After a decade of studying Humboldt squid in the Gulf of California, William Gilly was puzzled. Working at the Hopkins Marine Station of Stanford University, Gilly had established that Humboldt squid spend significant amounts of time in the oxygen minimum zone, a region of the water column in the eastern Pacific Ocean with a very low dissolved oxygen concentration. The squid still seemed able to make a living in these cold, oxygen-starved waters, and Gilly discovered that they did so by lowering their metabolism. But when Gilly started using electronic tags to monitor their movements, he was surprised to find that squid moved around just as much – and just as fast – in the oxygen minimum zone as they did in near-surface waters, which have much higher oxygen levels. ‘This was odd’, says Gilly. ‘On the one hand, we know that squid lower their metabolism in low oxygen conditions, but on the other hand we weren't seeing any effect on their behaviour.’ To solve this conundrum, Gilly and his colleagues decided to take another look at Humboldt squid swimming behaviour at different depths (p. 3175).
Fortunately, the team had access to a squid fishery in the Gulf of California, and easily caught some squid. Under the fin of each squid they tucked an electronic tag measuring depth, temperature and light once per second – a much better resolution than previous tags – and set the squid free again. After recovering nine tags, the team set about analysing the data. Sure enough, they found that the squid generally moved more slowly in the oxygen minimum zone than in the water layer just above the oxygen minimum zone, which has a higher oxygen level. When depth measurements indicated that squid were in the oxygen minimum zone, the team saw that the time squid spent moving at intermediate vertical speeds (0.3–1.0 m s−1) was reduced by about half, and the time spent travelling at higher velocities was even more drastically reduced. ‘This solved our puzzle’, says Gilly. ‘This is entirely consistent with the lower metabolism that we see in low oxygen conditions.’ The measurements from the previous tags simply weren't sensitive enough to detect the lower activity levels in oxygen-starved conditions.
But what were squid doing in the inhospitable oxygen minimum zone? ‘Their favourite prey is concentrated there, and given that they are voracious predators, it seems inconceivable that they're not eating’, says Gilly. To capture what the squid were doing at these depths, Gilly dressed squid in a ‘bathing suit’ holding a video and data-logging device. Unfortunately, they didn't dive deeply enough to provide footage of their antics in the oxygen minimum zone, but the accelerometer in the device did help Gilly's team build a more accurate picture of how Humboldt squid move. ‘They jet upwards at an angle of about 65 deg and then gently glide downwards at a much shallower angle’, explains Gilly. In fact, they spend the vast majority of time gliding, an efficient form of locomotion for the large creatures, which can weigh up to 20 kg.
Despite the lack of video evidence, Gilly believes that Humboldt squid eat all the time – at the surface at night, and near the oxygen minimum zone during the day. ‘Everything we saw in this study is consistent with that hypothesis,’ says Gilly, ‘and the next step is to test it more critically.’