The calanoid copepod (Acartia tonsa) is a species widely cultivatedin research laboratories and it has proven valuable as feed for fishlarvae in mariculture. The overall ecology of A. tonsa has been studied for decades, but reports on genetic responses to envi-ronmental stress in A. tonsa have only recently started to appear. For other copepod species, on the other hand, arow of studies have shown that stress-related genes, such as Hsp70and Hsp90, are expressed at elevated levels as response to, e.g., tem-perature stress andexposure to chemicals.