310 



Fishery Bulletin 98(2) 



North Atlantic (3.15 m) was a 

 virgin. The smallest mature fe- 

 male from the Southern Hemi- 

 sphere of 2.63 m was pregnant 

 (no. 23 in Table 1). 



Liver-, yolk-, and total mass of 

 embryos 



There was a significant posi- 

 tive relation between the HSI 

 and the length of near-term 

 embryos, rising from 3.8% in 

 a litter with a mean length 

 size of 59.9 cm to 7.0% in a 

 litter with a mean length size 

 of 68.8 cm. The slope was 

 0.34%/cm (SE=0.04%/cm, P< 

 0.001, /•2=o.83, n=17). The TL 

 range of the embryos in these 

 two litters was small (12.5 cm) 

 but they covered almost 50% of 

 the HSI range. The increasing 

 HSI was accompanied by a cor- 

 responding decrease in the amount of yolk in the 

 stomach from 29.1% to 10.9%- of total mass. The 

 yolk percentage versus TL regression was signifi- 

 cant with slope -1.64%/cm (SE= 0.33%/cm, P<0.001, 

 r2=0.62, /z=17). 



The mass-length relationship of embryos is com- 

 plicated owing to the highly variable mass of the 

 yolk-filled stomach. The scarce data indicated that 

 the yolk mass amounts to 60-70% of total mass for 

 mid-term litters and then decreases to around 10% 

 in a full-term litter (Fig. 5, y-axis on left). When the 

 mass of the yolk-filled stomach was subtracted from 

 the total mass, a power regression could be fitted to 

 net embryo mass (M,,^,,) (Fig. 5, y-axis on right). The 

 back-transformed equation was 



M„,„=8.198TL3"' 



(«=21, r2=0.98, TL ranges 0.36-0.71 m). 



The mass of the smallest free-swimming fish agreed 

 with the predicted mass based on the extrapolated 

 curve beyond the upper limit of the embryo data 

 (Fig. 5). The corresponding condition factors of short- 

 fin mako embryos (CF„,,,=M,„,/rL^) were between 7.3 

 and 7.9 kg/m'^ and were similar to postnatal values. 

 If the mass of the yolk-filled stomach was included, 

 the condition factor (CF=M/TL-^) reached values as 

 large as 26 kg/m-^ when the embryos were mid-term 

 and had the largest yolk stomachs (Fig. 6, /. oxyrin- 

 chus data). 



0.4 0.6 



Total length (m) 



0.8 



1.0 



Figure 5 



The relationships between 1) yolk '/i (left ,v-axis), 2) embryo mass with and without 

 yolk (right _v-axisl, and length of Isurus oxynnchus embryos. A power regression was 

 fitted to embryo mass without yolk data. n=yolk '^■i; • = embryo mass without yolk; + 

 = embryo mass with yolk; X = mass of free swimmers; S = Sanzo (1912); U = Uchida 

 et al. (1987); remaining data from this study including P = upper and lower limit of 

 mass range of litter with punctured yolk-stomachs. 



Discussion 



Embryonic development 



Our data documented early and late embryonic devel- 

 opment of the shortfin mako. As expected, it was simi- 

 ilar to that reported for other lamnids (Swenander, 

 1907; Lohberger, 1910; Otake, 1990; Gilmore, 1993; 

 Francis and Stevens, 2000) in which the embryos 

 hatch at about 6 cm TL and then feed on nutritive 

 eggs (oophagy) until the beginning of the last third 

 of the gestation period. Embryonic nutrition may 

 also include egg jelly absorbed through the external 

 gill filaments in the prehatching phase and uterine 

 milk in the posthatching phase ( Hamlett, et al. 1985; 

 Gilmore, 1993; Koob and Straus, 1998). 



After mating, the largest of the relatively small 

 ova are ovulated, fertilized, and packaged into egg 

 cases (one blastodisc ovum per egg case) as evident 

 in three cases in our study (Table 2). The diameter 

 of these blastodisc ova appears to be slightly larger 

 (~7 mm) than the nutritive ova (4-6 mm) and the 

 ova remaining in the ovary (4-6 mm); this larger 

 diameter may also apply to other lamnids and 

 lamnoids. Mating may stimulate the enlargement 

 of the largest ova in the ovary and trigger subse- 

 quent ovulation. The capsules with nutritive ova are 

 formed after the blastodisc ova have been encapsu- 

 lated. Blastodisc egg cases with only one fairly large 

 ovum were observed in the bigeye thresher shark, 

 Alopias superciliosus (Moreno and Moron, 1992). In 



