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Antarctic krill ( Euphausia superba) inhabit a region with strong seasonality in several parameters, such as photoperiod, light intensity, extent of sea ice, and food availability. In particular, seasonal changes in environmental light regimes have been shown to strongly influence krill metabolism, representing control signals for seasonal regulation of physiology of this key Southern Ocean species. Here, we report the identification of a cryptochrome gene, a cardinal component of the clockwork machinery in several organisms. EsCRY appears to be an ortholog of mammalian-like CRYs and clusters with the insect CRY2 subfamily. EsCRY has the canonical bipartite CRY structure, with a conserved N-terminal domain and a highly divergent C-terminus, that bears several binding motifs, some of them shared with insect CRY2 and others peculiar for EsCRY. We have evaluated the temporal expression of Escry both at mRNA and protein levels in individuals harvested from the Ross Sea at different times throughout the 24 h cycle during the Antarctic summer. We observed a daily fluctuation in abundance for Escry mRNA in the head, with high levels around 06:00 h, which is not mirrored by a cycle in the corresponding protein. Our findings represent a first step toward establishing the presence of an endogenous circadian time-keeping mechanism that might allow this organism to synchronize its physiology and behavior to the Antarctic light regimes. (Author correspondence: or rodolfo.costa@unipd.it) [ABSTRACT FROM AUTHOR]