60 



Fishery Bulletin 98(1) 



has been reported for shortfin makos and sandtigers 

 (Gilmore et al., 1983; Mollet et al."). The embryos 

 then rely on the yolk stored in their stomachs to 

 provide the energy needed for growth and respira- 

 tion during the rest of the gestation period. However 

 it is also possible that females continue ovulating, 

 and that the toothless embryos feed by swallowing 

 whole egg capsules or by squashing them in their 

 mouths. Whole egg capsules have been reported from 

 the stomachs of near-term embryos of the bigeye 

 thresher shark (Gilmore, 1983; Moreno and Moron, 

 1992), but not from near-term lamnid sharks. Clari- 

 fication of this point requires examination of the ova- 

 ries of near-term females to assess their ovulatory 

 state. 



Above 35 cm, the waste products of yoLk digestion 

 continue to accumulate in the intestine. The greenish 

 coloured waste is characteristic of oophagous sharks 

 (Swenander, 1907; Lohberger, 1910; Shann, 1923; 

 Springer, 1948; Uchida et al., 1996). The gritty na- 

 ture of the intestinal contents was also mentioned by 

 Swenander ( 1907) and has been reported to consist of 

 "crystal-like pieces" in white shark embryos (Uchida 

 et al., 1996). The composition of this material is un- 

 known. The liver grows most rapidly in the second 

 half of gestation as energy reserves are transferred 

 to it for storage. An increase in the relative weight of 

 the liver in larger embryos has also been observed in 

 shortfin makos (Mollet et al.'^). 



During the second half of gestation, several series 

 of "post-natal" teeth develop, but they are folded back 

 in the jaws and are nonfunctional. In white sharks, 

 some of these teeth are shed and swallowed by the em- 

 bryos (Francis, 1996; Uchida et al., 1996). The teeth 

 probably become erect near or soon after birth, as has 

 been found in near- term white shark embryos (Fran- 

 cis, 1996; Uchida et al., 1996). 



Typically, embryos in a porbeagle litter are of sim- 

 ilar size, but occasionally a large size range is en- 

 countered. Gauld ( 1989) found one litter with embryos 

 ranging ft-om 55.6 to 65.0 cm, and Shann (1923) re- 

 ported a litter with a range of 38.1-50.9 cm. The runts 

 in our two litters had small quantities of stomach and 

 intestinal contents, and small livers, but were other- 

 wise developing normally. This suggests that sibling 

 competition may occur when a dominant embryo with 

 its snout nuzzled into the anterior end of the uterus 

 consumes most of the egg capsules as they pass into 

 the uterus, leaving few for its sibling. However, all 

 four embryos are usually adequately nourished, and 

 the two embryos in each uterus are usually oriented 

 in opposite directions. This suggests that the direc- 

 tion of orientation within the uterus may be a problem 

 only if the mother is unable to produce enough egg 

 capsules to satisfy both embryos. - 



At birth, embryos may still have yolk in their stom- 

 achs. Near- term white shark embryos have been re- 

 ported with either empty (apart from some ingested 

 teeth and denticles) or yolk-filled stomachs (Francis, 

 1996; Uchida et al., 1996). Near-term shortfin mako 

 embryos "and new-bom sandtiger sharks may also 

 have small amounts of yolk in their stomachs (Cade- 

 nat, 1956; Bass et al., 1975; Gilmore et al., 1983; Mol- 

 let et al.'M. Along with the energy stored in the liver, 

 this yolk supplies the nutritional needs of the embryos 

 until they learn to feed. However, the livers of por- 

 beagle embryos never exceeded 10^ of the yolk-free 

 embryo weight, compared with 13.5-18.6'^ in near- 

 term white shark embryos (Francis, 1996; Uchida et 

 al, 1996). 



Hubbs ( 1923) reported a 9.1 kg (20 lb) embryo col- 

 lected in late August in Maine, USA. The weight is 

 clearly too large to be a porbeagle embryo because 

 they are not known to exceed 5 kg ( Fig. 10 ). Moss-^ sug- 

 gests that the embryo may have been ft-om a sandtiger 

 shark. 



The presence of an "umbilical scar" or "yolk sac scar" 

 in postnatal oophagous sharks has puzzled many sci- 

 entists who were aware that the embryos have no pla- 

 cental connection to their mothers ( Gilmore, 1983; Ste- 

 vens, 1983; Klimley, 1985; Cliff et al, 1990, 1996; Pratt 

 and Casey, 1990; Moreno and Moron. 1992; Francis, 

 1996; Uchida et al., 1996). Our observations show that 

 distension of the stomach stretches the abdominal 

 skin and separates subdermal muscle layers as far 

 forward as the fifth gill slits. As yolk is consumed the 

 stomach shrinks and the muscle layers return to their 

 original position, leaving a scar in the pectoral-gill re- 

 gion. The scar is sometimes faint or absent. 



In all lamnid sharks, embryos are nourished by 

 oophagy. Contrary to earlier suggestions, there is 

 no evidence that lamnid embryos indulge in uterine 

 cannibalism (adelphophagy), an extreme extension of 

 oophagy that has been confirmed only in the sandtiger 

 shark (Gilmore, 1993). All lamnids, and most ooph- 

 agous sharks, produce litters larger than two (one 

 per uterus) (Francis, 1996), providing strong circum- 

 stantial evidence that adelphophagy does not occur in 

 those species (Gilmore, 1993). One porbeagle embryo 

 had nonlethal abdominal lacerations, probably result- 

 ing from incidental damage inflicted by its larger sib- 

 ling while searching for egg capsules, which are about 

 the same size as the smaller embryo's abdomen. This 

 searching behavior could provide a mechanism for the 

 development of adelphophagy from oophagy. 



Unresolved questions 



Two puzzling features of the reproduction of porbea- 

 gles demand further investigation. The first concerns 



