MAXIMIZATION OF EVOLUTIONARY TRENDS FOR PLACENTAL VIVIPARITY IN THE SPADENOSE SHARK, SCOLIODON-LATICAUDUS

Placental viviparity has evolved in Scoliodon laticaudus to a degree t hat rivals some eutherian mammals. Its eggs are the smallest known of any shark. They have a diameter of 1 mm, a dry weight of 0.0654 +/- 0. 0100 mg and are nearly yolk-free. Implantation takes place at an early (3 mm) stage of development, and gestation is short (5-6 months). Com parison of the dry weight of the egg (0.065 mg) with the estimated dry weights of a mid-late term 90 mm embryo (910 mg) and a 152 mm neonate (3815.4 mg) reveals weight changes of 14219 x and 58338 x, respective ly. Its normalized brood weight, a measure of maternal nutrient invest ment, is 49.5 g . kg-1 female body weight for a six-month gestation. C omparisons with other species of placental and nonplacental sharks sho w that S. laticaudus has a highly advanced form of matrotrophy. Matern al nutrients appear to be acquired by placental transport and by imbib ition of uterine fluid. Hemotrophic placental nutrient transfer occurs across a unique uterine implantation site, termed the trophonematous cup, in which maternal blood appears to bathe the outer epithelium of the embryonic yolksac placenta. The latter is solid and filled with a three-dimensional network of capillaries and many free interstitial ce lls. The umbilical stalk contains the vitelline vessels but lacks a yo lk duct. Its surface is amplified by many long, villous appendiculae, which consist of a vascular core that ramifies into a massive surface capillary network invested by a simple squamous epithelium. The append iculae of S. laticaudus most likely are sites of gas exchange and poss ibly the uptake of small molecules. They are unlike the appendiculae d escribed in any other placental shark and exhibit design principles si milar to those of the uterine trophonemata of matrotrophic rays.