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Antarctic krill (Euphausia superba) is a highly abundant keystone species of the Southern Ocean ecosystem, directly connecting primary producers to high-trophic level predators. Sex ratios of krill vary remarkably between swarms and this phenomenon is poorly understood, as identification of krill sex relies on external morphological differences that appear late during development. Sex determination mechanisms in krill are unknown, but could include genetic, environmental or parasitic mechanisms. Similarly, virtually nothing is known about molecular sex differentiation. The krill genome has to date not been sequenced, and due to its enormous size and large amount of repetitive elements, it is currently not feasible to develop sex-specific DNA markers. To produce a reliable molecular marker for sex in krill and to investigate molecular sex differentiation we therefore focused on identifying sex-specific transcriptomic differences. Through transcriptomic analysis, we found large gene expression differences between testes and ovaries and identified three genes exclusively expressed in female whole krill from early juvenile stages onwards. The sex-specific expression of these three genes persisted through sexual regression, although our regressed samples originated from a krill aquarium and may differ from wild-regressed krill. Two slightly male-biased genes did not display sufficient expression differences to clearly differentiate sexes. Based on the expression of the three female-specific genes we developed a molecular test that for the first time allows the unambiguous sex determination of krill samples lacking external sex-specific features from juvenile stages onwards, including the sexually regressed krill we examined. [ABSTRACT FROM AUTHOR]

The Southern Ocean is expected to be significantly affected by future ocean acidification. Antarctic krill (Euphausia superba) is the key species of the Southern Ocean ecosystem. Understanding their behavioural responses to acidification is critical for assessing the impacts of ocean acidification on the ecosystem. Adult Antarctic krill reared in different holding tanks with various CO2 levels for 6 months prior to the experiments were tested for their behavioural responses to different carbon dioxide partial pressures (pCO2) (400, 1000, 1500, 2000, and 4000 μatm pCO2) in a two-channel flume. The time krill occupied either of the flume channels (with high or ambient CO2 levels) was highly variable in all tests. In most cases no significant preference to either side of the flume was found. The krill did not display any systematic discrimination to the sea water with different CO2 levels regardless of the CO2 levels that krill were acclimated for in the 6 months prior to the experiment. Poor ability to discriminate high CO2 waters may have an important implication to their life history in the future as ocean acidification rapidly progresses in parts of Southern Ocean. [ABSTRACT FROM AUTHOR]