One of the most intriguing images caught by the Okeanos Explorer was this portrait of a ghost shark, or chimaera*. Sporting a mask-like face that seems to combine the most haunting characteristics of the spook and demon bunny from Scream and Donnie Darko, this animal is one that would surely strike fear in any heart were one to encounter it mano-a-fisho at 4,500 feet. At the same time, there is a sculptural beauty in its features that is hard to define and rarely encountered elsewhere in the vertebrates, or backboned animals. And those eyes — those ghostly eyes . . . not to mention the spiked retractable sexual organ on their foreheads.
What makes the real-life chimaera extraordinary is its place in the tree of jawed fishes: it may be the most anciently formed lineage around today. Chimaeras split from sharks sometime in the Paleozoic, the several hundred million year stretch preceding the Mesozoic, or age of dinosaurs. It happened sometime before the Permo-Triassic extinction 250 million years ago (the greatest Earth has ever seen) that ended the Paleozoic, and the Cambrian explosion, “modern” life’s great diversificiation, which took place some 550 million years ago.
Specifically, it happened during the Devonian (ca. 420-360 million years ago), a time of great experimentation, a time when early armor-plated fishes like now-extinct placoderms roved the seas. It was just at this time that three major groups of jawed fish — the placoderms, cartilaginous fishes (chimaeras are in here) and ancestors of ray-finned fish (most everything else) — evolved from jawless fish that might have resembled lampreys or hagfish. The resulting group, the gnathostomes (lit. jawed mouths), represented a major milestone in vertebrate evolution. Jaws are great for crunching and munching. That is, for grasping relatively large prey so it can’t get away while you’re trying to eat it, and for magimixing your prey once teeth evolve on the jaw. The previous, less-enticing system can be summarized (not entirely, but relatively closely) by its section title in my college biology text: “Sucking Mud: The Rise of the Vertebrates”.
As cartilaginous fishes, chimeras share several features with the sharks and rays. All cartilaginous fishes, or Chondrichthyans (sharks, rays, and chimaeras), possess a cartilaginous skeleton**, though it seems they once possessed bone. For one, their sister group, the extinct placoderms, were armed to the teeth in bone (As an aside, the armoring of fishes seems to have taken the opposite path of the evolution of European armor: fish seem to have started with large bony plates and evolved toward the mail-like coats of scales seen today***. Contrast this with sharks, who are literally armed with teeth, since their placoid scales and all vertebrate teeth evolved from the same ancestral structure). And second, cartilaginous fishes (and all jawed fishes) seem to have evolved, like the placoderms, from bony jawless fish.
Chimaeras are the most primitive (i.e., split off from the rest of the groups earliest) of the cartilaginous fishes. They are crunchers of hard food, possessing, instead of rows of bristling, disposable razor blades like sharks, permanent bony plates which they use for crushing mollusk shells like nutcrackers. The vast majority are found in deep, dark water far from coasts.
Most notably, males of these species don’t possess a penis, but they do possess an arsenal of other interesting sexual organs. Like sharks, near their genital opening they possess a pair of “claspers”, which they use to grip the female in the pertinent location, and which have grooves for funneling sperm. Some chimaeras take this one step further by deploying what can only be described as a medieval-looking cephaloclasper that expands and retracts from the tops of their heads (view all these delightful accoutrements here). To my knowledge (which is very small), no one has actually ever seen this thing in use, so speculations on its actual purpose and function are just that. It is a good thing men don’t have such a thing, since they’ve already got enough problems with the ones they’ve got getting stuck in zippers, hotel rooms that rent by the hour, weekend “hikes on the Appalachian trail”, etc. If they had spiked, retractable sexual organs on their foreheads. . . well, God help us all.
Adding to the thrill of what must certainly be some of the most interesting sex in the vertebrates is the added difficulty that each one of these babies carries a spine in front of their dorsal fin loaded with a venom sac at its base. Which means the joke must go: How do chimaeras mate? Answer: Very. Carefully.
The equally fascinating product of all this maneuvering is something that is also known from the world of sharks: beautiful, leathery egg cases (see longitudinal view, left), also called mermaid’s purses. They are deposited on the seafloor where they harden and darken as the young chimaeras grow within. Some sharks have developed vivipary, or live birth, like humans, but that does not seem to have happened yet for chimaeras, and I can’t blame them. I know I’d rather lay a nice, rounded egg case and kick back with a margarita to watch on the blessed day. Stupid natural selection.
*Those who have studied Greek mythology will recall that the classical chimaera was a fire-breathing lioness with a snake for a tail and a goat’s head improbably sprouting from its back. Today the term is used in biology for individuals with cells that originally came from two different zygotes, or fertilized eggs. How or why the ghost shark acquired this name I have not been able to discover.
** but they do have calcified stiffenings of their backbones that function like vertebrae
*** Alive without breath,
As cold as death;
Never thirsty, ever drinking,
All in mail never clinking.
The Variety of Life (Tudge)
Life (Purves, Orians, and Heller, 4th ed.)