Thornback Guitarfish

The pectoral fin disc of the thornback guitarfish is heart-shaped, slightly longer than it is wide, and thick towards the front. The snout is short and broad, with a blunt tip protruding slightly from the disc. The eyes are small and widely spaced; the spiracles are larger than the eyes and lie closely behind. The wide nostrils are preceded by moderately large, broad flaps of skin. The mouth is wide and gently arched; there are a pair of creases running from the mouth corners to the nostrils, enclosing a roughly trapezoidal area. The lower lip is inscribed by a deep furrow that wraps around the mouth corners. The small teeth have low crowns that may be sharp to blunt, and are arranged in 68–82 rows in the upper jaw and 64–78 rows in the lower jaw. The five pairs of gill slits are small and located beneath the disc.

Endemic to the northeastern Pacific Ocean, the thornback guitarfish is found from Tomales Bay to Magdalena Bay, with additional isolated populations in the Gulf of California. It is reportedly very abundant in some coastal waters off California and Baja California, such as in Elkhorn Slough, and uncommon north of Monterey and in the Gulf of California. Bottom-dwelling in nature, this species is typically found close to shore in less than 6 m (20 ft) of water, though it has been recorded from as deep as 137 m (449 ft). It inhabits coastal habitats with muddy or sandy bottoms, including bays, sloughs, beaches, and lagoons, and can also be found in kelp beds and adjacent areas.

During the day, the thornback guitarfish spends much time partially buried in sediment. It may be encountered singly, in small groups, or in large aggregations that form seasonally in particular bays and sloughs. The diet of this ray consists of polychaete worms, crustaceans (including crabs, shrimps, and isopods), squids, and small bony fishes (including anchovies, sardines, gobies, sculpins, and surfperches). It can detect prey with its electroreceptive ampullae of Lorenzini, which are most sensitive to electric fields with a frequency of 5–15 Hz. In turn, the thornback guitarfish is preyed upon by sharks and the northern elephant seal (Mirounga angustirostris). Known parasites of this species include the tapeworm Echinobothrium californiense and the nematode Proleptus acutus.

Harmless and docile, the thornback guitarfish can be readily approached underwater, and fares well in public aquariums. Off the United States, this ray is common and faces no substantial threats: it is only occasionally caught incidentally by commercial and recreational fishers, and has no economic value. As most of its range lies within US waters, the species has been assessed as Least Concern overall by the International Union for Conservation of Nature (IUCN). However, in Mexican waters the thornback guitarfish population is small and fragmented, and the degree to which it is affected by fishing is uncertain. There, the IUCN has listed it locally under Data Deficient while noting its susceptibility to inshore lagoon fisheries and shrimp trawlers, and the urgent need for additional information to ensure its long-term regional survival.