Taxonomy

Rarity

The thornback ray is probably the most common ray a Southern California diver swims over, and almost nobody recognizes it. Anglers on Southern California piers catch more thornbacks than any other ray, and the species is described as plentiful and most populous along this stretch of coast. The habitat is the reason. Thornbacks want fine sand and mud in water shallower than 25 feet, which is the bottom under most shore dives between Santa Barbara and San Diego, and they are particularly common on the open sand at the edge of kelp. The International Union for Conservation of Nature assessed the species as Least Concern in 2015. No commercial fishery targets it, and the food value is poor enough that even the anglers who catch them regularly report never having eaten one.

Take is legal under the general California ocean finfish rule of 20 fish in combination with no more than 10 of any one species, and the ray has no minimum size. In practice they are released.

Seasonality

Location

The thornback ray ranges from Tomales Bay in northern California south to the Gulf of California, and it was long considered uncommon north of Monterey. That has shifted. Anglers have reported increasing numbers in the San Francisco Bay Area since about 2010, though no study explaining the change was found.

The bottom it wants is soft. Fine sand and mud off open beaches, inside bays, sloughs and lagoons, and out on the mud flats of coastal bays, where the species is described as abundant. It also holds on the sand beside kelp beds, which is where a diver on a kelp reef is most likely to find one. Depth records run to about 449 feet, but the usual water is much shallower than that, and most encounters happen in the first 25 feet.

A thornback spends most of its time buried. The ray settles onto the bottom and works sediment over its back until only the eyes break the surface of the sand, and it will hold that position while a diver passes directly overhead. They are usually alone or in small groups, and they gather in numbers in shallow bays and off beaches.

The body separates it from the guitarfish it gets confused with, and the published field key for California is unambiguous about how. On a thornback the head and pectoral fins form a heart-shaped disc, the snout is short and broadly rounded, and the back carries 1 to 3 rows of large hook-like thorns running down the midline and onto the tail. Those thorn rows are what the common name describes. The tail is long and stout and carries 2 large dorsal fins set well back, close to the tail fin. On a guitarfish the disc is wedge-shaped instead, the snout is pointed, and a single row of small thorns runs down the back. Anglers separate the 2 on feel as much as sight, and 3 rows of hooks along the spine is not subtle.

Deep Dive

The thornback ray belongs to the electric ray order and cannot generate electricity. That is not an error in the classification. Both FishBase and the World Register of Marine Species place the fanray family inside the order that also holds the true electric rays, and the thornback has no electric organ of any kind. What it does have is the other half of the equipment: ampullary electroreceptors, which are pores in the skin that detect the faint electrical fields produced by the muscles and nerves of living animals. The thornback's are tuned most sharply to signals between 5 and 15 cycles per second, which is the frequency range a buried clam or a burrowing worm produces.

That sense explains how the ray feeds. The diet is small crustaceans, marine worms and mollusks living in the sediment, none of which are visible to a fish gliding above the sand. The ray does not hunt by sight. It reads the electrical signature of an animal buried under the bottom, settles over it and digs. Anglers who catch them report they will take essentially anything found on the bottom, which fits a predator that finds food by detecting that it is alive rather than by recognizing what it is.

The thornback is harmless and this is the fact worth carrying into the water. It has no venomous spine, no sting and no defense beyond the thorns and the ability to disappear into sand. That distinction matters most in exactly the conditions where it is hardest to make, because the round stingray shares the same beaches and the same shallow sand, and a round stingray does have a spine and does use it. Pier anglers fishing at night are told to keep a good light for precisely this reason: to tell a non-harmful thornback from a round stingray before reaching for it. A diver shuffling into the shallows at a Southern California sand beach is running the same identification problem in the dark, and the shuffle-the-feet advice for stingrays covers both animals whether or not the identification gets made.

Reproduction is live-bearing. The young develop inside the mother, first on yolk and then on a nutrient-rich fluid secreted by the wall of the uterus, and litters run from 1 to 15 pups. Births are concentrated, arriving mainly in August, which means the shallow sand off Southern California beaches holds newborn thornbacks in late summer. Maturity begins at about 19 inches and the species reaches roughly 3 feet, though most caught from piers are under 2 feet and weigh 6 pounds at the outside. No maximum age has been published.

There is one more thing the name does, which is cause confusion. In Britain a thornback is a skate, a different fish in a different order, and this species is also sold and written about as the thornback guitarfish, which puts it in a third group it does not belong to. On this coast the animal with 3 rows of hooks on a heart-shaped disc, buried in the sand under the kelp, is the thornback ray.

Where to Find Them

Beaches whose habitats match this species

Matched by habitat: a beach is listed when its biome tags overlap this species' biomes.