Ecological and Genetic Insights Into Antarctic Fairy Shrimp, <i>Branchinecta gaini</i> Daday, 1910 (Branchiopoda: Anostraca) Populations on King George Island, Antarctica

ABSTRACT The Antarctic fairy shrimp ( Branchinecta gaini ) is the largest inland animal in Antarctica and a key component of its freshwater ecosystems. Despite its broad distribution, little is known about its population structure, habitat tolerance, and potential vulnerability to climate change. This study investigated the occurrence, genetic structure, and cyst bank dynamics of B. gaini across freshwater habitats on King George Island (KGI), South Shetland Islands. Field surveys were conducted at 35 freshwater sites during the summer of 2021–2022. The presence of B. gaini was determined through net sampling, and cyst density was measured from sediment samples using flotation and image analysis. Water chemistry and habitat characteristics were recorded. Genetic analyses were performed on mitochondrial cox 1 and 16S rRNA genes across 13 sites to assess population structure and phylogenetic relationships. Branchinecta gaini was detected at 17 sites, displaying wide habitat tolerance in ponds varying in size, salinity, and morphometry. Genetic analysis of 92 cox 1 sequences revealed three haplotypes, suggesting the absence of cryptic species. However, AMOVA indicated significant genetic differentiation among regions (Φ = 0.453, p &lt; 0.001). Cyst densities varied markedly, with an average of 1.08 cysts cm −3 . A young (~20‐year‐old) postglacial pond near the Ecology Glacier had a notable cyst density of 0.272 cysts cm −3 , indicating successful colonisation and supporting the monopolisation hypothesis. The highest cyst density (10.91 cysts cm −3 ) was recorded in a pond in the area of Ornithologists Creek. Branchinecta gaini demonstrates ecological opportunism, inhabiting a broad range of Antarctic freshwater habitats, including postglacial and coastal water bodies. While three haplotypes were detected, their regional distribution suggests some level of spatial structuring. Shared haplotypes across regions may reflect either historical connectivity or sporadic dispersal events. Environmental factors such as pH, sediment type, and water flow were associated with the presence and cyst abundance of B. gaini . Our findings confirm that B. gaini is an adaptable species capable of establishing populations in dynamic Antarctic freshwater systems. The presence of cyst banks in young ponds highlights the species' potential for rapid colonisation. Further research using higher‐resolution molecular tools is recommended to clarify dispersal mechanisms and assess long‐term population resilience under changing environmental conditions.