Thornback

Thornback, Platyrhinoidis triseriata 

The Thornback, Platyrhinoidis triseriata, whose common Spanish name is guitarra espinuda, is a member of the Thornback or Platyrhynidae Family, known collectively as guitarras espinudas in Mexico. This fish is also known as the California or Pacific Thornback. Its common name stems from the sharp spines on its back and tail. Globally, there are two members in the Platyrhynidae Family with the fish described herein being the only species in the genus Platyrhinoidis.

The Thornbacks have broad, oval, flattened, heart-shaped discs of approximately equal depth and length; their tail is also about the same size as the body length. They are uniformly gray-brown to olive-brown dorsally and off-white ventrally, and can adapt to blend into their surroundings. Their head has a blunt snout, very small eyes set close together, and large spiracles behind the eyes on the upper surface. Their mouth, nostril openings, and gill slits are on the underside. They are covered with non-venomous spines including 4 at the snout tip, 4 or 5 around each eye, small patches on each shoulder, and 3 rows of large hooked spines that run the length of the tail with the middle one extending onto the disc behind the head. They have two equally-sized dorsal fins and a thick tail with an elongated oval fin that lacks a lower lobe.

The Thornbacks are inshore fish found demersal over sandy and muddy bottoms in bays, lagoons, beach areas, sloughs, and in and around kelp forests at depths up to 450 feet. They reach a maximum length of 91 cm (36 inches). They can be found in large schools and spend the majority of their time half-submerged with only their eyes exposed. They feed on benthic invertebrates (clams, crabs, shrimp, squid, and worms) and small fish (anchovies, gobies, sardines, sculpin, and surfperch). They are preyed upon by large fish, small sharks, and Northern Elephant Seals. Reproduction is oviparous with each female producing 1 to 15 oblong eggs that are encased in capsules (Mermaid’s Purses) and attach themselves to the bottom substrate. Offspring measure 11.0 cm (4.3 inches) in length and are born alive as miniature adults. They have a lifespan of up to fifteen years. They are poorly studied and very little is known about their biology, ecology, and population dynamics.

In Mexican waters the Thornbacks have a limited distribution being found along the entire west coast of Baja and throughout the Sea of Cortez. They are absent from coastal waters from Mazatlán southward along the west coast of the mainland south to Guatemala.

The Thornback is most likely confused with the Banded Guitarfish, Zapteryx exasperata (pointed head profile; one row of thorns down middle of back).

Due to their small stature, the Thornbacks are not a commercial fishery target. They are a by-catch of other fisheries and are sold fresh as “skate”. They are also caught with some frequency from shore and off piers by recreational fishermen. From a conservation perspective they are considered of Least Concern. The Thornbacks are scientifically interesting as they are considered living fossils having remained unchanged for thousands of years.

f695-thornyback-1

f695-thornyback-2Thornback, Platyrhinoidis triseriata. Fish caught off the Oceanside Pier, Oceanside, California, April 2008. Disc width: 13 cm (5.2 inches); Disc length: 14 cm (5.7 inches); Total length: 28 cm (11 inches); Tail: 13 cm (5.2 inches).