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. The pelvic fins have curved outer margins and angular rear tips; males have long claspers. The tail is robust and much longer than the disc, with lateral skin folds running along either side. The two dorsal fins are similar in size and shape, being longer than tall with rounded posterior margins. The first dorsal fin lies closer to the caudal fin than the pelvic fins. The caudal fin is well-developed and almost elliptical, without a distinct lower lobe. …

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. Thornback guitarfish mate in late summer, and females give birth the following year at around the same time, peaking in August. It is aplacental viviparous, with developing embryos sustained until birth by yolk. Females bear litters of 1–15 pups every year; the newborn rays measure about 11 cm (4.3 in) long. Males and females reach sexual maturity at 37 and 48 cm (15 and 19 in) long respectively.