key strategy to preserve genetic diversity and support reef restoration. This study developed and validated a cryopreservation protocol for sperm of the Brazilian endemic reef‐building coral Mussismilia harttii , representing a milestone in coral conservation for the Southwestern Atlantic. Spermatozoa were exposed to dimethyl sulfoxide (DMSO) and methanol (MeOH) at concentrations of 10%, 15% and 20% and subjected to controlled slow freezing and ultra‐rapid freezing. Postthaw analyses assessed motility, mitochondrial activity and viability. The best results were obtained with 20% DMSO and controlled slow freezing, yielding 29.7% ± 0.84% motility and 83.6% ± 2.2% viability. Mitochondrial activity (MTT assay) was higher with 15% MeOH (1.63 ± 0.17) than with DMSO (0.74 ± 0.17). Cryopreserved sperm (20% DMSO + slow freezing) achieved a 100% fertilization rate, statistically equivalent to fresh sperm ( p = 0.4533). This optimized protocol enabled the establishment of the first coral sperm repository in the South Atlantic, currently storing 2.4 billion viable M. harttii spermatozoa. This repository provides a crucial resource to safeguard genetic diversity, strengthen assisted breeding programs and enhance the long‐term conservation and restoration capacity of Brazilian coral reefs.