I couldn’t believe my luck. I had just reached the bottom at 20 feet and was cruising toward the kelp forest in deeper water when I saw a prehistoric looking creature lounging on the sand. It was light brown, with a distinctive flat, heart shaped head and a long tail. There were three rows of spines on its back. The animal, which reminded me of an armored vehicle, was a thornback.
The creature tolerated having its photo taken for several minutes before moving on. (Yes, I know, paparazzi can be annoying!)
Thornbacks (Platyrhinoidis triseriata) are fish — cousins of sharks — with skeletons made of cartilage, not bone. They are members of the Family Rhinobatidae, which includes guitarfish. The thornback’s genus name, Platyrhinoidis, comes from the Greek platys, meaning broad, and rhinos, meaning snout. The species name, triseriata, is Latin for three rows and refers to the parallel rows of “thorns” that run from the middle of the creature’s back to just before its tail. While not venomous, the sharp spines, which look like the thorns found on rose stems, make these fishes less appetizing to some predators, though they don’t seem to deter sharks and northern elephant seals. The long tail has two dorsal fins and ends with a paddle-like caudal fin.
Thornbacks only get to be about three feet long and most are smaller. They are thought to live about 15 years. Though these fish aren’t aggressive, they also aren’t afraid of divers. And while they aren’t rare, they are rarely seen by divers because they spend most of their time in water less than 25 feet deep, “…on the mud and sandy bottoms of bays, sloughs, and coastal beaches and around kelp forests,” according to David A. Ebert’s Sharks, Rays and Chimaeras of California. A SoCal U/W photographer has reported seeing thornbacks on night dives off Redondo Beach.
Two of the books I consulted for research on thornbacks pointed out they are considered living fossils and have remained almost unchanged for hundreds of millions of years. They are found mainly from Monterey Bay to Baja and are most populous off Southern California. Like bat rays, large numbers of thornbacks congregate in certain areas in the summer, most probably for mating and pupping. Fertilization is internal and takes place in late summer, with the pups born live a year later. Thirteen baby thornbacks were born at the Chula Vista Nature Center in December 2009 and photos of them are posted on the internet. The four-inch long pups are miniature replicas of their parents and unbelievably cute. While adults have white undersides, those of the babies are pink and white.
Thornbacks are nocturnal, lying partially buried in the sand or mud during the day. Besides small fish, they also eat worms, mollusks and crustaceans, which they dig up with their snouts. They have small, pebblelike teeth.
Although quite a few researchers have studied thornbacks, little is known about their habits.
Some identification books claim these creatures are occasionally caught by fishermen; others say they are frequently caught by fishermen. I have no clue which of these statements is correct.