Abstract

Environmental variability causes variations in the biological traits of marine organisms, affecting their abundance, distribution, and morphology. By using two bio-oceanographic surveys in Maxwell Bay, King George Island (summer 2023), and South Bay, Doumer Island, Palmer Archipelago (summer 2022), West Antarctica, the influence of glacial meltwater was quantified on biological traits of the neritic calanoid copepods, Rhincalanus gigas and R. nasutus. Stations were categorized as glacial-influenced (GI) when the mean temperature and salinity were <1 °C and <33.9 g kg?1 (2022), and <2.2 °C and <33.9 g kg?1 (2023), respectively. Stations with warmer and saltier waters were considered as non-glacial influenced (NGI) zones. The abundance of adult females was similar for both species in the GI and NGI areas. Morphologically, both species were similar, varying from specimens with shorter urosome, and wider 2nd somite of the prosome, to specimens with larger urosome, and narrower 2nd somite of the prosome (PC1, 31 % of the variance). Also, copepods varied from specimens with a narrower body and a pointed urosome to individuals with a wider body and an expanded urosome (PC2, 27 %). The allometry was low but significant for R. gigas (3.46 %), and larger for R. nasutus (13.56 %), associated with a widening of the base of the cephalosome and a change of the urosome's direction. Discriminant Function Analysis detected significant shape differences between GI and NGI areas for the endemic R. gigas, but not for circumglobal R. nasutus. A Partial Least Squares analysis detected covariance between shape variations in Rhincalanus gigas females and hydrographic conditions, showing that individuals with larger (shorter) urosome were associated with warmer (colder) and less (more) oxygenated waters. These morphological variations may pose constraints on the relevant ecological traits of local populations or represent an expression of phenotypic plasticity in endemic Antarctic species. © 2025