Sunday, October 31, 2010

Deep-Sea Gigantism

Every child has heard of the great land animals: elephants, giraffes, anacondas, hippopotamuses. But few know of the deep sea creatures that are equally as strangely large. It's a phenomenon called deep-sea gigantism.

It's a manifestation of several factors. The first is the water itself. Water weighs one gram per cubic centimeter, which means that an animal like a fish, which also weighs approximately 1 g/mL, is effectively weightless. It does not have to support its own weight like a land animal does, thereby removing one of the largest barriers to scaling - the square-cube law. An organism's weight goes up with the cube of its size, but the cross section of its skeleton only goes up with the square of its size. Thus, the larger the animal, the larger its bones must be to support its own weight in air. But, marine animals are mostly except from the rule. A 100-foot blue whale does not require the immense bones of a 100-foot sauropod dinosaur like Seismosaurus. The whale has bones only a few inches thick; the dinosaur's leg bones are over a foot thick.

Other physical factors also come into play. Scarcer food and nutrients at great depth mean sexual maturity is delayed, meaning the organism will grow larger before its growth stops at maturity. The freezing-cold temperatures at depths of thousands of feet mean that larger animals have advantages in body temperature regulation and reducing the need to constantly keep moving - thus requiring fewer resources.

Some organisms also manage to grow large because they can use the massive energy available from hydrothermal vents called black smokers. Giant tube worms can reach almost 8 feet long (2.4 meters), tolerate extremely high levels of hydrogen sulfide (H2S) from vents, and survive depths up to several miles deep. They're basically giant cylinders that house bacteria which create nutrients like oxygen and carbon dioxide in a process called chemosynthesis, which the worm then feeds upon. The red tip of the worms contains a specialized hemoglobin that can carry oxygen with sulfides in the environment - most hemoglobins cannot.

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