An amphidiploid aquatic animal according to the present invention has genomes AB of different species and carries fertile XXXY sex chromosomes. Among a large number of aquatic animals of the first filial generation kept in a closed system, a nonreductive sperm of a male and a nonreductive egg of a female are selected. Then the nonreductive egg is fertilized with the nonreductive sperm to create an amphidiploid aquatic animal having fertile XXXY sex chromosomes. Since this amphidiploid has the XXXY sex chromosomes, it can be crossed with an egg AXBX of an F1 hybrid that produces nonreductive eggs, thus ensuring a stable creation of amphidiploids in the subsequent generations by natural crossbreeding. In the meiotic division, one set of the chromosomes of each species is assuredly distributed to each gamete. Therefore, the trait of the first generation (F1) will be perpetually maintained. Since no genetic separation takes place, two sets of the genomes of different species are always inherited to every individual. Therefore, inbreeding depression will not take effect even if relative mating is repeated.
