Life History
The common torpedo is aplacental viviparous, in which the embryos are sustained by yolk, supplemented by histotroph ("uterine milk") secreted by the mother. However, the organic content of the histotroph in this species is only 1.2%, much lower than in stingrays, and the embryo loses mass over the course of development as its initial yolk supply is expended for metabolic processes. Adult females have two functional ovaries and two functional uteruses, though the reproductive tract on the right side is more developed and consequently tends to carry more embryos. It has an annual reproductive cycle with well-defined seasonality, though the details differ between geographic regions. In the Mediterranean, mating occurs from December to February and birthing from late August to early September after a gestation period of 4–6 months; litter sizes range up to 19 and the newborns measure 8.0–9.7 cm (3.1–3.8 in) long. By contrast, off West Africa the gestation period lasts 6–8 months and birthing occurs from September to October; litter sizes range up to 28 and the newborns measure 10.2–12.5 cm (4.0–4.9 in) long. The litter size increases with female size.
Sexual segregation occurs during the breeding season as pregnant females move into shallow coastal waters. The common torpedo is unusual among sharks and rays in that birthing has been documented both at higher and at lower salinities than seawater, in estuaries and lagoons. Immediately after birth, this species is capable of producing electric shocks of up to four volts. Newborns grow rapidly, with a commensurate increase in electrogenic capacity: after four months, they almost double in weight and can generate 26 volts. Males and females mature sexually at similar lengths: 25 and 26 cm (9.8 and 10 in) respectively in the Tyrrhenian Sea, 18 and 22 cm (7.1 and 8.7 in) respectively off Egypt, 19 cm (7.5 in) for both off Tunisia, and 30 and 31 cm (12 and 12 in) respectively off Senegal.
Read more about this topic: Common Torpedo, Biology and Ecology
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