Thornback, Platyrhinoidis triseriata 

Thornback, 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).

Thornback, Platyrhinoidis triseriata. Fish caught off San Diego, California, June 2013. Total length: 66 cm (2 feet 2 inches). Tail: 13 cm (5.2 inches). Catch, photograph and identification courtesy of Eli (obsessiveangling.wordpress.com).

Thornback, Platyrhinoidis triseriata. Fish caught within San Diego Bay, San Diego, California, Octobert 2021. Total length: 68 cm (2 feet 3 inches). Catch, photograph and identification courtesy of Chris Moore, Peoria, Arizona.

Thornback, Platyrhinoidis triseriata. Fish caught in coastal waters off north of Guerrero Negro, Baja California, August 2017. Total length: 70 cm (2 feet 4 inches). Catch and photograph courtesy of Barry Mastro, Escondido, California.

The Thornback, Platyrhinoidis triseriata, is a member of the Thornback or Platyrhynidae Family, that is also known as the California Thornback and the Pacific Thornback and in Mexico as guitarra espinuda. 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 Thornback has a 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 2 equally-sized dorsal fins and a thick tail with an elongated oval fin that lacks a lower lobe.

The Thornback is an inshore demersal fish found over sandy and muddy bottoms in bays, lagoons, beach areas, sloughs, and in and around kelp forests at depths up to 135 m (450 feet). They reach a maximum of 91 cm (3 feet 0 inches) in length. 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 lifespans of up to 15 years. The Thornback is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.

The Thornback is a resident of Mexican waters of the Pacific Ocean but has 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, Sinaloa, 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).

From a conservation perspective the Thornback is currently considered to be of Least Concern. They are scientifically interesting as they are considered living fossils having remained unchanged for thousands of years. Due to their small stature, the Thornback is 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.