Blue sharks in the Gulf Stream, in the western Atlantic, probably spend all day eating squid.
At first, that doesn't seem remarkable. Every big predator likes to eat squid, and sharks are no exception. And there are plenty of squid in the Gulf Stream, particularly Illex illecebrosus, which could arguably be referred to as the Humboldt squid of the western Atlantic. Illex illecebrosus and Dosidicus gigas are in the same family of large, muscular oceanic squid--Ommastrephidae--and they exhibit similar daily vertical migrations.
And that's why that first sentence up there is so very interesting to me! Illex and Dosidicus both spend the night near the surface and the day at depth. The difference is that when Dosidicus goes deep during the day, it manages to hide from predators, but Illex doesn't. Why not?
Dosidicus lives in the eastern Pacific, where if you dive down a few hundred meters you find yourself in the Oxygen Minimum Zone, an area of depleted oxygen where most large predators can't survive. Dosidicus is an exception, due to specialized adaptations we don't fully understand yet, so it can use the OMZ as a refuge from other predators who can't stand low oxygen--like sharks.
Illex lives in the western Atlantic, where there is no pronounced OMZ. There's plenty of oxygen even hundreds of meters below the surface. So when they dive deep during the day, their predators can follow, and what results is a sad story for Illex and a happy story for blue sharks.
The beautiful thing is that this isn't just some boring arbitrary difference that you have to memorize, that the eastern Pacific has an OMZ and the western Atlantic doesn't. No, you can derive this difference from basic principles of oceanography!
Knowing which way the Earth spins, and knowing about Coriolis forces, you can figure out that currents flow clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. The result is that on the eastern side of oceans, currents flow from the poles towards the equator. On either side of the equator, currents flow from east to west. Then on the western side of the oceans, currents flow from the equator towards the poles.
The net result is that warm equatorial water is constantly getting pushed from east to west, so the western side of every ocean has a big pool of warm water sitting on top of the deep cold water. The eastern side of every ocean has very little warm water sitting on top of the deep cold water.
Deep cold water is highly nutritious for phytoplankton. You can derive why, but for now let's just take it as given. Sunlight is also important, if you are phytoplankton. So phytoplankton are much better off on the eastern side of oceans, where the deep cold water is not being oppressed by a huge pool of warm water, and can get closer to the surface where the sunlight is.
Nutrients + sunlight = productivity! The phytoplankton can bloom like crazy, but everything that lives must die, and huge phytoplankton blooms mean lots of little phytoplankton bodies sinking down into the depths.
I'll just rip through the end of this derivation so we can get back to the squid, but all of this is explained in beautiful elegant detail in Mark Denny's How the Ocean Works.
The dead phytoplankton are decomposed by bacteria as they sink, bacteria use up oxygen as they break down the bodies, and bam: there's your oxygen minimum zone in the depths of the eastern ocean.
So, thanks to Coriolis, nutrients, sunlight, and rotting corpses, Dosidicus has an environmental refuge from predation but Illex does not.
Daytime Dining On Squid: An Oceanography Lesson