We evaluated the effect of pH on larval development in larval Pacific oyster(Crassostrea gigas) and blood cockle(Arca inflata Reeve).The larvae were reared at pH 8.2(control),7.9,7.6,or 7.3beginning 30 min or 24 h post fertilization.Exposure to lower pH during early embryonic development inhibited larval shell formation in both species.Compared with the control,larvae took longer to reach the D-veliger stage when reared under pH 7.6 and 7.3.Exposure to lower pH immediately after fertilization resulted in significantly delayed shell formation in the Pacific oyster larvae at pH 7.3 and blood cockle larvae at pH 7.6 and 7.3.However,when exposure was delayed until 24 h post fertilization,shell formation was only inhibited in blood cockle larvae reared at pH 7.3.Thus,the early embryonic stages were more sensitive to acidified conditions.Our results suggest that ocean acidification will have an adverse effect on embryonic development in bivalves.Although the effects appear subtle,they may accumulate and lead to subsequent issues during later larval development. We evaluated the effect of pH on larval development in larval Pacific oyster (Crassostrea gigas) and blood cockle (Arca inflata Reeve). The larvae were reared at pH 8.2 (control), 7.9, 7.6, or 7.3 endoscopy 30 min or 24 h post Fertilization. Post to lower pH during early embryonic development inhibited larval shell formation in both species. Compared with the control, larvae took longer to reach the D-veliger stage when reared under pH 7.6 and 7.3. Post to lower pH immediately after fertilization resulted in Severely delayed shell formation in the Pacific oyster larvae at pH 7.3 and blood cockle larvae at pH 7.6 and 7.3. When, when exposure was delayed until 24 h post fertilization, shell formation was only inhibited in blood cockle larvae reared at pH 7.3.Thus, the early embryonic stages were more sensitive to acidified conditions. Our results suggest that ocean acidification will have an adverse effect on embryonic development in bivalves. Although the effects appeared subtle, they may ac cumulate and lead to subsequent issues later later larval development.