“Nail Disease”, an emerging syndrome in the Pacific oyster Magallana gigas cultivated in the Baja California Peninsula, Mexico: the challenge of determining its causal agent(s) and the importance of early warning

• Unusual mortalities of Magallana gigas in aquaculture facilities have been associated with Nail Disease. • Nail Disease is a Syndrome linked to various microbial agents that cause severe shell deformities. • The microbial communities and histopathological features involved differ markedly from those described by Shell Disease. • Nail Disease is an emerging condition that raises concern due to its potential impact on oyster aquaculture. In 2024, aquaculture producers reported unprecedented cases of shell malformations and mortality in Pacific oysters ( Magallana gigas ) in Baja California Peninsula, Mexico. Due to the shell malformations, they named this condition “nail disease.” Sixty oysters from the main affected location were used for shell, bacteriological, fungal, and histopathological analysis. Shell malformations may reach 90% of the surface. Bacteriological identification based on 16S rRNA gene sequencing revealed the presence of Staphylococcus saprophyticus and Flagellimonas marinaquae . Shell decalcification exposed one predominant fungal hyphae with septate, branched filaments bearing spores, while the other occasional form consisted of straight, aseptate hyphae, similar to Ostracoblabe implexa . Histopathological analysis revealed infiltration and necrosis, as well as chitinous material and bacteria. Shell malformations resemble those associated with “shell disease,” for which O. implexa has been proposed as the etiological agent. In this case, the predominant fungi do not match the morphology of O. implexa . Genetic studies are required to determine their identity. Nail disease appears to involve a complex interaction of microbial agents and changes in the shell-associated microbial community. This dysbiosis likely disrupts the homeostasis of the shell environment, initiating a host immune response that manifests as shell malformation. The nail disease presents a challenge in determining the causative agent(s) using Koch’s postulates in mean time it must be considered a syndrome. Our results serve as an early warning to the community regarding the emergence of a new disease. Timely communication is a key when addressing emerging diseases, independently if the specific causal agent has been determinate.