FISHERY BULLETIN: VOL 86, NO. 1 



of abundance oC their prey (Brinton and 

 Townsend 1980). Some M . japanica , however, in- 

 cluding individuals both large and small, were 

 found in this region throughout December. Like 

 M. tharstoni , winter catch data are not compara- 

 ble because of differences in fishing effort, when 

 M . Japanica is apparently caught less frequently. 



Mobula miinkiatia 



NotJirbartolo-di-Sciara 1987 



Local name: tortilla 



Twenty-four specimens, 10 males (DW range 

 686-900 mm) and 14 females (719-1,097 mm), 

 were caught at four stations (Punta Arena de la 

 Ventana, Ensenada de los Muertos, Bahia de los 

 Frailes, and Isla El Pardito) and adjacent waters, 

 between December 1982 and October 1984. Size- 

 frequency distributions for M. munkiana (Fig. 10) 

 revealed that female mean size was greater than 

 male, although not significantly (T-value 

 = 1.724, df = 22,P > 0.1). Seasonal differences in 

 size-frequency distribution could not be examined 

 because M . munkiana were only collected during 

 the fall and winter. All 10 freshly captured speci- 

 mens were weighed (size range: 686-1,097 mm 

 DW; 4.1-11.8 kg). Their WT/DW relationship is 

 described by the following equation: 



WT= 1.041 X 10-«(DW)2 34 

 r = 0.95. 



WT is given in kg, DW in mm. 



The largest specimen in the sample, a female, 

 was one of the largest "tortillas" ever seen. There 

 are no data on size at birth, as no embryos were 

 found. Lack of knowledge of the size of the young 

 of the year also prevented insight on size segrega- 



>- 

 o 



z 



LlJ 



o 



UJ 



q: 

 u. 



LlI 



O 



CE 

 < 



10 



5 - 



p?rn r-p^ 



Xtot = 886.5 ±33.4 mm (M = 24) 

 X^^ = 853.8 ± 39.5mm (N = IO) 

 X^^ =909.9 ±47. 1 mm (N:|4) 



D TOTAL 

 m is 

 ?? 



mm 



650 750 850 950 1050 

 DISC WIDTH (mm) 



Figure 10. — Size-frequency distribution.s oi' Mubula munki- 

 ana (means ±2 SE). 



tion. Term-embryos in M . rochebrunei , a closely 

 related, similar sized species from west Africa, 

 were 340-350 mm wide (Cadenat 1960). Since 

 Mobula at birth has a DW of about 1/3 of the 

 adult, size at birth would be about 350 mm DW. 

 This information argues in favor of size segrega- 

 tion in M. munkiana. Male to female ratio was 

 0.71 (N = 24), insignificantly different from 1 (x" 

 test P > 0.05). Both sexes were caught in the 

 same net sets, indicating that males and females 

 school together, and that there was no sex segre- 

 gation, either geographic or behavioral. 



A dried, twisted male carcass, for which mor- 

 phometries could not be obtained, with a calcu- 

 lated DW of 895 mm, had long, well-developed 

 claspers, markedly protruding beyond the pelvic 

 fins. Based on other mobulid species, this condi- 

 tion indicates sexual maturity. A second speci- 

 men, with a disc 686 mm wide, had small and 

 pliable claspers, and the ratio between clasper 

 length and pelvic fin length, both measured from 

 tip to anterior margin of vent, was 0.88. At this 

 ratio, both M. thurstoni and M . japanica are im- 

 mature. Two M . munkiana , 871 and 872 mm DW, 

 differed greatly in the relative size of their 

 claspers: one possessed slightly longer claspers 

 than the pelvic fins (ratio = 1.10), and an incipi- 

 ent hardening of the cartilage was apparent; in 

 the other specimen the claspers were much 

 shorter than the pelvics (ratio = 0.84), and still 

 soft. This information suggested that male sexual 

 maturity in M. munkiana began at about 870 mm 

 DW. 



The largest female specimen was sexually ma- 

 ture, as it had a large, flaccid left uterus, and the 

 enlarged left ovary consisted mainly of about 30 

 macroscopic ova (size range 2-15.2 mm). 



Ten stomachs were examined: four were empty, 

 three contained unidentifiable whitish matter, 

 and three were full of planktonic crustaceans. A 

 list of prey species ranked by decreasing 9rIRI is 

 given in Table 6. Mysidium sp. appeared to be the 

 main staple of M. munkiana's diet. Mobula 

 munkiana is thus similar to the wintering young 

 of M. thurstoni. One of the stomachs contained 

 coarse coral fragments and small gastropod 

 shells, perhaps ingested by the ray while foraging 

 on mysids near a sandy substrate. The west 

 African species M . rochebrunei appeared to have 

 similar feeding habits, as Cadenat (1960) found 

 mysids and a few larger postlarval stomatopods 

 in the stomachs of several specimens. 



I have no record of remoras associating with 

 M. munkiana. The only parasitic crustacean 



60 



