Figure 1. Interzonal migrating copepods predominating in the subarctic Pacific and its marginal waters. They carry out day-night (diel) and/or seasonal vertical migration down to 2000 m. (Photo by T. Kobari)
	Figure 2. Reproductive conditions of adult female Neocalanus cristatus (left: gonad developing, middle: egg spawning, right: after egg release “spent”). Spent females become transparent due to the consumption of all body tissues for egg production. (Photo by T. Kobari)

Humans regularly travel to 10000-m above the land during our airplane travel across the globe, but we do not take many trips the other direction- down into the deep sea. It takes a special submarine to withstand the high pressure and cold temperatures of the deep ocean, and few humans have made the trip. However, small animals the size of a grain of rice can reach these depths easily. They are Neocalanus copepods, tiny crustaceans that are an important part of the ocean food web in the subarctic Pacific. They are part of a larger, vertically migrating community of “micro-submarines” in the sea

There are 3 species of Neocalanus in the subarctic Pacific Ocean, first reported 80 years ago by Russian scientists. Around the waters at our K2 study site young animals grow up during winter through early summer. They feed on a variety of particulate organic matter, including diatoms, flagellates and ciliates, and also sinking aggregates and feces. The Neocalanus build up a large amount of lipid in their bodies as a result of their feeding, so much so that summer zooplankton net samples appear very ‘oily’. The fatty bodies of the Neocalanus copepods are important food for sardine, saury, and salmon, which are common Japanese seafood dishes. After enough lipid accumulation, the Neocalanus migrate down to deep waters below 500-m depth. This is quite a distance for such a small animal to travel. Because they travel from the surface sunlit zone to the deep mesopelagic zone, they are called “interzonal migrating” copepods. Very little food is available in these deep waters, so the copepods consume all their body tissues for production of eggs (spawning) after which their body becomes transparent (see figure 2), and they eventually die. The eggs produced at depth float toward the surface where they then hatch.

In the arctic to subarctic waters, interzonal migrating copepods are predominant members of zooplankton community. Their seasonal migration is an important part of the “biological pump”, or the transport of organic matter from the surface to the deep waters of the ocean. This is because Neocalanus copepods migrate deep and then die at depth. When they arrive at depth, they have achieved their largest body size and the most acclimated lipids of any of their life stages. The latter results in a high body carbon:nitrogen (C:N) ratio, which results in relatively more carbon transported to depth. We have been fortunate to use a high technology tool, the IONESS net (courtesy of Dr. Kitamura of the Japan Agency for Marine-Earth Science and Technology-JAMSTEC), to sample Neocalanus at different depths and study their migration.

At present, we can predict the weather next week from space. But, how much do we understand about the deep ocean? There are still many mysteries.

— Toru Kobari ( Kagoshima University, Japan )

—Debbie Steinberg (Virginia Institute of Marine Science)