Smoothtail Mobula, Mobula thurstoni
The Smoothtail Mobula, Mobula thurstoni, whose common Spanish name is manta doblada, is a species in the Eagle Ray or Myliobatidae Family, known collectively as águilas marinas in Mexico. This fish is also known as the Bentfin Devil Ray. Globally, there are 12 species in the genus Mobula, of which five are found in Mexican waters, one in the Atlantic, three in the Pacific, and one in both oceans.
The Smoothtail Mobulas are moderately-sized devil rays. They are dark gray to olive-gray dorsally, white ventrally, and silvery near their wing tips with a dark greenish patch usually visible on each side near the posterior edge of their pectoral fins. Their dorsal fin has a white tip. Their head is projecting and relatively short with a pair of horn-like projections on either side. Their mouth is underside and equipped with teeth set in bands on both jaws forming a low pavement-like mosaic. Their disc is slightly less than twice as wide as it is long with the pectoral fins forming large triangular shaped “wings”. Their dorsal fin is small and found at the base of the tail with its height equaling 80% of its length. Their pectoral fins are broad and triangular and have a prominent double bend at the front margins and concave rear margins. Their tail is moderately long, whip-like, and approximately 60% of disc width and has a flattened base. Tail length versus disc width is an important marker for accurate identification, however, many mobulas lose parts of their tail fairly frequently, which lessens the usefulness of this parameter. Their upper disc is sparsely covered with small blunt denticles. They have 15 to 20 filter plates around each gill cover. They do not have a tail spine or a caudal fin.
The Smoothtail Mobulas are found circumglobally in both coastal tropical and subtropical oceanic waters at depths up to 330 feet. They reach a maximum disc width of 1.90 meters (6 feet and 3 inches) and can weigh up to 54 kg (120 pounds). They travel pelagically as single fish or in large groups of several hundred individuals. They are known for their acrobatics above water, including somersaults. They segregate by size and sex during the winter months and feed on planktonic crustaceans. In Mexico, they most commonly consume Krill, Nyctiphanes simplex. The southern Sea of Cortez has been documented as an important feeding and mating location for this species. Reproduction occurs via ovoviviparity with internal fertilization. Each female has one pup per annum measuring 65 cm (26 inch) to 85 cm (33 inch). Embryos are initially fed on yolk then receive additional nourishment from the mother by indirect adsorption of uterine fluid enriched with mucus, fat, and protein. They are born live as miniature adults. They are an exceedingly rare and poorly studied species with very limited information available about their lifestyle and behavioral patterns including specific details on catch, age, growth, movement patterns, diet, habitat use, and range. They have a lifespan of up to 14 years.
In Mexican waters the Smoothtail Mobulas have a limited distribution being found in the lower 85% of the Sea of Cortez and along the coast of the mainland south to Guatemala.
The Smoothtail Mobula is very similar in appearance to and difficult to separate from the four other Mobulinae Rays found in Mexican waters of the Pacific namely, the Giant Manta, Manta birostris (mouth opens at front), the Pygmy Devil Ray, Mobula munkiana (dark outer half of disc underside), the Sicklefin Devil Ray, Mobula tarapacana (tail shorter than one half disc width; disc width greater than 3 meters), and the Spinetail Mobula, Mobula japanica (tail longer than disc width; disc width greater than 3 meters).
The Smoothtail Mobulas are caught primarily as a by-catch in tuna gill nets and purse seines and by longline fishermen. They are also targeted by both artisanal and large-scale fisheries. Although their meat is considered marginal for human consumption and only used locally as shark bait or in pet food, they have become the target of increasing commercial interest due to the Chinese Medicinal Trade. Specifically, the feathery gill plates, which number five pairs per fish and encircle each gill slit, have become in vogue by the creation of an artificial market commanding exorbitant prices ($130 per pound) that promote gill plates for the treatment of a wide variety of human diseases including asthma, skin rashes, chicken pox, even cancer and as an overall anti-inflammatory that can detox the blood stream by removing toxins and boosting the immune system. From a conservation perspective they are currently considered Near Threatened, having a low global population and being heavily targeted by large, unregulated, and unmonitored global fisheries. Globally, they are uncommon and their highly fragmented populations have decreased by at least 50% over the last ten years. Monitoring of this species is poor and plagued by a lack of historical data, poor record keeping, and an abundance of misidentification. The sustainability of this species is also hampered by current extreme fishing pressure coupled with the exceedingly low reproductive rate of one pup per year on average and low post-release survival levels. They are protected nationally in Mexico and have been added to the global list of fish at high risk of extinction but many are still retained illegally.
Smoothtail Mobula, Mobula thurstoni, Juvenile. Fish caught off Point Palmilla, Baja California Sur, April 2013. Total length: 68 cm (27 inches); Disc: 62 cm (24 inches) x 36 cm (14 inches); Tail: 35 cm (14 inches).