FISHKKY BULI.KTIN VOL 86. NO 1 



the larger sizes were insufficient, and therefore 

 the maximum egg diameter was not l^^nown. It 

 appears from the scanty available data that fe- 

 male M. thurstoni began to mature at a DW of 

 about 1,500 mm (the point at which the slope of 

 the curve becomes steeper). 



Mobulo thurstoni, like all mobulids, is a vivi- 

 parous matrotroph ( Wourms 1981 ), the near-term 

 embryo being three orders of magnitude larger 

 than the mature egg. Uteri and nidamental 

 glands of 68 females were inspected. No adujt 

 female exammed from March through June iN 

 = 55) was pregnant. Embryos were found in July 

 and October. Two embryos from females caught 

 in July were near-term. Four embryos found in 

 October were in an early stage of development. 

 All females (A^ = 4) inspected in October were 

 pregnant, and all embryos were in the same de- 

 velopmental stage, suggesting coordinated breed- 

 ing activity. The largest female (DW 1,801 mm) 

 had a single embryo, which appeared to be in the 

 final stage of fetal development, fully pigmented, 

 the yolk sac completely absorbed, and the umbili- 

 cus a mere scar (Fig. 5A). The embryo's DW was 

 630 mm; its WT, 3.4 kg. It occupied the left uterus 

 with the rostrum pointed forward. Its pectoral 

 fins were folded dorsally, the right pectoral on 

 top. The cephalic fins were almost totally un- 

 rolled and extended ventrally towards the mid- 

 line of the body. The uterus, with the embryo 

 inside, occupied roughly one third of the female's 

 pleuroperitoneal cavity. Lack of space inside the 

 cavity and the distended skin on the abdomen 

 made it apparent that no other embryo had been 

 recently expelled or aborted. Uniparity appears to 

 be a common pattern within the genus Mobiila 

 (Hill 1862; Gill 1908; Coles 1913, 1916b; Barnard 

 1925; Setna and Sarangdhar 1950; Cadenat 1960; 

 Wallace 1967; Capape and Zaouali 1976; 

 Figueiredo 1977). Only Risso (1826) asserted that 

 Cephaloptera giorna (= M. mohular) may have 

 one or two young, but his statement was not doc- 



umented. Since the smallest free-swimming spec- 

 imen noted had a DW of 864 mm, the average size 

 at birth for M. thurstoni is probably between 650 

 and 850 mm DW, and a WT of approximately 4.5 

 kg. The second largest female (DW 1,799 mm) 

 had also only one embryo in her left uterus. The 

 embryo was unpigmented, with disc 210 mm 

 wide, and weighed 173 g. Like the term-embryo, 

 its rostrum was pointing forward; unlike it, how- 

 ever, its pectoral fins were folded ventrally. 



Mating, parturition, and early mobulid life his- 

 tory take place in the shallower portion of a popu- 

 lation's range, not an uncommon elasmobranch 

 feature. McLaughlin and O'Gower (1971) dis- 

 cussed inshore movements in the mating Port 

 Jackson shark Heterodontus portusjacksoni , as 

 did Springer (1960) for the sandbar shark Eu- 

 lamia milberti (= Carcharhinus plumbeus). One- 

 year-old gray reef sharks, Carcharhinus am- 

 bhrhynchos, were observed in French Polynesia 

 in shallower waters than adults by Nelson and 

 Johnson (1980). A similar result was reported for 

 the hammerhead shark, Sphyrna lewini , by 

 Clarke (1971) in Hawaii and by Klimley ( 1983) in 

 the Gulf of California. Bullis (1967) hypothesized 

 an upward movement to shallower depths for 

 newborn marbled cat sharks, Galeus arae. There 

 is likely an advantage for juveniles to remain in 

 relatively protected areas during the earlier 

 stages of their life, when they are most vulnera- 

 ble to predation (Springer 1967). 



Examination of 139 stomachs indicated that 

 M. thurstoni was extremely specialized in its 

 feeding habits. Eighty one (58.3%) stomachs were 

 empty, or had only traces of food (<1 g wet 

 weight). The remaining 58 stomachs (41.7%) had 

 quantifiable contents. All recognizable prey 

 items were planktonic crustaceans (with the ex- 

 ception of a few fish eggs, one nematod, and a 

 small coleopteran, probably ingested accidentally 

 when it was floating). They were listed, ranked by 

 decreasing ^IRI, in Table 1. Mobula thurstoni 



Table 1 —Prey species found m 57 stomachs of Mobula thurstoni, ranked by decreasing Index 

 of Relative Importance (IRI). N = percentage of prey species by number; M = percentage of 

 prey species by mass; F = percent frequency of occurrence of prey species. 



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