13,000 to 20,000 feet (4,000–6,000 m)
Translated literally from the Greek, abyssopelagic means “bottomless sea.” Abyss is one of the more evocative words in the English language to describe an unfathomable chasm, a yawning gulf, an immeasurably profound depth or void. At 2½ to almost 4 miles (4–6 km), the abyss is extraordinarily deep, though it is still not the deepest part of the sea.
The abyss spans a vast area of the ocean bottom—the majority of the abyssal hills (the single most common geological feature on Earth) and the abyssal plains. Together, they comprise much of the ocean bottom—all but the ridges, which are the extensive volcanic areas bordering all the plates, and the trenches, which dip well below the abyssal plains in the western Atlantic and Pacific oceans to depths below 20,000 feet (6,000 m).
Here, we begin the zone of the tiny and no-eyed monsters. Despite the steadily diminishing size of many fish and invertebrates, however, some invertebrates show the opposite trend and become larger. The deep-water giant red mysid Gnathophausia ingens reaches a length of nearly 14 inches (35 cm), fifteen times the size of mysids in the upper waters. The isopod Bathynomus giganteus grows to 16½ inches (42 cm), comparatively gargantuan proportions, while the amphipod Alicella gigantea can be 7½ inches (19 cm) long. The copepod Gaussia princeps achieves a size of slightly less than half an inch (1 cm), nearly 10 times the size of the average free-swimming calanoid copepod near the surface. Bright red shrimp can be 12 inches (30 cm) long, with antennae twice that length. Some sea urchins on the seafloor at this depth are 12 inches (30 cm) in diameter, up to 10 times the diameter of shallow-water species.
Gigantism may arise for several reasons. First, scarce food and low temperatures reduce the growth rate and increase both the time required for sexual maturity and longevity, leading to larger size. Second, gigantism may simply represent a peculiarity of metabolism at high pressure.
The ultimate textbook example of gigantism, however, is Architeuthis dux, the giant squid. The scientific name says it all. The Greek word arkhi means “chief” or “most important” and teuthis means “squid,” and the Latin dux means “leader”; thus “chief squid leader.” While some squid species are merely an inch (2.5 cm) long, the giant squid has taken gigantism to an awesome evolutionary end. The longest of the relatively few giant squid that have washed ashore was a 57-foot (17.4 m) female from New Zealand waters, although researchers caution that the measured length can vary depending on the elasticity of the tentacles. Thought to be the largest of all invertebrates in length, this mollusk can weigh up to about a ton (900 kg). Its eight shorter arms have hundreds of suckers, and its two nearly 40-foot-long (12 m) tentacles each boast at least a hundred serrated suckers at the end.
Gigantism may arise for several reasons. First, scarce food and low temperatures reduce the growth rate and increase both the time required for sexual maturity and longevity, leading to larger size. Second, gigantism may simply represent a peculiarity of metabolism at high pressure. In any case, natural selection almost certainly plays a role. Large size, delayed sexual maturity and a longer life would all be useful to a deep-sea creature. Larger young could avoid predators, feed on a wider range of food sizes and search over a broader area for food and mates, and greater longevity would give the animals more time to find mates.
However, some squid achieve large sizes rapidly and die relatively young. The giant squid’s presumed maximum longevity of no more than five years exceeds some other squid species but seems brief relative to a whale’s 50- to 200-year life span. A whale might produce only one calf every two to five years over a few decades, depending on the species. During its short life, the giant squid may be extraordinarily fruitful, producing thousands of offspring, as do other squid.
Gigantism affects only certain species at this depth. Most invertebrates in the deep are smaller than their shallow-sea counterparts. In fact, the overall trend in the deeper parts of the sea and on the bottom is toward miniaturization and often extreme miniaturization. In recent years, oceanographers have begun collecting specimens using 0.3-millimeter screens, noting that few species appear in 1-millimeter screens, which are much finer than the dredges of old. Despite the tales of sea monsters, the deep sea is mainly a “small organism” habitat.
It is eerily quiet down here. The noise that rattled the iMonstercam’s hydrophone in the deep-water sound channels and the distant cries of whales now appear faint.
In terms of pressure, we are moving into the land of collapsed vehicles and shrunken heads. The pressures that seem to force many creatures to stay small are evident in the “experiments” conducted by deep-sea oceanographers. Some oceanographers who spend a lot of time at sea dropping research packages over the sides of ships attach a piece of Styrofoam and send it down to see how small it is when the package is hauled back to the surface. Of course, the deeper the package is dropped, the smaller the Styrofoam when it returns. Styrofoam cups turn into twisted thimbles.
The joke became a lesson in physics, as seasoned oceanographers began to initiate their graduate students in the ritual of measuring, attaching and then dropping the Styrofoam. The results soon attained the status of prized souvenirs, coveted by young researchers eager to show how deep their expedition had plumbed. Some oceanographers bring along a Styrofoam head or two, the sort used to display hairpieces and hats, demonstrating what the deep does to something the size of a human head. The result—equivalent to the shrunken heads that New Guinea natives once collected—often produces nervous laughter as the tiny head is pulled up from the deep.
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