水母素（英语：Aequorin，又译埃奎明，分子量：相对分子约35000）是一种生物性发光蛋白质，属荧光素酶，具蛋白质一般特性，双缩脲和茚三酮反应均呈阳性，遇硫酸铵时沉淀。高温时荧光迅速消失。pH低于4时即使在0℃也不稳定，但在10mmolEDTA中相对稳定。溶液在紫外线照射下无荧光，但在加入Cu后即显明亮蓝色荧光。水母素EDTA溶液在波长280nm处有吸收峰。等电点pH3.5以下（因水母素吸附于纸上，故用纸电泳很难测准）。在溶液中存在钙离子的情况下，水母素可以通过催化其底物腔肠素发生氧化反应而发光。由于其发光受钙离子浓度的影响，因此可以用它来检测生物体内的钙。

Aequorin is a calcium-activated photoprotein isolated from the hydrozoan Aequorea victoria. Though the bioluminescence was studied decades before, the protein was originally isolated from the animal by Osamu Shimomura. In the animals, the protein occurs together with the Green fluorescent protein to produce green light by resonant energy transfer, while aequorin by itself generates blue light.