FISHERY BULLETIN: VOL. 86, NO. 1 



Mob III (I tarapacatm (Philippi 1892) 

 Local name: vaquetilla 



Mobula tarapacann is not a common species in 

 the study area. Seven specimens were collected, 

 one of which, a premature male embryo, was ex- 

 pelled by a large female while she was being 

 landed. Of the postnatal individuals, two were 

 male (DW range 2,476-2,494 mm), and four were 

 female (2,704-3,052 mm). All were caught in 

 Bahia de la Ventana between 9 June and 30 Octo- 

 ber 1983. All but two of the specimens were 

 weighed. The following equation describes the 

 WT/DW relationship for M. tarapacana (where 

 WT is given in kg, DW in mm): 



WT = 2.378 X 10'*^(DW)2-92 

 r = 0.998. 



Although all sampled postnatal M . tarapacana 

 were large, smaller individuals are known from 

 the area, as can be seen in photographs taken at 

 Punta Arena de la Ventana in summer 1981 

 (courtesy Felipe Galvan Magaha, CICIMAR, La 

 Paz, Mexico; also Greg B. Deets"^). This informa- 

 tion argues against geographical size segregation 

 of M. tarapacana . Data on the embryo provide no 

 indication of size at birth, since it was still far 

 from term. Pale pigmentation was apparent only 

 around the head and pelvic regions, and the ex- 

 ternal yolk sac was present (Fig. 5B). The em- 

 bryo, expelled tail first, was alive at birth. Judg- 

 ing by its size it had filled the left uterus 

 completely and must have been the sole develop- 

 ing embryo. 



The small size of the sample does not permit 

 any clear inference on size at sexual maturity for 

 M . tarapacana . Some indication, however, can be 

 obtained by comparison with similar species. Of 

 the two postnatal males, the specimen with a disc 

 2,476 mm wide appeared to be immature: no sem- 

 inal fluid was found in the ducti deferentes, the 

 testes were small and apparently little developed, 

 and the ratio between clasper length and pelvic 

 fin length was 0.94. Conversely, the second post- 

 natal male, with a disc of approximately the same 

 width (2,494 mm), possessed claspers longer than 

 pelvics (ratio = 1.14), and the testes were well de- 

 veloped. Thus, sexual maturity in male M . tara- 

 pacana begins around a DW of 2,400-2,500 mm. 



The specimen with a DW of 2,704 mm, one of two 

 nonpregnant females, had a bulky left ovary, con- 

 taining numerous large eggs; the largest, 32 mm 

 in diameter, weighed 12 g. Similar features ap- 

 peared to be associated with sexual maturity in 

 female M. thurstoni and M. Japanica. The left 

 ovary of another specimen, DW 2,831 mm, was 

 smaller, and the diameter of the largest ovum 

 was 18.6 mm, indicating that a DW of 2,700- 

 2,800 mm denotes a transitional stage for female 

 M. tarapacana , in which both mature and non- 

 mature individuals can occur. 



Twelve echeneidids were recovered from 

 M . tarapacana . Three were Remora remora (size 

 range: 108-229 mm SL), and nine were R. 

 albescens (74-159 mm SL). The following crusta- 

 cean parasites were also found: one cymothoid 

 isopod (still in an unidentifiable aegathoid stage) 

 and Pupulina [lores (Copepoda: Caligidae) on the 

 skin, Entepherus laminipes (Copepoda: Cecropi- 

 dae) on the branchial filter plates, and Eu- 

 dactylina sp. (Copepoda: Eudactylinae) in the 

 gills. 



Mobula tarapacana is strictly a summer and 

 fall visitor to this region (Fig. 6C). This species is 

 often found farther from the coast than 

 M . thurstoni and M. japanica, and may have 

 more pelagic habits. Four of the five stomachs 

 examined (all from specimens caught in summer) 

 were almost empty. Only traces of food were 

 found among the folds of the stomach epithelium. 

 Prey included four species of copepods (Acartia 

 sp. , Pontella sp., Tetnora discaudata , and Undin- 

 ula vulgaris ), hiperiid amphipods, one brachi- 

 uran (family Calappidae), one euphausiid, two 

 caridean decapods (one of which belonged to the 

 family Alpheidae), megalopa and stomatopod lar- 

 vae, and a fish egg. The fifth stomach, from a late 

 October capture, contained the remains of 27 

 fishes (probably carangids 15-30 cm long, and a 

 smaller anchovy-like species). Small tetraodon- 

 tids had been found before in the stomach of a 

 M . tarapacana caught in Bahia de la Ventana 

 (Felipe Galvan Magaha'*). On this basis it is im- 

 possible to determine whether M. tarapacana is a 

 specialized ichthyophagous ray, with the few 

 crustacean items accidentally ingested while 

 swimming, or a generalized feeder. The mesh size 

 of this species' branchial filter plates is indeed 

 greater than in other Mobula species 

 (Notarbartolo-di-Sciara 1987). However, filter- 



•*Greg B. Deets, Long Beach State University, CA, pens, com- 

 mun. 1984. 



^Felipe Galvan Magana, CICIMAR, La Paz, Mexico, pers. 

 commun. 1983. 



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