Why Do Deep Sea Creatures Evolve Into Giants?

Below 400 meters food becomes quite scarce in the ocean where as sunlight tapers off photosynthetic algae and plankton disappear too and without this crucial part of the food chain life gets markedly harder for deep sea animals where instead most of them have to rely on detritus that rains down from shallower waters a phenomenon called marine snow demonstrating how vertical organic flux from photic zone creates detrital food web in aphotic depths with marine snow composed of dead plankton fecal pellets and decomposing tissue forming backbone of deep ocean ecosystem despite extreme scarcity.

Marine snow is the backbone for all life in the deep but it can’t support a great deal of biomass where the number of creatures in the deep is sparse and the food web is strained where many animals don’t eat marine snow directly but rely on eating those that do and thus the pressure of predation is extremely high where any small fish crustacean or cephalopod has a big target on its back from any larger predator demonstrating how intense predation pressure in food-limited deep sea creates strong selection for gigantism enabling transition from vulnerable prey status to predator-immune top predator eliminating major source of mortality through size refuge strategy.

Enter the giant squid both an icon and an enigma of the deep where by the turn of the 21st century the giant squid remained one of the few extant megafauna to have never been photographed alive where what we know about the giant squid largely comes from specimens that have washed ashore where it wasn’t until 2004 that the first photographs of a live giant squid in its natural habitat were taken in the twilight zone at a depth of about 1000 meters demonstrating how elusive deep-sea megafauna remained unobserved until recent technological advances enabled in-situ documentation at bathypelagic depths.

The colossal squid sometimes called the Antarctic squid is the largest invertebrate in the world where they’re shorter in length than the giant squid at only 10 meters but can weigh between 500 and 700 kilograms lurking in depths over 2000 meters where the colossal squid seems to be exceedingly efficient where one study estimated the metabolic rate to be so low that they only burn 45 calories per day and only require 0.03 kilograms of food per day demonstrating how extreme metabolic depression enables survival in food-scarce bathypelagic zone with single toothfish sustaining 500kg squid for 200 days representing 300-600 fold lower energy requirements than similarly sized ocean predators.

The colossal squid takes something called Kleiber’s law to the next level where Kleiber’s law states that metabolism doesn’t scale linearly with body size as you might predict where a horse that weighs a thousand times more than a mouse doesn’t require a thousand times more energy demonstrating how metabolic rate scales with mass to three-quarters power not linearly representing universal biological scaling law applying from blue whales to individual cells driven by physics and geometry of circulatory systems surface area to volume ratios and fractal blood vessel architecture creating size-dependent efficiency advantage.

Bergmann’s rule states that animals found in cold environments will be larger than those found in warm environments where this rule however has historically only been found to be definitively true for endotherms the warm-blooded animals like birds and mammals where it’s a surprisingly controversial area of science but it’s clear that certain species of ectotherm are indeed larger at colder temperatures demonstrating how temperature-dependent gigantism applies robustly to endotherms for thermoregulation but controversially extends to some ectotherms through different mechanisms possibly including oxygen solubility viscosity effects and reduced metabolic costs.

The Greenland shark is the largest fish in the Arctic ocean and one of the largest sharks on earth seven meters long and weighing as much as fourteen hundred kilograms where it lives at depths over two thousand meters where the water temperature is between negative two and seven degrees Celsius where Greenland sharks are the longest living vertebrates in the world so old that there are likely individuals swimming in the sea that were alive before Christopher Columbus voyaged to the Americas demonstrating how extreme cold temperatures and minimal metabolism enable vertebrate lifespans exceeding 500 years with reproductive maturity delayed until 150 years representing unprecedented longevity achieved through protein carbon dating of non-degrading eye lens tissue.

In the hadal trenches six thousand to eleven thousand meters deep the temperature varies between one and four degrees Celsius where there is no light and the pressure can reach levels eleven hundred times that of the surface where at these depths it seems like life should be impossible but instead the deeper you go the more you’ll find colossal amphipods crawling along the bottom demonstrating how hadal zone extreme conditions paradoxically support largest amphipods ever discovered with Alicella gigantea supergiant reaching 34 centimeters representing 17-170 fold size increase over shallow water 5-20mm relatives showing most extreme bathymetric gigantism gradient known.

These amphipods and isopods are scavengers and detritivores eating any decomposing material they can find where their large body size may help them store as much food and energy as possible when they can find it where relatably when giant isopods find a significant food source they gorge themselves to the point of compromising their locomotive ability demonstrating how extreme fasting capacity enabled by large body energy storage allows giant isopods to survive over 5 years without food in captivity while size also provides travel distance advantage for searching scarce deep-sea food sources across vast abyssal plains.

One hadal amphipod Hirondellea gigas was found to have a unique cellulase enzyme one that seemed like its purpose was to break down plant matter where this was surprising because absolutely zero plant life can live in the hadal depths and the amphipods wouldn’t need this enzyme to digest bits of algae in the form of marine snow demonstrating how pressure-optimized cellulase enzyme enables hadal amphipods to digest driftwood that sinks to deepest ocean trenches converting sawdust and wood pulp directly into glucose with enzymatic activity increasing under high hydrostatic pressure providing unexpected food source in most resource-limited ecosystem on Earth.