In situ hybridization showing Mnemiopsis photoprotein mRNA expression, and Mnemiopsis opsin mRNA expression. (A) Photoprotein expression: Panels A-D and F are lateral views; E is a surface view; and G-L are aboral views. The blastopore is indicated with an asterisk. Tb = tentacle bulb. (B) Opsin expression: Panels A-D and K are lateral views; E, F-J, and L-N are aboral views. The blastopore is indicated with an asterisk. Ao = apical organ. White arrows indicate expression in regions of putative photocytes. Black arrows indicate expression in four putative photoreceptors in apical organ. hpf: hours post-fertilization.