KISHEKY BULLETIN: VOL 86, NO. 1 



fed mostly on adult and juvenile euphausiids, 

 Nyctiphanes .s///?p/c.v. The niysid Mysidium sp. 

 (underscribed, Thomas E. Bowman^) was second 

 in order of importance. Mysids and euphausiids 

 were never found together in the same stomach. 

 The overall importance of the two food items dif- 

 fered by two orders of magnitude. All other prey 

 species found in the stomachs were rare and prob- 

 ably fortuitously ingested. These included one 

 zoea larva and the following copepod species: 

 Undinula vulgaris, Eucalanus subcrassus, E. 

 subtenuis, Ternora discaudata, Scolecithrix 

 danae, Nannocalanus minor, Euchaeta remana, 

 Euchaeta sp., and Labidocera diandra. Diet 

 varied with season (Fig. 6A), with mysids being 

 dominant from December through March, and eu- 

 phausiids during the warmer months. Diet varied 

 with predator size (Table 2): smaller individuals 

 fed both on euphausiids and mysids; the larger 

 rays fed only on euphausiids. This result probably 

 reflects the predominance of smaller rays during 

 winter, when fewer euphausiids are available, 

 rather than an ontogenetic change in food prefer- 

 ences. 



Table 2. — Size differences in the diet of Mobula thurstoni. 



Two remoras (Echeneididae) were occasionally 

 found on large M. thurstoni: Remora remora (3 

 specimens; range: 98-200 mm SL), and R. 

 albescens (3 specimens; 93-100 mm SL). Crus- 

 tacean parasites were encountered: Pupulina 

 minor (Copepoda: Caligidae), Ecthrogaleus den- 

 ticulatus (Copepoda: Pandaridae) sparsely on the 

 skin, and Ecthrogaleus disciarai (Benz and Deets 

 1987) in large patches on the dorsal surface, En- 

 tepherus laminipes (Copepoda: Cecropidae) from 

 the branchial filter plates, Eudactylina oliveri 

 (Copepoda: Eudactylinae) from the gill lamellae, 

 and Kroeyerina sp. (Copepoda: Kroyeriidae) from 

 the olfactory lamellae. 



Mobula thurstoni was usually observed at the 

 surface in coastal waters of Bahia de la Ventana, 

 Cueva de Leon, and Bahia de los Muertos, often 

 within a few hundred meters of land and occa- 



2Thoma.s E. Bowman, Smithsonian Institution, Washington, 

 D.C., pers. commun. 1984. 



sionally as far as 6 km. When sighted offshore, it 

 was sometimes found over considerable depths 

 O500 m), although it appeared to be more abun- 

 dant in shallower, neritic waters. Mobula 

 thurstoni was always caught in the shallower 

 part of the nets, usually at a depth of < 100 m. The 

 greatest part of the catch, however, was surface- 

 dwelling rays. Beginning in mid-April, numerous 

 M. thurstoni were consistently seen in the early 

 morning hours cruising slowly at the surface. 

 They would frequently pause, conspicuous on 

 calmer days, with the tips of their pectoral fins 

 protruding out of the water. This behavior is well 

 known in mobulids (Norman and Eraser 1937); it 

 has been observed also in connection with mating 

 activities in M. olfersi ( = M. hypostoma ) by Coles 

 (1910). During such occasions, fishermen could 

 easily approach the rays and harpoon them, be- 

 fore startling them and causing them to dive. Re- 

 peated captures within the same aggregation re- 

 vealed that rays of various sizes and both sexes 

 could be found together. While at the surface, 

 M. thurstoni was usually solitary or in small, 

 nonpolarized groups (2-6), rather than in larger 

 aggregations or schools. The species was fre- 

 quently seen jumping out of the water in spectac- 

 ular, often reiterated somersaults; it was recog- 

 nized by the distinctive ventral markings. 



It is not known to what extent mobulids make 

 use of the sea bottom. Beebe and Hollister (1935) 

 observed a group of 12 small devilfish (most likely 

 Mobula ) lying on the sandy substrate off Frigate 

 Islet, in the British West Indies. Bigelow and 

 Schroeder (1953) speculated that Manta spends 

 much of its time resting quietly on the seafloor. 

 During an experiment organized in conjunction 

 with Sea World of San Diego, aimed at establish- 

 ing whether M. thurstoni could survive in a con- 

 fined environment, five young specimens were 

 captured with gill nets and kept in a large pen (6 

 m in diameter) anchored in 2.5 m of water in 

 Ensenada de los Muertos. None of the rays sur- 

 vived 24 hours of captivity; the reasons for their 

 deaths were not clear, although the particularly 

 stressful capturing method appeared as a likely 

 cause. During that experiment the negatively 

 buoyant rays (sinking tail-first as soon as they 

 stopped swimming) spent a great deal of time 

 resting on the bottom, and were able to circulate 

 water through their gills while resting, by a syn- 

 chronized maneuvering of the oral valve and of 

 the gill covers (as judged from the flow made vis- 

 ible by the numerous particles suspended in the 

 water). A frequent method of turning around 



54 



