Why the Earth Has Fewer Species Than We Think

In 2012, I ran a trip sampling the nautilus populations along Australia’s Great Barrier Reef explicitly to see if nautiluses living on marine protected areas of the reef are as rare as from places where they are fished for their pretty shells (such as in the Philippines and Indonesia). Work along the Great Barrier Reef in the 1990s had shown that two different and accepted species are present. One, Nautilus pompilius, is the most widespread of all the nautiluses across their vast Pacific and Indian Ocean range. The second, Nautilus stenomphalus, is found only on the Great Barrier Reef. It differs from the more common N. pompilius in having a hole right at the center of its shell. (In N. pompilius, there is a thick calcareous plug.) There are also marked differences in shell coloration and pattern of stripes on the shell. But when the Australian species was first brought up from its 1,000-foot habitat alive, in the late 20th century, scientists were astonished to find that N. stenomphalus has markedly different anatomy as well on its thick “hood,” a large fleshy area that protects the interior guts and other anatomical soft parts when the animal pulls into its shell. In N. pompilius the hood is covered with low bumps of flesh, like warts. Meanwhile the N. stenomphalus hood is covered with a forest of brushy projections that rise above the hood like a thick carpet of twiggy moss, or tiny trees of flesh; the coloration of the hood is also radically different.

The 2012 trip was to sample the DNA of the two “species” as well as to better understand how many nautiluses and the morphologically different was identical. No genetic difference, yet radically different morphology. The best way to interpret this is to go back to one of the most useful analogies in evolution: of a ball rolling down a slope composed of many gullies. Which gully the ball rolls down (corresponding to the ultimate anatomy or “phenotype” of the grown animal) is controlled by the direction of the push of the ball. In evolution, the ultimate morphological fate of an organism is caused by some aspect of the environment the organism is exposed to early in life—or, in the case of the nautiluses, while they slowly develop in their large egg over the course of an entire year before hatching. Perhaps it is a difference in temperature. Perhaps it is forces that the embryo encounters prehatching, or when newly hatched, the small nautiluses (one inch in diameter, with eight complete chambers) find different food, or perhaps they are attacked and survive, i.e., have two different kinds of predators. That’s why and are not two species. They are a single species with epigenetic forces leading to the radically different shell and soft parts. Increasingly it appears that perhaps there are fewer, not more, species on Earth than science has defined.