Observations on twinning in the sea lamprey (Petromyzon marinus) in the laboratory

Forty-five sets of separate, normal twins were produced from four portions of the eggs from one female sea lamprey (Petromyzon marinus) each fertilized by a different male. The percentage of twinning in the different groups of embryos ranged from 1.0 to 1.7. Although the development of some of the twins was slightly slower than average, that of most was within the normal range for sea lamprey embryos. The twins were smaller than non-twins, and one twin was usually smaller than the other. Although the discovery of Siamese twins in fish is not uncommon, separate, normal twins have seldom been reported.