PRATT REPRODUCTION IN BLUE SHARKS 



The embryos taken during June and July were 

 full term (mean lengths 37.1-46.6 cm). Some were 

 larger than any reported in the literature (42-46.6 

 cm) except for one 49 cm embryo reported by Be- 

 neden (1871) and cited by Tucker and Newnham 

 (1957). 



The range of smaller embryo sizes reported by 

 other authors as "full term" may result from 

 examination of young "in utero" or aborted on 

 deck. Judging from my January sample, after only 

 4-5 mo gestation embryo blue sharks have lost 

 branchial gills and yolk sacs, and appear to be 

 full-term replicas of the adults. When returned to 

 the ocean they are active and quickly swim away. 

 Premature embryos (up to 30 cm) have a propor- 

 tionally thin body for the size of the head giving 

 them a tadpolelike appearance. Full-term em- 

 bryos (Figure 21) have a girth that equals or ex- 

 ceeds the head circumference. The pregnant 

 female sampled in January carried 82 embryos 

 averaging 13 cm long. Suda (1953) indicates this 

 264 cm female could be between 4 and 5 mo preg- 

 nant. Only the report of Gubanov and Grigor'yev 

 (1975) of 135 young exceeds this observation. 

 Minimum number of young could not be deter- 

 mined due to reports of premature parturition 

 while the fish were being boated. 



Gubanov and Grigor'yev (1975) agreed with a 

 proposition of Liibbert and Ehrenbaum (1936) 

 that embryo blue sharks develop and are born in 

 stages. No evidence was found in the Atlantic to 

 support this hypothesis. Embryos occurred in the 



same relative stage of development in each 

 female. This is apparent in Figure 22. The 37 cm 

 (.V ) embryos ventral to the 232 cm pregnant female 

 appear slightly smaller due to camera parallax. 

 Nearly every litter contains one stunted or decom- 

 posing embryo. The explanation may be failure to 

 attain placentation, dislodgement, or tangling of 

 the umbilical cords. A stunted embryo is 10th from 

 the left in the ventral row of embryos (Figure 22). 

 The smallest free-swimming young have been 

 observed in the Pacific by Francis Williams (see 

 footnote 6) at 35 cm and Strasburg ( 1958) at about 

 38 cm. The smallest Atlantic specimen is Bigleow 

 and Schroeder's (1948) report of 44 cm. These 

 lengths resemble lengths of the largest embryo 

 sizes, and available evidence suggests that size at 

 birth for the blue shark is between 35 and 44 cm. 

 The pupping season can be interpolated from Fig- 

 ure 20 to occur from March to July. The apparent 

 lack of "young-of-the-year" blue sharks suggests 

 an offshore pupping. The blue shark is the most 

 prolific of the large oceanic sharks (Bigelow and 

 Schroeder 1948), yet I have seen only one free- 

 swimming fish that was <1 m FL. Blue sharks in 

 the first and second year of life must, therefore, 

 inhabit an unknown niche. 



Structure and Function of 

 the Oviducal Gland 



The oviducal gland (Figure 23) as defined by 

 Metten (1941) has also been referred to as the 



FIGURE 21— Ovary and 43 cm FL full- 

 term embryo from 220 cm FL gravid 

 blue shark. tf.,* 



461 



