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



53 



B 



iX 



Figure 9 



Anterior view of the heads of (A) 9.6-cm embryo, and (B) 26.4 cm female 

 embryo showing the functional fangs. (Photo A. by G. Duhamel.) 



swollen, and the upper body and pectoral fins had be- 

 come pigmented (Fig. 8B). At 40.3 cm, pigmentation 

 was essentially the same as in postnatal porbeagles, 

 the swelling of the head had disappeared, and the yolk 

 stomach had begun to shrink (Fig. 8C). At 58.0 cm the 

 juvenile body form had been attained, apart from an 

 enlarged abdomen (Fig. 8D). Other embryos around 

 this size and larger had a more streamlined shape, 

 with little noticeable abdominal distension. 



Distension of the abdomen during early develop- 

 ment causes the subdermal muscle layers to split 

 along the ventral midline, extending anteriorly as far 

 as the fifth gill slits. The expanding stomach protrudes 

 between the muscle layers and stretches the abdomi- 

 nal skin. Later, the stomach shrinks back inside the 

 muscle layers, and the stretched skin returns to its 

 original shape. A distinct "scar" remains in the ventral 

 midline in the area between the origins of the pectoral 



fins and the fifth gill slits, marking the anteriormost 

 point of the split muscle layers. 



Small embryos had large, erect, tubular, recurved 

 "fangs" in both jaws (Fig. 9, A and B). These teeth, 

 which were quite unlike those found in postnatal por- 

 beagles, were clearly functional. In the Kerguelen em- 

 bryos (9.6-10.4 cm), the tooth formula was (1-t- 1/1+1), 

 and the lower teeth were massive in relation to mouth 

 size (Fig. 9A). Larger embryos (19.8-38.3 cm) had 

 more functional upper teeth (3+3/1+1) (Fig. 9B). Ad- 

 ditional minute teeth were visible under a light micro- 

 scope, but they appeared vestigial and nonfunctional 

 and were not included in the tooth formula. Replace- 

 ment fangs were present behind the functional series, 

 but they were irregularly spaced and usually located 

 between the functional teeth. Oval scars on the gum of 

 both jaws external to functional fangs indicated that 

 fangs are progressively shed and replaced. The largest 



