Reproductive biology of the female shortfin mako, Isurus oxyrinchus Rafinesque, 1810, with comments on the embryonic development of lamnoids

Reproductive data from 95 mature female shortfin mako sharks, Isurus oxyrin chus Rafinesque, 1810, including 35 pregnant females, together with data on 450 postnatal fish were collected from around the world. Size at birth was approximately 70 cm total length (TL) and litter size varied from 4 to 25. increasing with maternal size. Embryo length-at-capture data predicted a g estation period of 15-18 months and late winter to midspring parturition in both hemispheres. A temporal analysis of uterus width index and gonadosoma tic index of pregnant and postpartum females indicated that the reproductiv e cycle is three years. The median TL-at-maturity of females from the weste rn North Atlantic (2.98 m) was greater than that of females from the Southe rn Hemisphere (2.73 m) and they were 16-19% heavier in the TL range of 2.5- 3.5 m. Recently ovulated females and a litter with 2.6-3.3 cm TL embryos having ex ternal gills, a large yolk sac, and still inside their egg cases, are descr ibed. We describe a litter of embryos (52.0 cm TL) with huge yolk-filled st omachs. Litters of 59.9- and 68.8-cm-TL embryos showed a decline in the mas s of the yolk-filled stomach from 29.1% to 10.9% of total mass and an incre ase in hepatosomatic indices from 3.7% to 7.0% as gestation advances. When the mass of the yolk-filled stomach was excluded, the mass-length relations hip of shortfin mako embryos could be fitted with a power regression simila r to that for postnatal fish. The condition factor of lamnid embryos (inclu ding yolk-stomach mass) reaches a maximum between 20 and 35 kg/m(3) when th e embryos are midterm and have the largest yolk stomachs. The condition fac tor of alopiid embryos remains constant, indicating that no large yolk-fill ed stomach develops.