Foureyes are live-bearers with internal fertilization and ma-trotrophic (type 2) gestation and development. The male gonopodium swings out in a horizontal plane, but only in one direction, either to the left or to the right. The vent of the female is covered by a hinged scale called a "foricula," which opens to the left or right. Thus, right-handed males must mate with left-handed females and vice versa, an odd situation that does not seem to impede their reproduction. The gestation period is about 20 weeks, with a brood size of 10 to 20 fry. The young are large, up to 2 in (5 cm) at birth. This large size is remarkable, considering that sexual maturity is reached at about 3.5 in (9 cm).
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