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The largest animal daily migration in the planet occurs in the water column of the oceans. Every day, massive numbers of zooplanktonic and micronektonic organisms move from deeper waters to surface waters during dusk and in the opposite way during dawn. This process is known as daily vertical migration (DVM). This means that animals must dynamically respond to multiple vertical environmental gradients. Gradients in temperature, food and oxygen concentration are likely the most influential factors affecting animal behavior and physiology. Apparently, the ecophysiological responses of euphausiids (Crustacea: Euphausiacea), commonly known as krill, that occur during their DVM involve changes in the oxidative stress indicators. Euphausiid species that perform the deepest migrations and cross the oxygen minimum layer (OML) exhibit the highest antioxidant enzyme activities at surface. Considering the currently known zoogeographic patterns, the location of the highest densities of each species and contrasting their interspecific DVM range between the cold and warm environmental conditions in the north and central part of the Gulf of California, two scenarios of their antioxidant responses can be identified. The subtropical (N. simplex) and temperate (N. difficilis) krill species show physiological signs of oxidative stress when they migrate to deeper low oxygen layers, while the tropical species (E. eximia) shows signs of oxidative stress when collected in layers near the surface (0-50 m depth). These interspecific physiological capabilities likely have been shaping, in the long term, the current zoogeographical patterns of these three krill species. E. eximia is an endemic species of the Eastern Tropical Pacific and has different physiological patterns that can be explained by their larger gills/cephalothorax surface ratio in comparison with the rest of the reported species of the genus Euphausia.

Palabras clave: Krill; Eufáusacea; Migración vertical

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