Looking like something out of a science fiction movie, the anglerfish uses a natural lure to draw its next meal nearer.
Photograph by Bruce Robison/Corbis

The angry-looking deep sea anglerfish has a right to be cranky. It is quite possibly the ugliest animal on the planet, and it lives in what is easily Earth’s most inhospitable habitat: the lonely, lightless bottom of the sea.

There are more than 200 species of anglerfish, most of which live in the murky depths of the Atlantic and Antarctic oceans, up to a mile below the surface, although some live in shallow, tropical environments. Generally dark gray to dark brown in color, they have huge heads and enormous crescent-shaped mouths filled with sharp, translucent teeth. Some angler fish can be quite large, reaching 3.3 feet (1 meter) in length. Most however are significantly smaller, often less than a foot.

Their most distinctive feature, worn only by females, is a piece of dorsal spine that protrudes above their mouths like a fishing pole—hence their name. Tipped with a lure of luminous flesh this built-in rod baits prey close enough to be snatched. Their mouths are so big and their bodies so pliable, they can actually swallow prey up to twice their own size.

The male, which is significantly smaller than the female, has no need for such an adaptation. In lieu of continually seeking the vast abyss for a female, it has evolved into a permanent parasitic mate. When a young, free-swimming male angler encounters a female, he latches onto her with his sharp teeth. Over time, the male physically fuses with the female, connecting to her skin and bloodstream and losing his eyes and all his internal organs except the testes. A female will carry six or more males on her body.

This spiny turtle, like other turtles, has an upper shell that forms as ribs widen and fuse into a bony plate.
Never mind Aesop and his fables. Japanese scientists are telling a new story of how the turtle got its shell. A shield from the elements and from predators, as well as a mineral reserve in low-oxygen environments, the turtle’s shell is unique in vertebrate anatomy. Still, a turtle’s embryo starts out looking like any spined animal’s—say, a chicken’s or a mouse’s. But about a third of the way through in-ovo development, says Shigeru Kuratani of the Riken Center for Developmental Biology, “an anatomical rule is violated” that remaps the animal’s physique. The ribs grow over the shoulder blades instead of under them as humans’ do, forcing the body wall to fold in on itself. What would have been an internal rib cage fuses into a bony plate under the skin and becomes a part of the turtle’s outer armor. In 2008 the fossil record delivered elegant support for this theory—and for another, more disputed one: that shells evolved from the bottom up. With a belly plate but an incomplete upper shell, 220-million-year-old Odontochelys semitestacea, found in China, seems an in-between form—one that looks a lot like an early stage in modern turtle development. More bony finds may someday tell the rest of the turtle’s story. —Jennifer S. Holland

Photo: Joel Sartore. Art: Hiram Henriquez
Source: Hiroshi Nagashima, Riken Center for Developmental Biology