Changes in Thermal Tolerance of Eggs of Wrasse　Halichoeres poecilopterus during Development

Effects of short-term thermal shock on the developmental fish eggs were investigated using the wrasse Halichoeres Poecilopterus（Japanese name：Kyusen）．An average of 28（rangng from 20 to 69 eggs was exposed to 9 test temperatures（△t＝7～15℃）for 15 min at 25 different stages from shortly after fertilization until hatching，and returned immediately to the initial incubation temperature（23.0士0．2℃），nearly equal to the value at ferti1ization．From the results of hatching rate, the median hatchable shock temperature（MHST）was calculated for both total and viable hatch at every tested stage． Both MHSTs depended on the developmental stage at the time of the shock，and were lower at the early cleavage and blastopre closing stages than at any of the others．Namely，the eggs at these two stages were the most susceptible to the short-term thermal shock