By terrestrial standards, the Japanese flying squid (Todarodes pacificus) is rather shy. Little is known about the elusive animals’ reproduction, despite tons of them appearing on dining tables every year. Pandey Puneeta from Hokkaido University, Japan, says, ‘A full understanding of its life history is hampered because observation is difficult at the depths at which spawning occurs’, adding that the squid were assumed to spawn in shallow coastal waters above the continental shelf surrounding Japan. Previous attempts to observe spawning in the field and the lab had led Yasunori Sakurai to suggest that the squid migrated above the layer of water separating the cold deep waters from the warmer surface waters (the thermocline) to spawn, with the egg masses sinking down later into the thermocline where the eggs would develop prior to hatching. However, when the Hakodate Research Centre for Fisheries and Oceans opened in Japan in June 2014, Sakurai finally had access to a 6 m deep, 300 m3 tank where he and Puneeta could test the theory.
Having collected male and female squid by hand jigging and in traps in the waters off Hokkaido, Puneeta pumped warm and cold seawater into the colossal reinforced concrete tank to create a thermocline between 2.5 and 3.5 m deep, ready to observe the females as they spawned. And then Puneeta repeated the experiment in water where the temperature was a uniform 22°C across the entire depth.
Filming the females in the water with a thermocline, Puneeta and her colleagues Sakurai, Dharmamony Vijai, Hae-Kyun Yoo and Hajime Matsui saw the squid descend into the cool water beneath the thermocline where they produced egg masses ranging in diameter from 17 to 80 cm containing 38,000–200,000 eggs. Explaining that the animals could only use their fins to swim as they produced the egg masses – because the mantles were used to pump water into the jelly surrounding the egg mass – Puneeta describes how each animal cradled the egg mass in their arms and tentacles as it was extruded. In addition, the squid sank gradually to near the bottom of the tank during the 7 min it took for them to produce each egg mass. However, she never saw the squid sit in the posture that Sakurai had suggested they adopt before spawning. Puneeta also noticed that initially the egg masses were elongated, although they eventually became more spherical and then their density changed and they floated up into the thermocline, where the eggs developed and hatched 5–6 days later.
However, the eggs that were laid in the tank with uniform temperature water did not hatch successfully. They failed to become buoyant, remaining at the bottom of the tank until 2–3 days later, when the egg masses collapsed and were consumed by microorganisms.
The team suspects that in addition to spawning in shallow coastal waters, the squid may head further offshore to spawn. They also suggest that the thermocline is responsible for maintaining the buoyancy of the egg mass and preserving the outer jelly layer that protects the eggs from microorganism damage.