Francis and Stevens: Reprodurtion, embryonic development, and growth of Lamna nasus 



51 



Figure 6 



Four empty egg capsules found in the uteri and in the mouth of an embryo, from a pregnant porbeagle 

 from Macquarie Island. Also shown are several loose ova found in one of the egg capsules. The largest 

 egg case was 75 mm long. 



extend from Ap:il to September. For aging pur- 

 poses, we defined the theoretical birth date as 1 

 June. Based on the lengths of the smallest juve- 

 nile and the largest embryo, the length at birth 

 is 58—67 cm. If a growth rate of 7.48 cm per 

 month is maintained by Southern Hemisphere 

 porbeagles throughout gestation, the gestation 

 period is about 8—9 months. However, the un- 

 explained variability in Fig. 7 compromises 

 our ability to accurately estimate the gestation 

 period. 



Embryonic development 



Porbeagle embryos develop the distended yolk 

 stomach that is characteristic of all oophagous 

 lamnid sharks (Fig. 8). In the Kerguelen em- 

 bryos (9.6-10.4 cm), such distension was al- 

 ready apparent. The caudal fin was notably 

 curved, with the upper lobe much longer than 

 the lower lobe, and there were no external gill 

 filaments. At 19.8-20.7 cm, the bulging yolk 

 stomach was the most noticeable feature, along 

 with a marked lateral expansion of the head 

 and branchial region (Fig. 8A). The body lacked pig- 

 mentation (except for the eyes), and appeared pink 

 because of the presence of blood vessels under the 



skin. At 34.2 cm, the yolk stomach had become enor- 

 mously distended, and measured 22.6 cm long by 15.9 

 cm high; the branchial and throat regions remained 



