The tale of an endemic shrimp's exceptional osmoregulation and the ancient Athalassic mangrove oasis.

The hyperarid mangrove in the Middle East is characterised by the absence of rivers or freshwater inputs and is one of the most extreme settings of this ecosystem on Earth. Endemic to Qatar's hyperarid mangroves, a Palaemon shrimp is uniquely confined to a sole mangrove site in the Arabian Gulf. Within these mangrove channels, we unveiled brine groundwater sources exceeding 70 ppt salinity, contrasting the local marine standard of 42 ppt. Concurrently, a mysid species typically linked to salt pans and groundwater coexists. Stable isotopic analysis implied the existence of a predator-prey dynamic between this mysid species and the studied shrimp. Then, investigating the endemic shrimp's adaptation to extreme salinity, we conducted osmolarity experiments and phylogenetic studies. Our findings demonstrate that this shrimp transitions from hypo- to hyper-osmoregulation, tolerating salinities from 18 to 68 ppt-an unprecedented osmoregulatory capacity among caridean shrimps. This speciation pattern likely arises from the species osmolarity adaptation, as suggested for other Palaemon congeners. Phylogenetic analysis of the studied Palaemon, along with the mangrove's geological history, suggests a profound evolutionary interplay between the ecosystem and the shrimp since the Eocene. This study proposes the hyperarid mangrove enclave as an Athalassic mangrove oasis-a distinctive, isolated ecosystem within the desert landscape.