One of the five Aetideid copepods was distributed at depths greater than 1,000 m throughout the year and had a low population density, while the remaining four species were concentrated at depths of 600-700 m during the midnight sun (May-August). On the other hand, during the polar night (November-January), the depth of distribution varied among species, with two species becoming shallower to 300-400 m, one species remaining at around 600 m, and one species becoming deeper to 800 m (see figure below). The different depths of distribution in the polar night for each species of the same family were interpreted to reduce competition for food during the season when light is scarce, the amount of particles settling in the deep sea is low, and food is scarce.
The timing of reproduction, inferred from seasonal changes in population structure, differed among species with different distribution depths. The reproduction of the two species whose distribution depths become shallower during the polar night was found to be in the late polar night, while the reproduction of the two species that remain in deep water even during the polar night was found to be in the early polar night.
