Incidence of conjoined twins in tilapia after thermal shock induction of polyploidy

Conjoined twins (polyembryonic fish) were observed in batches of blue tilap ia Oreochromis aureus (Steindachner) and Nile tilapia O. niloticus (L.) emb ryos incubated at 28 degrees C. Heat shock (41.8 degrees C) treatment of fe rtilized eggs for 3-4 min applied between 4 min and 27 min after fertilizat ion increased the twinning rate by three or four times over that of unshock ed eggs. Two basic morphological variations - ventrally or laterally conjoi ned twins - were observed. Twins did not survive beyond 21 days after hatch ing. The proportion of twins in heat-shocked egg batches of both species co mbined ranged from 0.32% to 0.40%.