Most squid, Sthenoteuthis included, have less than a year to be born, grow, and reproduce before they expire. This translates into an incredibly high metabolism. Young squid in captivity can eat nearly half their body weight five times per day 2.5 times their body weight per day! That would be like me eating 450lbs of food! Adult Sthenoteuthis are easily the most common cephalopods we see on blackwater dives. They can be seen as the large shoals the edges of our lights as we drift through the pelagios on our bi-weekly Black Water Dive.
This week we get to meet the anomaly that is the purpleback flying squid (Sthenoteuthis oualaniensis). They are called “flying squid” because, like Exocoetids (aka flying fish) they can leap and glide across the surface of the water to escape predators that skirt the edges of our lights as we drift through the pelagios. They spend their days hiding in a region between 200-1000 meters deep in the midwater known as the oxygen minimum zone. This is an area where animals with higher metabolic demands cannot sustain their bodily functions due to low levels of available oxygen. So while the low oxygen levels keep the squid safe from their most fearsome predators like tuna, the squid itself requires large concentrations of oxygen as well, thus the conundrum. It turns out that squid kept in tanks containing low-oxygen water can shut their metabolism down to a tenth of the normal rate of consumption. This means that the squid may not only be hiding from predators in the oxygen minimum zone, but it may be using the cold, dark, low-oxygen environment of the OMZ to shut down its metabolism and wait out daily periods of low-foraging opportunity when the squid would otherwise just starve.
Pan-tropical spotted dolphins (Stenella attenuata) often come in to forage on the schools of Sthenoteuthis attracted to our lights. One night, a dolphin pod came in to dine and hit the squid school just under the water’s surface. I watched this shiny object drifting down under the scene and recognized it as a squid’s eyeball drifting into the blackness.