When you’ve been diving as long as I have (31 years) you may think you’ve seen it all. Then something happens that proves you’ve just begun to unravel the sea’s mysteries. On an August trip to San Clemente Island, that happened to me. Indeed, it happened to an entire boatload of divers. It was a beautiful, sunny weekend with flat calm, wonderfully warm (67 degrees) water and visibility of about 40 feet. I’d been in the Encore’s galley, when I came out on deck Guillaume Chanfreau was holding up a skeleton. He’d found it lying on the sand at the edge of a kelp bed in about 60 feet of water. After videotaping it, he brought it back to the boat, hoping someone could tell him what it was.

Nobody could, not even experienced boat captain, Pam Driver, who knows everything. Speculation, however, abounded. On first glance, I thought the skeleton might be that of a wolf eel. I quickly discarded that idea. Wolf eels aren’t normally found off Clemente. Even more important, the head was the wrong shape. Maybe it was from a bat ray.

Some on board thought the skeleton was from a sea lion and the head wasn’t a head at all but a pelvis. I couldn’t think of a sea creature that had a tail that long. I took several photos of the remains and, when I got back home, e-mailed one of them to Guillaume. He’s an assistant professor of chemistry and biochemistry at UCLA, where there are people who could solve our puzzle.

Not long after I sent the photo, Guillaume forwarded an e-mail from Peter Adam, a Ph.D. candidate in ecology and evolutionary biology who often teaches a lab in shark anatomy. Peter said, “Looks to me as though the skeleton is that of a fairly large shark (six feet or so), but I wouldn’t be able to get it down to species without a closer look. The thing at the top is a chondrocranium — the cartilage that surrounds the brain — and the rest is a partial vertebral column.”

That blew me away. Several years ago I wrote a series of articles on sharks. I acquired books recommended by shark experts and spent hours pouring through them. There were no photos of a shark skeleton in any of those books. While I knew sharks had “cartilaginous” skeletons, I had no idea what they looked like. It never occurred to me their spinal columns would resemble ours.

According to Richard Ellis, writing in the Book of Sharks, “…a shark has no bones. The skull, spinal column, and some fin supports are all cartilaginous, and since there is no rib cage or other supporting structure, the shark is held together… by its muscle and skin.”

After it was photographed and examined by the curious, the smelly, slimy skeleton was returned to the sea. Peter Adam thinks it was probably from a leopard shark, but we’ll never know for sure. Triakis semifasciata is a member of the Family Triakidae (houndsharks) and Order Carcharhiniformes (ground sharks). Leopards are found off the West Coast from Oregon to Baja California, including the northern Sea of Cortez, usually over sand or mud bottoms in 20 feet of water or less. During certain times of the year, these sharks have congregated in the shallow waters of Big Fisherman Cove, Catalina, a short walk from the Hyperbaric Chamber. Congregations have also been sighted off San Diego beaches.

Leopards are easily spooked and avoid divers. They have never been known to attack humans. While leopard sharks can reach six feet in length, five feet is more common. They have sharp, three pointed teeth and their favorite foods are crabs, shrimp, bony fish, fish eggs, clam necks and innkeeper worms (something else I’ve never seen). The young develop in eggs inside the female and are born alive after they hatch.

I have never encountered a leopard shark while diving but I have seen them underwater — these sharks do very well in captivity. The photo of the live shark that illustrates this article was taken in Monterey Bay Aquarium.