Conservation Status:  Safe for Now

AquaticThornback

Platyrhinoidis triseriata Cartilaginous FishesRays

Aquarium of the Pacific - Online Learning Center - Species Print Sheet

Conservation Status:  Safe for Now

AquaticThornback

(Platyrhinoidis triseriata)Cartilaginous Fishes • Rays

Thornback
Thornback at Elkhorn Slough, California | Mischa Lochton/Pelagic Shark Research Foundation
Thornback
Thornback at Elkhorn Slough, California | Mischa Lochton/Pelagic Shark Research Foundation

Species Overview

There are three species of thornbacks but only one, Platyrhinoidis triseriata, is found in California where it is a native fish, often called the. California or Pacific thornback. A thornback is a small to moderate sized, inshore fish that while distantly related to rays, is more closely related to guitarfish. For this reason the preferred common name of this species is thornback, not thornback ray. The name is derived from the sharp protective spines on the back and tail. These elasmobranchs are plentiful in southern California where they are frequently caught by both sport and commercial fishers.

Species In-Depth | Print full entry

At the Aquarium

Currently, we do not have thornback rays. This information about the tthornbacks is supplied for readers interested in the species.

Geographic Distribution

Central eastern Pacific coast from Monterey Bay, California to northern Baja California, Mexico. Most common in southern California, sometimes seen in central California, rare in northern limits of range. Scattered groups in Gulf of California.

Habitat

Although reported to a depth of 137 m (450 ft), thornbacks are most commonly found in inshore water 7.6 m (25 ft) or less in depth. They are usually found in bays, lagoons, coastal sandy beach areas, sloughs, and in and around kelp forests. They prefer sandy or mud bottoms in temperate and subtropical waters.

Physical Characteristics

Thornbacks have a flat head and body. The disk-shaped, short, broad body and pectoral fins together create a distinctly heart-shaped form. The tail is sturdy and somewhat flattened terminating in an oval shaped caudal fin with essentially no lower lobe. An identifying characteristic is a pattern of one to three parallel rows of very sharp, pale, curved or hooked, prominent spines or thorns that extend from the back to usually well down the tail. Their spines are not venomous.

The upper surfaces of these fish are generally an olive drab color, but may be brownish. Their underside is creamy white. These colors allow the animal to blend with colors of the substrates it most commonly occupies.

Size

Maximum length for this fish has been reported as 91 cm (35.8 in). Most specimens are much smaller, not often exceeding 61 cm (24 in).

Diet and Feeding

These cartilaginous fish are opportunistic feeders. Their diet includes a large range of food items. Shrimp, squid, crabs, clams, and a variety of worms make up most of their invertebrate diet. They also feed on a number of small fishes including sculpin, sardines, anchovies, surfperch, and gobies.

Reproduction

Males become sexually mature at about 37 cm (14.5 in) and females at about 48 cm (18.9 in). Thornbacks breed in late summer and birth takes place the following summer, most commonly in August.

Females are ovoviviparous. One to fifteen eggs are deposited in sand or mud. The eggs are oblong capsules with stiff pointed horns at the corners. Pups are 11 cm (4.3 in) long when they emerge from the egg capsule. They do not receive any maternal care.

Behavior

Thornbacks are demersal spending most of their time on or near the bottom. They are frequently observed partially covered or almost buried in the substrate. They burrow into or cover their bodies with bottom material, remaining partly buried with only their spines exposed. They are known to congregate in large numbers in favorite areas in certain coastal bays and sloughs.

They are not especially shy and will permit divers to come quite close as long as there is no apparent threat.

Adaptation

They are color adapted to their environment with earth tones on the upper side of the body and white on the bottom side (countershading). (See photo). They blend well with the substrate on which they most commonly live.

They avoid taking silt into the gill chamber while buried in sand or mud by breathing through their spiracles on the top of the head instead of their nostrils located on the underside.

Longevity

Their life span is believed to be about 15 years.

Conservation

There has never been a commercial fishery targeting this species and it has no commercial value although thornbacks are reported to be sold in fish markets as skate. They are subject to bycatch in other fisheries and are sometimes caught by sport fishers from piers and boats. Thornbacks are edible but most specimens are so small they are rejected as impractical as a food source.

Small sharks and northern elephant seals are known to be predators and it is likely that some larger fishes feed on this species. The dorsal spines may discourage some otherwise interested foragers.

At the present time there are no conservation measures in place. The IUCN assesses the California population of this species as Least Concern. The Mexican population is listed as Data Deficient.

Amazing Facts

Thornbacks are true living fossils. They have remained almost unchanged for thousands of years.

Although thornbacks are a native California fish and common in many areas of southern California coastal waters, little is known about their biology, ecology, and population dynamics. The Pelagic Research Foundation located in Santa Cruz, California is conducting research on thornbacks at Elkhorn Slough near Monterey, California.