Fig. 4

Embryo morphology, heart rates of sensitive, resistant, and crossed embryos differ by developmental stages. a F. heteroclitus embryos at developmental stages 31, 35, and 36. LV: liver; UB: urinary bladder; CA: cartilage; AT: atrium; VT: ventricle. Stage 31: Heart chambers are differentiated under the cephalic region left atrium and the right ventricle rostrally. The liver circulation through the sinusoids can be observed. The bilobed urinary bladder contains visible precipitate. Stage 35: Pre-Hatching. Continuous elongation of the head causes further separation of the ventral head region from the yolk. The eyes are large and move frequently. The liver increases in size and obscures the view of the gall bladder. The tail can reach beyond the caudal border of the eye. The tip of the tail can reach beyond the otic vesicle and the embryo often rotates within the chorion. Stage 36: Hatching. The more frequent opening of the lower jaw indicates the beginning of the hatching, during which the hatching enzyme is released from unicellular glands in mucosa of buccal and pharyngeal regions. The chorion eventually lyses and the embryo breaks free tail-first. The tail straightens immediately upon hatching, and the embryo can swim within the hatching medium. b, c Heart rates of sensitive, resistant, and crossed embryos (n = 20 per family structure) whose parents were collected at Magotha Bay, VA and Elizabeth River, VA. d, e Heart rates differences among sensitive, resistant, and crossed embryos (n = 14 per family structure) whose parents were collected at Tuckerton, NJ and Newark Bay, NJ. Heart rates were determined at stage 31 (b, d) and 35 (c, e). *p < 0.05; **p < 0.01; ***p < 0.001; ****p < 0.0001