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The sheer depth of course is astounding, and the fact that life can exist at such depths is also astounding, but the most astounding thing to me is to see terre

The sheer depth of course is astounding, and the fact that life can exist at such depths is also astounding, but the most astounding thing to me is to see terrestrial animals diving hundreds of meters, and aquatic mammals diving thousands, and moreover large creatures like sharks apparently living comfortably in the upper and middle regions of the midnight zone, alongside the skeletal and bioluminescent creatures we usually think of as inhabiting that area. Why?! What revolutionary pressure selected for the ability to dive hundreds of meters underwater from the surface or close to it, presumably to where food is more scarce? Is it easier to hunt down there? Less competition from less highly adapted animals? It seems like such an extreme adaptation that you have to presume it predates our modern familiar species of penguins, pinnipeds and cetacea.

darkerside 3y ago
As a thought exercise, what evolutionary pressure drove deep sea creatures to make the tremendous effort required to leave the depths of the ocean in the first place?
kuhewa 3y ago
The deep scattering layer which is made up of various organisms from planktonic to laternfish and larger squid undergoes a diel vertical migration from near the surface at night to well into the mesopelagic zone (200-1000m) during the day, and while the ultimate driver involves illuminance, the evolutionary pressures are complex and involve bottom up and top down forcing (e.g. following food or avoiding predators). Higher trophic level predators have adapted to bounce dive or undergo their own DVM to match and have to deal with cold, low oxygen (although hypoxia can be ecologically limiting in some areas), but the pressure, which isn't that big of a deal especially if you have limited air spaces. Once you can dive to 600-1000 m the pressure differential is not that much greater to go a bit deeper compared to at shallower depths (relative difference between 800-1600 m is the same as 0-10m). There appear to be other functional behaviours besides foraging that push the physiological limits of these animals, for example the webpage has the typical daytime depths of swordfish of 550m which is accurate and they will stay there from dawn to dusk typically, but very rarely they will bounce dive 1000m - 1400m+. Possibly, although not confirmed, related to the need to calibrate navigational faculties.

darkerside 3y ago

As a thought exercise, what evolutionary pressure drove deep sea creatures to make the tremendous effort required to leave the depths of the ocean in the first place?

kuhewa 3y ago

The deep scattering layer which is made up of various organisms from planktonic to laternfish and larger squid undergoes a diel vertical migration from near the surface at night to well into the mesopelagic zone (200-1000m) during the day, and while the ultimate driver involves illuminance, the evolutionary pressures are complex and involve bottom up and top down forcing (e.g. following food or avoiding predators). Higher trophic level predators have adapted to bounce dive or undergo their own DVM to match and have to deal with cold, low oxygen (although hypoxia can be ecologically limiting in some areas), but the pressure, which isn't that big of a deal especially if you have limited air spaces. Once you can dive to 600-1000 m the pressure differential is not that much greater to go a bit deeper compared to at shallower depths (relative difference between 800-1600 m is the same as 0-10m). There appear to be other functional behaviours besides foraging that push the physiological limits of these animals, for example the webpage has the typical daytime depths of swordfish of 550m which is accurate and they will stay there from dawn to dusk typically, but very rarely they will bounce dive 1000m - 1400m+. Possibly, although not confirmed, related to the need to calibrate navigational faculties.