Hello, friends! Welcome back to Sciency Words, a special series here on Planet Pailly where we talk about those weird and wacky terms scientists use. This week’s Sciency Word is:
ABYSSAL GIGANTISM
In the deepest, darkest abyss of the ocean, animals have a tendency to grow to gigantic sizes. This tendency is known as abyssal gigantism. It’s also known as deep-sea gigantism.
Based on what Google Ngram Viewer has to show us, it looks like these terms (both abyssal and deep-sea gigantism) first appeared in the 1950’s, but people have obviously known that giant things live in the ocean for far longer than that. Common examples of abyssal gigantism include the giant squid, the giant oarfish, and the Japanese spider crab. All of these animals live in the deep, deep, deeeeeep ocean, and they all grow larger—considerably larger—than their shallow-water cousins.
What causes abyssal gigantism? That’s not entirely clear. As you might imagine, marine biologists have a tough time studying creatures that live that far down underwater. But based on what I’ve read about this so far, the two most common explanations seem to be:
- Keeping warm: Bigger animals can retain more of their own body heat. That’s important if you live in extremely cold environments, like the deep oceans. This is related to an ecological principle known as Bergmann’s rule.
- Being metabolically efficient: Bigger animals tend to be more metabolically efficient, as modeled by something called Kleiber’s law. In other words, big animals need less food relative to their size than smaller animals do. That’s important if you live in an environment where food is scarce, like the deep oceans.
I have to admit I still have a lot to learn about this topic, and some of the things I read were a little confusing to me. For example, I’ve read contradictory things about oxygen levels in the deep ocean and how that might factor into abyssal gigantism.
But that’s not the important thing. You see, it’s not just that animals can grow to gigantic sizes in the deep ocean; it’s that they must. For one reason or another, there’s evolutionary pressure on deep sea animals to get bigger and bigger and bigger. And that’s got me thinking….
Next time on Planet Pailly, let’s revisit that very deep, very dark, very cold subsurface ocean on Europa.
Planet Pailly 
One of the things that surprised me, although it stands to reason, is that most life in the sea is near the continents and islands. The deep sea is something of a desert in terms of ecology. It makes you wonder how likely life could be on (in) one of the ocean exoplanet worlds, where the ocean is effectively bottomless.
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As I understand it, you need liquid water in direct contact with rock in order to get life. Otherwise, where would life get its minerals? A lot of those ocean exoplanets have oceans so deep that there’s probably a layer of ice between the liquid water layer and the rock layer. If that’s true, then those worlds are probably lifeless.
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Never considered that, but it makes sense. We might imagine volcanic processes spewing material into the water, but on worlds where the ocean is hundreds of kilometers thick, with that high pressure ice layer, nothing seems likely to get up out of the abyss.
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fascinating
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